diff --git a/Cargo.lock b/Cargo.lock
index a06f5021..1f976cd2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -500,15 +500,6 @@ dependencies = [
  "either",
 ]
 
-[[package]]
-name = "itertools"
-version = "0.11.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b1c173a5686ce8bfa551b3563d0c2170bf24ca44da99c7ca4bfdab5418c3fe57"
-dependencies = [
- "either",
-]
-
 [[package]]
 name = "itertools"
 version = "0.12.0"
@@ -650,6 +641,7 @@ name = "nac3artiq"
 version = "0.1.0"
 dependencies = [
  "inkwell",
+ "itertools 0.12.0",
  "nac3core",
  "nac3ld",
  "nac3parser",
diff --git a/nac3artiq/Cargo.toml b/nac3artiq/Cargo.toml
index 1b4788f2..133ff283 100644
--- a/nac3artiq/Cargo.toml
+++ b/nac3artiq/Cargo.toml
@@ -9,6 +9,7 @@ name = "nac3artiq"
 crate-type = ["cdylib"]
 
 [dependencies]
+itertools = "0.12"
 pyo3 = { version = "0.20", features = ["extension-module"] }
 parking_lot = "0.12"
 tempfile = "3.8"
diff --git a/nac3artiq/src/lib.rs b/nac3artiq/src/lib.rs
index 59b3a91d..42e7b38d 100644
--- a/nac3artiq/src/lib.rs
+++ b/nac3artiq/src/lib.rs
@@ -13,6 +13,7 @@ use inkwell::{
     targets::*,
     OptimizationLevel,
 };
+use itertools::Itertools;
 use nac3core::codegen::{CodeGenLLVMOptions, CodeGenTargetMachineOptions, gen_func_impl};
 use nac3core::toplevel::builtins::get_exn_constructor;
 use nac3core::typecheck::typedef::{TypeEnum, Unifier};
@@ -470,7 +471,7 @@ impl Nac3 {
 
         if let Err(e) = composer.start_analysis(true) {
             // report error of __modinit__ separately
-            return if e.contains("<nac3_synthesized_modinit>") {
+            return if e.iter().any(|err| err.contains("<nac3_synthesized_modinit>")) {
                 let msg = Self::report_modinit(
                     &arg_names,
                     method_name,
@@ -481,12 +482,15 @@ impl Nac3 {
                 );
                 Err(CompileError::new_err(format!(
                     "compilation failed\n----------\n{}",
-                    msg.unwrap_or(e)
+                    msg.unwrap_or(e.iter().sorted().join("\n----------\n"))
                 )))
             } else {
-                Err(CompileError::new_err(format!(
-                    "compilation failed\n----------\n{e}"
-                )))
+                Err(CompileError::new_err(
+                    format!(
+                        "compilation failed\n----------\n{}",
+                        e.iter().sorted().join("\n----------\n"),
+                    ),
+                ))
             }
         }
         let top_level = Arc::new(composer.make_top_level_context());
diff --git a/nac3artiq/src/symbol_resolver.rs b/nac3artiq/src/symbol_resolver.rs
index bb5f6b09..a5440a12 100644
--- a/nac3artiq/src/symbol_resolver.rs
+++ b/nac3artiq/src/symbol_resolver.rs
@@ -15,7 +15,7 @@ use pyo3::{
     PyAny, PyObject, PyResult, Python,
 };
 use std::{
-    collections::HashMap,
+    collections::{HashMap, HashSet},
     sync::{
         Arc,
         atomic::{AtomicBool, Ordering::Relaxed}
@@ -1172,17 +1172,21 @@ impl SymbolResolver for Resolver {
         })
     }
 
-    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, String> {
+    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, HashSet<String>> {
         {
             let id_to_def = self.0.id_to_def.read();
             id_to_def.get(&id).copied().ok_or_else(String::new)
         }
         .or_else(|_| {
-            let py_id =
-                self.0.name_to_pyid.get(&id).ok_or(format!("Undefined identifier `{id}`"))?;
-            let result = self.0.pyid_to_def.read().get(py_id).copied().ok_or(format!(
-                "`{id}` is not registered with NAC3 (@nac3 decorator missing?)"
-            ))?;
+            let py_id = self.0.name_to_pyid.get(&id)
+                .ok_or_else(|| HashSet::from([
+                    format!("Undefined identifier `{id}`"),
+                ]))?;
+            let result = self.0.pyid_to_def.read().get(py_id)
+                .copied()
+                .ok_or_else(|| HashSet::from([
+                    format!("`{id}` is not registered with NAC3 (@nac3 decorator missing?)"),
+                ]))?;
             self.0.id_to_def.write().insert(id, result);
             Ok(result)
         })
diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index 6615a696..fc71c8e0 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -1649,7 +1649,7 @@ pub fn gen_expr<'ctx, G: CodeGenerator>(
                     let fun = ctx
                         .resolver
                         .get_identifier_def(*id)
-                        .map_err(|e| format!("{} (at {})", e, func.location))?;
+                        .map_err(|e| format!("{} (at {})", e.iter().next().unwrap(), func.location))?;
                     return Ok(generator
                         .gen_call(ctx, None, (&signature, fun), params)?
                         .map(Into::into));
diff --git a/nac3core/src/codegen/test.rs b/nac3core/src/codegen/test.rs
index de28ec4e..c3a8c719 100644
--- a/nac3core/src/codegen/test.rs
+++ b/nac3core/src/codegen/test.rs
@@ -63,12 +63,14 @@ impl SymbolResolver for Resolver {
         unimplemented!()
     }
 
-    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, String> {
+    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, HashSet<String>> {
         self.id_to_def
             .read()
             .get(&id)
             .cloned()
-            .ok_or_else(|| format!("cannot find symbol `{}`", id))
+            .ok_or_else(|| HashSet::from([
+                format!("cannot find symbol `{}`", id),
+            ]))
     }
 
     fn get_string_id(&self, _: &str) -> i32 {
diff --git a/nac3core/src/symbol_resolver.rs b/nac3core/src/symbol_resolver.rs
index 5224659d..54a243a6 100644
--- a/nac3core/src/symbol_resolver.rs
+++ b/nac3core/src/symbol_resolver.rs
@@ -1,6 +1,6 @@
 use std::fmt::Debug;
 use std::sync::Arc;
-use std::{collections::HashMap, fmt::Display};
+use std::{collections::HashMap, collections::HashSet, fmt::Display};
 use std::rc::Rc;
 
 use crate::typecheck::typedef::TypeEnum;
@@ -296,7 +296,7 @@ pub trait SymbolResolver {
     ) -> Result<Type, String>;
 
     /// Get the top-level definition of identifiers.
-    fn get_identifier_def(&self, str: StrRef) -> Result<DefinitionId, String>;
+    fn get_identifier_def(&self, str: StrRef) -> Result<DefinitionId, HashSet<String>>;
 
     fn get_symbol_value<'ctx>(
         &self,
@@ -341,7 +341,7 @@ pub fn parse_type_annotation<T>(
     unifier: &mut Unifier,
     primitives: &PrimitiveStore,
     expr: &Expr<T>,
-) -> Result<Type, String> {
+) -> Result<Type, HashSet<String>> {
     use nac3parser::ast::ExprKind::*;
     let ids = IDENTIFIER_ID.with(|ids| *ids);
     let int32_id = ids[0];
@@ -379,10 +379,12 @@ pub fn parse_type_annotation<T>(
                 let def = top_level_defs[obj_id.0].read();
                 if let TopLevelDef::Class { fields, methods, type_vars, .. } = &*def {
                     if !type_vars.is_empty() {
-                        return Err(format!(
-                            "Unexpected number of type parameters: expected {} but got 0",
-                            type_vars.len()
-                        ));
+                        return Err(HashSet::from([
+                            format!(
+                                "Unexpected number of type parameters: expected {} but got 0",
+                                type_vars.len()
+                            ),
+                        ]))
                     }
                     let fields = chain(
                         fields.iter().map(|(k, v, m)| (*k, (*v, *m))),
@@ -395,16 +397,22 @@ pub fn parse_type_annotation<T>(
                         params: HashMap::default(),
                     }))
                 } else {
-                    Err(format!("Cannot use function name as type at {loc}"))
+                    Err(HashSet::from([
+                        format!("Cannot use function name as type at {loc}"),
+                    ]))
                 }
             } else {
                 let ty = resolver
                     .get_symbol_type(unifier, top_level_defs, primitives, *id)
-                    .map_err(|e| format!("Unknown type annotation at {loc}: {e}"))?;
+                    .map_err(|e| HashSet::from([
+                        format!("Unknown type annotation at {loc}: {e}"),
+                    ]))?;
                 if let TypeEnum::TVar { .. } = &*unifier.get_ty(ty) {
                     Ok(ty)
                 } else {
-                    Err(format!("Unknown type annotation {id} at {loc}"))
+                    Err(HashSet::from([
+                        format!("Unknown type annotation {id} at {loc}"),
+                    ]))
                 }
             }
         }
@@ -427,7 +435,9 @@ pub fn parse_type_annotation<T>(
                     .collect::<Result<Vec<_>, _>>()?;
                 Ok(unifier.add_ty(TypeEnum::TTuple { ty }))
             } else {
-                Err("Expected multiple elements for tuple".into())
+                Err(HashSet::from([
+                    "Expected multiple elements for tuple".into()
+                ]))
             }
         } else {
             let types = if let Tuple { elts, .. } = &slice.node {
@@ -444,11 +454,13 @@ pub fn parse_type_annotation<T>(
             let def = top_level_defs[obj_id.0].read();
             if let TopLevelDef::Class { fields, methods, type_vars, .. } = &*def {
                 if types.len() != type_vars.len() {
-                    return Err(format!(
-                        "Unexpected number of type parameters: expected {} but got {}",
-                        type_vars.len(),
-                        types.len()
-                    ));
+                    return Err(HashSet::from([
+                        format!(
+                            "Unexpected number of type parameters: expected {} but got {}",
+                            type_vars.len(),
+                            types.len()
+                        ),
+                    ]))
                 }
                 let mut subst = HashMap::new();
                 for (var, ty) in izip!(type_vars.iter(), types.iter()) {
@@ -472,7 +484,9 @@ pub fn parse_type_annotation<T>(
                 }));
                 Ok(unifier.add_ty(TypeEnum::TObj { obj_id, fields, params: subst }))
             } else {
-                Err("Cannot use function name as type".into())
+                Err(HashSet::from([
+                    "Cannot use function name as type".into(),
+                ]))
             }
         }
     };
@@ -483,10 +497,14 @@ pub fn parse_type_annotation<T>(
             if let Name { id, .. } = &value.node {
                 subscript_name_handle(id, slice, unifier)
             } else {
-                Err(format!("unsupported type expression at {}", expr.location))
+                Err(HashSet::from([
+                    format!("unsupported type expression at {}", expr.location),
+                ]))
             }
         }
-        _ => Err(format!("unsupported type expression at {}", expr.location)),
+        _ => Err(HashSet::from([
+            format!("unsupported type expression at {}", expr.location),
+        ])),
     }
 }
 
@@ -497,7 +515,7 @@ impl dyn SymbolResolver + Send + Sync {
         unifier: &mut Unifier,
         primitives: &PrimitiveStore,
         expr: &Expr<T>,
-    ) -> Result<Type, String> {
+    ) -> Result<Type, HashSet<String>> {
         parse_type_annotation(self, top_level_defs, unifier, primitives, expr)
     }
 
diff --git a/nac3core/src/toplevel/composer.rs b/nac3core/src/toplevel/composer.rs
index d47be350..e3fb80b4 100644
--- a/nac3core/src/toplevel/composer.rs
+++ b/nac3core/src/toplevel/composer.rs
@@ -362,7 +362,7 @@ impl TopLevelComposer {
         }
     }
 
-    pub fn start_analysis(&mut self, inference: bool) -> Result<(), String> {
+    pub fn start_analysis(&mut self, inference: bool) -> Result<(), HashSet<String>> {
         self.analyze_top_level_class_type_var()?;
         self.analyze_top_level_class_bases()?;
         self.analyze_top_level_class_fields_methods()?;
@@ -374,7 +374,7 @@ impl TopLevelComposer {
     }
 
     /// step 1, analyze the type vars associated with top level class
-    fn analyze_top_level_class_type_var(&mut self) -> Result<(), String> {
+    fn analyze_top_level_class_type_var(&mut self) -> Result<(), HashSet<String>> {
         let def_list = &self.definition_ast_list;
         let temp_def_list = self.extract_def_list();
         let unifier = self.unifier.borrow_mut();
@@ -417,10 +417,12 @@ impl TopLevelComposer {
                         } =>
                     {
                         if is_generic {
-                            return Err(format!(
-                                "only single Generic[...] is allowed (at {})",
-                                b.location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "only single Generic[...] is allowed (at {})",
+                                    b.location
+                                ),
+                            ]))
                         }
                         is_generic = true;
 
@@ -459,10 +461,12 @@ impl TopLevelComposer {
                             })
                         };
                         if !all_unique_type_var {
-                            return Err(format!(
-                                "duplicate type variable occurs (at {})",
-                                slice.location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "duplicate type variable occurs (at {})",
+                                    slice.location
+                                ),
+                            ]))
                         }
 
                         // add to TopLevelDef
@@ -481,11 +485,11 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = analyze(class_def, class_ast) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
         Ok(())
     }
@@ -493,7 +497,7 @@ impl TopLevelComposer {
     /// step 2, base classes.
     /// now that the type vars of all classes are done, handle base classes and
     /// put Self class into the ancestors list. We only allow single inheritance
-    fn analyze_top_level_class_bases(&mut self) -> Result<(), String> {
+    fn analyze_top_level_class_bases(&mut self) -> Result<(), HashSet<String>> {
         if self.unifier.top_level.is_none() {
             let ctx = Arc::new(self.make_top_level_context());
             self.unifier.top_level = Some(ctx);
@@ -542,11 +546,13 @@ impl TopLevelComposer {
                     }
 
                     if has_base {
-                        return Err(format!(
-                            "a class definition can only have at most one base class \
-                        declaration and one generic declaration (at {})",
-                            b.location
-                        ));
+                        return Err(HashSet::from([
+                            format!(
+                                "a class definition can only have at most one base class \
+                                declaration and one generic declaration (at {})",
+                                b.location
+                            ),
+                        ]))
                     }
                     has_base = true;
 
@@ -565,10 +571,12 @@ impl TopLevelComposer {
                     if let TypeAnnotation::CustomClass { .. } = &base_ty {
                         class_ancestors.push(base_ty);
                     } else {
-                        return Err(format!(
-                            "class base declaration can only be custom class (at {})",
-                            b.location,
-                        ));
+                        return Err(HashSet::from([
+                            format!(
+                                "class base declaration can only be custom class (at {})",
+                                b.location,
+                            ),
+                        ]))
                     }
                 }
                 Ok(())
@@ -581,16 +589,16 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = get_direct_parents(class_def, class_ast) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
 
         // second, get all ancestors
         let mut ancestors_store: HashMap<DefinitionId, Vec<TypeAnnotation>> = HashMap::default();
-        let mut get_all_ancestors = |class_def: &Arc<RwLock<TopLevelDef>>| {
+        let mut get_all_ancestors = |class_def: &Arc<RwLock<TopLevelDef>>| -> Result<(), HashSet<String>> {
             let class_def = class_def.read();
             let (class_ancestors, class_id) = {
                 if let TopLevelDef::Class { ancestors, object_id, .. } = &*class_def {
@@ -615,11 +623,11 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = get_all_ancestors(class_def) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
 
         // insert the ancestors to the def list
@@ -657,7 +665,9 @@ impl TopLevelComposer {
                             stmt.node,
                             ast::StmtKind::FunctionDef { .. } | ast::StmtKind::AnnAssign { .. }
                         ) {
-                            return Err("Classes inherited from exception should have no custom fields/methods".into());
+                            return Err(HashSet::from([
+                                "Classes inherited from exception should have no custom fields/methods".into()
+                            ]))
                         }
                     }
                 } else {
@@ -680,7 +690,7 @@ impl TopLevelComposer {
     }
 
     /// step 3, class fields and methods
-    fn analyze_top_level_class_fields_methods(&mut self) -> Result<(), String> {
+    fn analyze_top_level_class_fields_methods(&mut self) -> Result<(), HashSet<String>> {
         let temp_def_list = self.extract_def_list();
         let primitives = &self.primitives_ty;
         let def_ast_list = &self.definition_ast_list;
@@ -703,12 +713,12 @@ impl TopLevelComposer {
                     &mut type_var_to_concrete_def,
                     (&self.keyword_list, &self.core_config),
                 ) {
-                    errors.insert(e);
+                    errors.extend(e);
                 }
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
 
         // handle the inherited methods and fields
@@ -752,15 +762,16 @@ impl TopLevelComposer {
 
         let mut subst_list = Some(Vec::new());
         // unification of previously assigned typevar
-        let mut unification_helper = |ty, def| {
+        let mut unification_helper = |ty, def| -> Result<(), HashSet<String>> {
             let target_ty =
                 get_type_from_type_annotation_kinds(&temp_def_list, unifier, &def, &mut subst_list)?;
-            unifier.unify(ty, target_ty).map_err(|e| e.to_display(unifier).to_string())?;
+            unifier.unify(ty, target_ty)
+                .map_err(|e| HashSet::from([e.to_display(unifier).to_string()]))?;
             Ok(())
         };
         for (ty, def) in type_var_to_concrete_def {
             if let Err(e) = unification_helper(ty, def) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         for ty in subst_list.unwrap() {
@@ -787,7 +798,7 @@ impl TopLevelComposer {
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
 
         for (def, _) in def_ast_list.iter().skip(self.builtin_num) {
@@ -806,7 +817,7 @@ impl TopLevelComposer {
     }
 
     /// step 4, after class methods are done, top level functions have nothing unknown
-    fn analyze_top_level_function(&mut self) -> Result<(), String> {
+    fn analyze_top_level_function(&mut self) -> Result<(), HashSet<String>> {
         let def_list = &self.definition_ast_list;
         let keyword_list = &self.keyword_list;
         let temp_def_list = self.extract_def_list();
@@ -842,11 +853,13 @@ impl TopLevelComposer {
                             if !defined_parameter_name.insert(x.node.arg)
                                 || keyword_list.contains(&x.node.arg)
                             {
-                                return Err(format!(
-                                    "top level function must have unique parameter names \
-                                    and names should not be the same as the keywords (at {})",
-                                    x.location
-                                ));
+                                return Err(HashSet::from([
+                                    format!(
+                                        "top level function must have unique parameter names \
+                                        and names should not be the same as the keywords (at {})",
+                                        x.location
+                                    ),
+                                ]))
                             }
                         }
 
@@ -869,17 +882,17 @@ impl TopLevelComposer {
                         arg_with_default
                             .iter()
                             .rev()
-                            .map(|(x, default)| -> Result<FuncArg, String> {
+                            .map(|(x, default)| -> Result<FuncArg, HashSet<String>> {
                                 let annotation = x
                                     .node
                                     .annotation
                                     .as_ref()
-                                    .ok_or_else(|| {
+                                    .ok_or_else(|| HashSet::from([
                                         format!(
                                             "function parameter `{}` needs type annotation at {}",
                                             x.node.arg, x.location
-                                        )
-                                    })?
+                                        ),
+                                    ]))?
                                     .as_ref();
 
                                 let type_annotation = parse_ast_to_type_annotation_kinds(
@@ -897,7 +910,7 @@ impl TopLevelComposer {
                                 let type_vars_within =
                                     get_type_var_contained_in_type_annotation(&type_annotation)
                                         .into_iter()
-                                        .map(|x| -> Result<(u32, Type), String> {
+                                        .map(|x| -> Result<(u32, Type), HashSet<String>> {
                                             if let TypeAnnotation::TypeVar(ty) = x {
                                                 Ok((Self::get_var_id(ty, unifier)?, ty))
                                             } else {
@@ -934,9 +947,9 @@ impl TopLevelComposer {
                                                 primitives_store,
                                                 unifier,
                                             )
-                                            .map_err(
-                                                |err| format!("{} (at {})", err, x.location),
-                                            )?;
+                                            .map_err(|err| HashSet::from([
+                                                format!("{} (at {})", err, x.location),
+                                            ]))?;
                                             v
                                         }),
                                     },
@@ -965,7 +978,7 @@ impl TopLevelComposer {
                             let type_vars_within =
                                 get_type_var_contained_in_type_annotation(&return_ty_annotation)
                                     .into_iter()
-                                    .map(|x| -> Result<(u32, Type), String> {
+                                    .map(|x| -> Result<(u32, Type), HashSet<String>> {
                                         if let TypeAnnotation::TypeVar(ty) = x {
                                             Ok((Self::get_var_id(ty, unifier)?, ty))
                                         } else {
@@ -1007,9 +1020,9 @@ impl TopLevelComposer {
                         ret: return_ty,
                         vars: function_var_map,
                     }));
-                    unifier.unify(*dummy_ty, function_ty).map_err(|e| {
-                        e.at(Some(function_ast.location)).to_display(unifier).to_string()
-                    })?;
+                    unifier.unify(*dummy_ty, function_ty).map_err(|e| HashSet::from([
+                        e.at(Some(function_ast.location)).to_display(unifier).to_string(),
+                    ]))?;
                 } else {
                     unreachable!("must be both function");
                 }
@@ -1024,11 +1037,11 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = analyze(function_def, function_ast) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
         Ok(())
     }
@@ -1041,7 +1054,7 @@ impl TopLevelComposer {
         primitives: &PrimitiveStore,
         type_var_to_concrete_def: &mut HashMap<Type, TypeAnnotation>,
         core_info: (&HashSet<StrRef>, &ComposerConfig),
-    ) -> Result<(), String> {
+    ) -> Result<(), HashSet<String>> {
         let (keyword_list, core_config) = core_info;
         let mut class_def = class_def.write();
         let (
@@ -1092,25 +1105,23 @@ impl TopLevelComposer {
                             if !defined_parameter_name.insert(x.node.arg)
                                 || (keyword_list.contains(&x.node.arg) && x.node.arg != zelf)
                             {
-                                return Err(format!(
-                                    "top level function must have unique parameter names \
+                                return Err(HashSet::from([
+                                    format!("top level function must have unique parameter names \
                                     and names should not be the same as the keywords (at {})",
-                                    x.location
-                                ));
+                                    x.location),
+                                ]))
                             }
                         }
 
                         if name == &"__init__".into() && !defined_parameter_name.contains(&zelf) {
-                            return Err(format!(
-                                "__init__ method must have a `self` parameter (at {})",
-                                b.location
-                            ));
+                            return Err(HashSet::from([
+                                format!("__init__ method must have a `self` parameter (at {})", b.location),
+                            ]))
                         }
                         if !defined_parameter_name.contains(&zelf) {
-                            return Err(format!(
-                                "class method must have a `self` parameter (at {})",
-                                b.location
-                            ));
+                            return Err(HashSet::from([
+                                format!("class method must have a `self` parameter (at {})", b.location),
+                            ]))
                         }
 
                         let mut result = Vec::new();
@@ -1139,12 +1150,12 @@ impl TopLevelComposer {
                                         .node
                                         .annotation
                                         .as_ref()
-                                        .ok_or_else(|| {
+                                        .ok_or_else(|| HashSet::from([
                                             format!(
                                                 "type annotation needed for `{}` at {}",
                                                 x.node.arg, x.location
-                                            )
-                                        })?
+                                            ),
+                                        ]))?
                                         .as_ref();
                                     parse_ast_to_type_annotation_kinds(
                                         class_resolver,
@@ -1181,7 +1192,9 @@ impl TopLevelComposer {
                                         None => None,
                                         Some(default) => {
                                             if name == "self".into() {
-                                                return Err(format!("`self` parameter cannot take default value (at {})", x.location));
+                                                return Err(HashSet::from([
+                                                    format!("`self` parameter cannot take default value (at {})", x.location),
+                                                ]));
                                             }
                                             Some({
                                                 let v = Self::parse_parameter_default_value(
@@ -1191,9 +1204,9 @@ impl TopLevelComposer {
                                                 Self::check_default_param_type(
                                                     &v, &type_ann, primitives, unifier,
                                                 )
-                                                .map_err(|err| {
-                                                    format!("{} (at {})", err, x.location)
-                                                })?;
+                                                .map_err(|err| HashSet::from([
+                                                    format!("{} (at {})", err, x.location),
+                                                ]))?;
                                                 v
                                             })
                                         }
@@ -1279,7 +1292,7 @@ impl TopLevelComposer {
                     // which should be fine since type within method_type will be subst later
                     unifier
                         .unify(method_dummy_ty, method_type)
-                        .map_err(|e| e.to_display(unifier).to_string())?;
+                        .map_err(|e| HashSet::from([e.to_display(unifier).to_string()]))?;
                 }
                 ast::StmtKind::AnnAssign { target, annotation, value: None, .. } => {
                     if let ast::ExprKind::Name { id: attr, .. } = &target.node {
@@ -1325,11 +1338,13 @@ impl TopLevelComposer {
                             for type_var_within in type_vars_within {
                                 if let TypeAnnotation::TypeVar(t) = type_var_within {
                                     if !class_type_vars_def.contains(&t) {
-                                        return Err(format!(
-                                            "class fields can only use type \
-                                            vars over which the class is generic (at {})",
-                                            annotation.location
-                                        ));
+                                        return Err(HashSet::from([
+                                            format!(
+                                                "class fields can only use type \
+                                                vars over which the class is generic (at {})",
+                                                annotation.location
+                                            ),
+                                        ]))
                                     }
                                 } else {
                                     unreachable!("must be type var annotation");
@@ -1337,26 +1352,32 @@ impl TopLevelComposer {
                             }
                             type_var_to_concrete_def.insert(dummy_field_type, parsed_annotation);
                         } else {
-                            return Err(format!(
-                                "same class fields `{}` defined twice (at {})",
-                                attr, target.location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "same class fields `{}` defined twice (at {})",
+                                    attr, target.location
+                                ),
+                            ]))
                         }
                     } else {
-                        return Err(format!(
-                            "unsupported statement type in class definition body (at {})",
-                            target.location
-                        ));
+                        return Err(HashSet::from([
+                            format!(
+                                "unsupported statement type in class definition body (at {})",
+                                target.location
+                            ),
+                        ]))
                     }
                 }
                 ast::StmtKind::Assign { .. } => {}, // we don't class attributes
                 ast::StmtKind::Pass { .. } => {}
                 ast::StmtKind::Expr { value: _, .. } => {} // typically a docstring; ignoring all expressions matches CPython behavior
                 _ => {
-                    return Err(format!(
-                        "unsupported statement in class definition body (at {})",
-                        b.location
-                    ))
+                    return Err(HashSet::from([
+                        format!(
+                            "unsupported statement in class definition body (at {})",
+                            b.location
+                        ),
+                    ]))
                 }
             }
         }
@@ -1369,7 +1390,7 @@ impl TopLevelComposer {
         unifier: &mut Unifier,
         _primitives: &PrimitiveStore,
         type_var_to_concrete_def: &mut HashMap<Type, TypeAnnotation>,
-    ) -> Result<(), String> {
+    ) -> Result<(), HashSet<String>> {
         let (
             _class_id,
             class_ancestor_def,
@@ -1420,9 +1441,11 @@ impl TopLevelComposer {
                                     type_var_to_concrete_def,
                                 );
                             if !ok {
-                                return Err(format!(
-                                    "method {class_method_name} has same name as ancestors' method, but incompatible type"
-                                ));
+                                return Err(HashSet::from([
+                                    format!(
+                                        "method {class_method_name} has same name as ancestors' method, but incompatible type"
+                                    ),
+                                ]))
                             }
                             // mark it as added
                             is_override.insert(*class_method_name);
@@ -1457,9 +1480,11 @@ impl TopLevelComposer {
                     // find if there is a fields with the same name in the child class
                     for (class_field_name, ..) in &*class_fields_def {
                         if class_field_name == anc_field_name {
-                            return Err(format!(
-                                "field `{class_field_name}` has already declared in the ancestor classes"
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "field `{class_field_name}` has already declared in the ancestor classes"
+                                ),
+                            ]))
                         }
                     }
                     new_child_fields.push(to_be_added);
@@ -1483,7 +1508,7 @@ impl TopLevelComposer {
 
     /// step 5, analyze and call type inferencer to fill the `instance_to_stmt` of
     /// [`TopLevelDef::Function`]
-    fn analyze_function_instance(&mut self) -> Result<(), String> {
+    fn analyze_function_instance(&mut self) -> Result<(), HashSet<String>> {
         // first get the class constructor type correct for the following type check in function body
         // also do class field instantiation check
         let init_str_id = "__init__".into();
@@ -1588,9 +1613,9 @@ impl TopLevelComposer {
                     };
                     constructors.push((i, signature, definition_extension.len()));
                     definition_extension.push((Arc::new(RwLock::new(cons_fun)), None));
-                    unifier.unify(constructor.unwrap(), signature).map_err(|e| {
+                    unifier.unify(constructor.unwrap(), signature).map_err(|e| HashSet::from([
                         e.at(Some(ast.as_ref().unwrap().location)).to_display(unifier).to_string()
-                    })?;
+                    ]))?;
                     return Ok(());
                 }
                 let mut init_id: Option<DefinitionId> = None;
@@ -1618,9 +1643,9 @@ impl TopLevelComposer {
                     ret: self_type,
                     vars: contor_type_vars,
                 }));
-                unifier.unify(constructor.unwrap(), contor_type).map_err(|e| {
+                unifier.unify(constructor.unwrap(), contor_type).map_err(|e| HashSet::from([
                     e.at(Some(ast.as_ref().unwrap().location)).to_display(unifier).to_string()
-                })?;
+                ]))?;
 
                 // class field instantiation check
                 if let (Some(init_id), false) = (init_id, fields.is_empty()) {
@@ -1632,12 +1657,14 @@ impl TopLevelComposer {
                         let all_inited = Self::get_all_assigned_field(body.as_slice())?;
                         for (f, _, _) in fields {
                             if !all_inited.contains(f) {
-                                return Err(format!(
-                                    "fields `{}` of class `{}` not fully initialized in the initializer (at {})",
-                                    f,
-                                    class_name,
-                                    body[0].location,
-                                ));
+                                return Err(HashSet::from([
+                                    format!(
+                                        "fields `{}` of class `{}` not fully initialized in the initializer (at {})",
+                                        f,
+                                        class_name,
+                                        body[0].location,
+                                    ),
+                                ]))
                             }
                         }
                     }
@@ -1650,11 +1677,11 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = analyze(i, def, ast) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n---------\n"));
+            return Err(errors)
         }
 
         for (i, signature, id) in constructors {
@@ -1869,9 +1896,9 @@ impl TopLevelComposer {
                                     if let TypeEnum::TObj { obj_id, .. } = &*base {
                                         *obj_id
                                     } else {
-                                        return Err(format!(
-                                            "Base type should be a class (at {loc})"
-                                        ));
+                                        return Err(HashSet::from([
+                                            format!("Base type should be a class (at {loc})"),
+                                        ]))
                                     }
                                 };
                                 let subtype_id = {
@@ -1881,9 +1908,11 @@ impl TopLevelComposer {
                                     } else {
                                         let base_repr = inferencer.unifier.stringify(*base);
                                         let subtype_repr = inferencer.unifier.stringify(*subtype);
-                                        return Err(format!(
-                                            "Expected a subtype of {base_repr}, but got {subtype_repr} (at {loc})"
-                                        ));
+                                        return Err(HashSet::from([
+                                            format!(
+                                                "Expected a subtype of {base_repr}, but got {subtype_repr} (at {loc})"
+                                            ),
+                                        ]))
                                     }
                                 };
                                 let subtype_entry = defs[subtype_id.0].read();
@@ -1893,9 +1922,11 @@ impl TopLevelComposer {
                                     if m.is_none() {
                                         let base_repr = inferencer.unifier.stringify(*base);
                                         let subtype_repr = inferencer.unifier.stringify(*subtype);
-                                        return Err(format!(
-                                            "Expected a subtype of {base_repr}, but got {subtype_repr} (at {loc})"
-                                        ));
+                                        return Err(HashSet::from([
+                                            format!(
+                                                "Expected a subtype of {base_repr}, but got {subtype_repr} (at {loc})"
+                                            ),
+                                        ]))
                                     }
                                 } else {
                                     unreachable!();
@@ -1918,12 +1949,14 @@ impl TopLevelComposer {
                                 &mut |id| format!("typevar{id}"),
                                 &mut None,
                             );
-                            return Err(format!(
-                                "expected return type of `{}` in function `{}` (at {})",
-                                ret_str,
-                                name,
-                                ast.as_ref().unwrap().location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "expected return type of `{}` in function `{}` (at {})",
+                                    ret_str,
+                                    name,
+                                    ast.as_ref().unwrap().location
+                                ),
+                            ]))
                         }
 
                         instance_to_stmt.insert(
@@ -1947,11 +1980,11 @@ impl TopLevelComposer {
                 continue;
             }
             if let Err(e) = analyze_2(id, def, ast) {
-                errors.insert(e);
+                errors.extend(e);
             }
         }
         if !errors.is_empty() {
-            return Err(errors.into_iter().sorted().join("\n----------\n"));
+            return Err(errors)
         }
         Ok(())
     }
diff --git a/nac3core/src/toplevel/helper.rs b/nac3core/src/toplevel/helper.rs
index 6399f6da..1367543e 100644
--- a/nac3core/src/toplevel/helper.rs
+++ b/nac3core/src/toplevel/helper.rs
@@ -204,13 +204,13 @@ impl TopLevelComposer {
     pub fn get_class_method_def_info(
         class_methods_def: &[(StrRef, Type, DefinitionId)],
         method_name: StrRef,
-    ) -> Result<(Type, DefinitionId), String> {
+    ) -> Result<(Type, DefinitionId), HashSet<String>> {
         for (name, ty, def_id) in class_methods_def {
             if name == &method_name {
                 return Ok((*ty, *def_id));
             }
         }
-        Err(format!("no method {method_name} in the current class"))
+        Err(HashSet::from([format!("no method {method_name} in the current class")]))
     }
 
     /// get all base class def id of a class, excluding itself. \
@@ -221,7 +221,7 @@ impl TopLevelComposer {
     pub fn get_all_ancestors_helper(
         child: &TypeAnnotation,
         temp_def_list: &[Arc<RwLock<TopLevelDef>>],
-    ) -> Result<Vec<TypeAnnotation>, String> {
+    ) -> Result<Vec<TypeAnnotation>, HashSet<String>> {
         let mut result: Vec<TypeAnnotation> = Vec::new();
         let mut parent = Self::get_parent(child, temp_def_list);
         while let Some(p) = parent {
@@ -242,7 +242,7 @@ impl TopLevelComposer {
             if no_cycle {
                 result.push(p);
             } else {
-                return Err("cyclic inheritance detected".into());
+                return Err(HashSet::from(["cyclic inheritance detected".into()]));
             }
         }
         Ok(result)
@@ -272,11 +272,13 @@ impl TopLevelComposer {
     }
 
     /// get the `var_id` of a given `TVar` type
-    pub fn get_var_id(var_ty: Type, unifier: &mut Unifier) -> Result<u32, String> {
+    pub fn get_var_id(var_ty: Type, unifier: &mut Unifier) -> Result<u32, HashSet<String>> {
         if let TypeEnum::TVar { id, .. } = unifier.get_ty(var_ty).as_ref() {
             Ok(*id)
         } else {
-            Err("not type var".to_string())
+            Err(HashSet::from([
+                "not type var".to_string(),
+            ]))
         }
     }
 
@@ -338,7 +340,7 @@ impl TopLevelComposer {
         )
     }
 
-    pub fn get_all_assigned_field(stmts: &[Stmt<()>]) -> Result<HashSet<StrRef>, String> {
+    pub fn get_all_assigned_field(stmts: &[Stmt<()>]) -> Result<HashSet<StrRef>, HashSet<String>> {
         let mut result = HashSet::new();
         for s in stmts {
             match &s.node {
@@ -355,10 +357,12 @@ impl TopLevelComposer {
                         }
                     } =>
                 {
-                    return Err(format!(
-                        "redundant type annotation for class fields at {}",
-                        s.location
-                    ))
+                    return Err(HashSet::from([
+                        format!(
+                            "redundant type annotation for class fields at {}",
+                            s.location
+                        ),
+                    ]))
                 }
                 ast::StmtKind::Assign { targets, .. } => {
                     for t in targets {
@@ -410,7 +414,7 @@ impl TopLevelComposer {
     pub fn parse_parameter_default_value(
         default: &ast::Expr,
         resolver: &(dyn SymbolResolver + Send + Sync),
-    ) -> Result<SymbolValue, String> {
+    ) -> Result<SymbolValue, HashSet<String>> {
         parse_parameter_default_value(default, resolver)
     }
 
@@ -467,14 +471,14 @@ impl TopLevelComposer {
 pub fn parse_parameter_default_value(
     default: &ast::Expr,
     resolver: &(dyn SymbolResolver + Send + Sync),
-) -> Result<SymbolValue, String> {
-    fn handle_constant(val: &Constant, loc: &Location) -> Result<SymbolValue, String> {
+) -> Result<SymbolValue, HashSet<String>> {
+    fn handle_constant(val: &Constant, loc: &Location) -> Result<SymbolValue, HashSet<String>> {
         match val {
             Constant::Int(v) => {
                 if let Ok(v) = (*v).try_into() {
                     Ok(SymbolValue::I32(v))
                 } else {
-                    Err(format!("integer value out of range at {loc}"))
+                    Err(HashSet::from([format!("integer value out of range at {loc}")]))
                 }
             }
             Constant::Float(v) => Ok(SymbolValue::Double(*v)),
@@ -482,9 +486,11 @@ pub fn parse_parameter_default_value(
             Constant::Tuple(tuple) => Ok(SymbolValue::Tuple(
                 tuple.iter().map(|x| handle_constant(x, loc)).collect::<Result<Vec<_>, _>>()?,
             )),
-            Constant::None => Err(format!(
-                "`None` is not supported, use `none` for option type instead ({loc})"
-            )),
+            Constant::None => Err(HashSet::from([
+                format!(
+                    "`None` is not supported, use `none` for option type instead ({loc})"
+                ),
+            ])),
             _ => unimplemented!("this constant is not supported at {}", loc),
         }
     }
@@ -497,37 +503,51 @@ pub fn parse_parameter_default_value(
                         let v: Result<i64, _> = (*v).try_into();
                         match v {
                             Ok(v) => Ok(SymbolValue::I64(v)),
-                            _ => Err(format!("default param value out of range at {}", default.location)),
+                            _ => Err(HashSet::from([
+                                format!("default param value out of range at {}", default.location)
+                            ])),
                         }
                     }
-                    _ => Err(format!("only allow constant integer here at {}", default.location))
+                    _ => Err(HashSet::from([
+                        format!("only allow constant integer here at {}", default.location),
+                    ]))
                 }
                 ast::ExprKind::Name { id, .. } if *id == "uint32".into() => match &args[0].node {
                     ast::ExprKind::Constant { value: Constant::Int(v), .. } => {
                         let v: Result<u32, _> = (*v).try_into();
                         match v {
                             Ok(v) => Ok(SymbolValue::U32(v)),
-                            _ => Err(format!("default param value out of range at {}", default.location)),
+                            _ => Err(HashSet::from([
+                                format!("default param value out of range at {}", default.location),
+                            ])),
                         }
                     }
-                    _ => Err(format!("only allow constant integer here at {}", default.location))
+                    _ => Err(HashSet::from([
+                        format!("only allow constant integer here at {}", default.location),
+                    ]))
                 }
                 ast::ExprKind::Name { id, .. } if *id == "uint64".into() => match &args[0].node {
                     ast::ExprKind::Constant { value: Constant::Int(v), .. } => {
                         let v: Result<u64, _> = (*v).try_into();
                         match v {
                             Ok(v) => Ok(SymbolValue::U64(v)),
-                            _ => Err(format!("default param value out of range at {}", default.location)),
+                            _ => Err(HashSet::from([
+                                format!("default param value out of range at {}", default.location),
+                            ])),
                         }
                     }
-                    _ => Err(format!("only allow constant integer here at {}", default.location))
+                    _ => Err(HashSet::from([
+                        format!("only allow constant integer here at {}", default.location),
+                    ]))
                 }
                 ast::ExprKind::Name { id, .. } if *id == "Some".into() => Ok(
                     SymbolValue::OptionSome(
                         Box::new(parse_parameter_default_value(&args[0], resolver)?)
                     )
                 ),
-                _ => Err(format!("unsupported default parameter at {}", default.location)),
+                _ => Err(HashSet::from([
+                    format!("unsupported default parameter at {}", default.location),
+                ])),
             }
         }
         ast::ExprKind::Tuple { elts, .. } => Ok(SymbolValue::Tuple(elts
@@ -538,17 +558,21 @@ pub fn parse_parameter_default_value(
         ast::ExprKind::Name { id, .. } if id == &"none".into() => Ok(SymbolValue::OptionNone),
         ast::ExprKind::Name { id, .. } => {
             resolver.get_default_param_value(default).ok_or_else(
-                || format!(
-                    "`{}` cannot be used as a default parameter at {} \
-                    (not primitive type, option or tuple / not defined?)",
-                    id,
-                    default.location
-                )
+                || HashSet::from([
+                    format!(
+                        "`{}` cannot be used as a default parameter at {} \
+                        (not primitive type, option or tuple / not defined?)",
+                        id,
+                        default.location
+                    ),
+                ])
             )
         }
-        _ => Err(format!(
-            "unsupported default parameter (not primitive type, option or tuple) at {}",
-            default.location
-        ))
+        _ => Err(HashSet::from([
+            format!(
+                "unsupported default parameter (not primitive type, option or tuple) at {}",
+                default.location
+            ),
+        ]))
     }
 }
diff --git a/nac3core/src/toplevel/test.rs b/nac3core/src/toplevel/test.rs
index 2b8b03ad..19dc2165 100644
--- a/nac3core/src/toplevel/test.rs
+++ b/nac3core/src/toplevel/test.rs
@@ -64,8 +64,9 @@ impl SymbolResolver for Resolver {
         unimplemented!()
     }
 
-    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, String> {
-        self.0.id_to_def.lock().get(&id).cloned().ok_or_else(|| "Unknown identifier".to_string())
+    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, HashSet<String>> {
+        self.0.id_to_def.lock().get(&id).cloned()
+            .ok_or_else(|| HashSet::from(["Unknown identifier".to_string()]))
     }
 
     fn get_string_id(&self, _: &str) -> i32 {
@@ -551,9 +552,9 @@ fn test_analyze(source: Vec<&str>, res: Vec<&str>) {
 
     if let Err(msg) = composer.start_analysis(false) {
         if print {
-            println!("{}", msg);
+            println!("{}", msg.iter().sorted().join("\n----------\n"));
         } else {
-            assert_eq!(res[0], msg);
+            assert_eq!(res[0], msg.iter().next().unwrap());
         }
     } else {
         // skip 5 to skip primitives
@@ -735,9 +736,9 @@ fn test_inference(source: Vec<&str>, res: Vec<&str>) {
 
     if let Err(msg) = composer.start_analysis(true) {
         if print {
-            println!("{}", msg);
+            println!("{}", msg.iter().sorted().join("\n----------\n"));
         } else {
-            assert_eq!(res[0], msg);
+            assert_eq!(res[0], msg.iter().next().unwrap());
         }
     } else {
         // skip 5 to skip primitives
diff --git a/nac3core/src/toplevel/type_annotation.rs b/nac3core/src/toplevel/type_annotation.rs
index 54922776..82b5bf99 100644
--- a/nac3core/src/toplevel/type_annotation.rs
+++ b/nac3core/src/toplevel/type_annotation.rs
@@ -82,7 +82,7 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
     // the key stores the type_var of this topleveldef::class, we only need this field here
     locked: HashMap<DefinitionId, Vec<Type>>,
     type_var: Option<Type>,
-) -> Result<TypeAnnotation, String> {
+) -> Result<TypeAnnotation, HashSet<String>> {
     let name_handle = |id: &StrRef,
                        unifier: &mut Unifier,
                        locked: HashMap<DefinitionId, Vec<Type>>| {
@@ -109,10 +109,12 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
                     if let TopLevelDef::Class { type_vars, .. } = &*def_read {
                         type_vars.clone()
                     } else {
-                        return Err(format!(
-                            "function cannot be used as a type (at {})",
-                            expr.location
-                        ));
+                        return Err(HashSet::from([
+                            format!(
+                                "function cannot be used as a type (at {})",
+                                expr.location
+                            ),
+                        ]))
                     }
                 } else {
                     locked.get(&obj_id).unwrap().clone()
@@ -120,11 +122,13 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
             };
             // check param number here
             if !type_vars.is_empty() {
-                return Err(format!(
-                    "expect {} type variable parameter but got 0 (at {})",
-                    type_vars.len(),
-                    expr.location,
-                ));
+                return Err(HashSet::from([
+                    format!(
+                        "expect {} type variable parameter but got 0 (at {})",
+                        type_vars.len(),
+                        expr.location,
+                    ),
+                ]))
             }
             Ok(TypeAnnotation::CustomClass { id: obj_id, params: vec![] })
         } else if let Ok(ty) = resolver.get_symbol_type(unifier, top_level_defs, primitives, *id) {
@@ -133,10 +137,14 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
                 unifier.unify(var, ty).unwrap();
                 Ok(TypeAnnotation::TypeVar(ty))
             } else {
-                Err(format!("`{}` is not a valid type annotation (at {})", id, expr.location))
+                Err(HashSet::from([
+                    format!("`{}` is not a valid type annotation (at {})", id, expr.location),
+                ]))
             }
         } else {
-            Err(format!("`{}` is not a valid type annotation (at {})", id, expr.location))
+            Err(HashSet::from([
+                format!("`{}` is not a valid type annotation (at {})", id, expr.location),
+            ]))
         }
     };
 
@@ -147,7 +155,9 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
          mut locked: HashMap<DefinitionId, Vec<Type>>| {
             if ["virtual".into(), "Generic".into(), "list".into(), "tuple".into(), "Option".into()].contains(id)
             {
-                return Err(format!("keywords cannot be class name (at {})", expr.location));
+                return Err(HashSet::from([
+                    format!("keywords cannot be class name (at {})", expr.location),
+                ]))
             }
             let obj_id = resolver.get_identifier_def(*id)?;
             let type_vars = {
@@ -170,12 +180,14 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
                     vec![slice]
                 };
                 if type_vars.len() != params_ast.len() {
-                    return Err(format!(
-                        "expect {} type parameters but got {} (at {})",
-                        type_vars.len(),
-                        params_ast.len(),
-                        params_ast[0].location,
-                    ));
+                    return Err(HashSet::from([
+                        format!(
+                            "expect {} type parameters but got {} (at {})",
+                            type_vars.len(),
+                            params_ast.len(),
+                            params_ast[0].location,
+                        ),
+                    ]))
                 }
                 let result = params_ast
                     .iter()
@@ -201,11 +213,12 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
                 if no_type_var {
                     result
                 } else {
-                    return Err(format!(
-                        "application of type vars to generic class \
-                    is not currently supported (at {})",
-                        params_ast[0].location
-                    ));
+                    return Err(HashSet::from([
+                        format!(
+                            "application of type vars to generic class is not currently supported (at {})",
+                            params_ast[0].location
+                        ),
+                    ]))
                 }
             };
             Ok(TypeAnnotation::CustomClass { id: obj_id, params: param_type_infos })
@@ -311,7 +324,9 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
             if let ast::ExprKind::Name { id, .. } = &value.node {
                 class_name_handle(id, slice, unifier, locked)
             } else {
-                Err(format!("unsupported expression type for class name (at {})", value.location))
+                Err(HashSet::from([
+                    format!("unsupported expression type for class name (at {})", value.location)
+                ]))
             }
         }
 
@@ -324,13 +339,16 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
             };
             let underlying_ty = underlying_ty[0];
 
-            let value = SymbolValue::from_constant(value, underlying_ty, primitives, unifier)?;
+            let value = SymbolValue::from_constant(value, underlying_ty, primitives, unifier)
+                .map_err(|err| HashSet::from([err]))?;
 
             if matches!(value, SymbolValue::Str(_) | SymbolValue::Tuple(_) | SymbolValue::OptionSome(_)) {
-                return Err(format!(
-                    "expression {value} is not allowed for constant type annotation (at {})",
-                    expr.location
-                ))
+                return Err(HashSet::from([
+                    format!(
+                        "expression {value} is not allowed for constant type annotation (at {})",
+                        expr.location
+                    ),
+                ]))
             }
 
             Ok(TypeAnnotation::Constant {
@@ -339,7 +357,9 @@ pub fn parse_ast_to_type_annotation_kinds<T>(
             })
         }
 
-        _ => Err(format!("unsupported expression for type annotation (at {})", expr.location)),
+        _ => Err(HashSet::from([
+            format!("unsupported expression for type annotation (at {})", expr.location),
+        ])),
     }
 }
 
@@ -351,7 +371,7 @@ pub fn get_type_from_type_annotation_kinds(
     unifier: &mut Unifier,
     ann: &TypeAnnotation,
     subst_list: &mut Option<Vec<Type>>
-) -> Result<Type, String> {
+) -> Result<Type, HashSet<String>> {
     match ann {
         TypeAnnotation::CustomClass { id: obj_id, params } => {
             let def_read = top_level_defs[obj_id.0].read();
@@ -361,11 +381,13 @@ pub fn get_type_from_type_annotation_kinds(
             };
 
             if type_vars.len() != params.len() {
-                return Err(format!(
-                    "unexpected number of type parameters: expected {} but got {}",
-                    type_vars.len(),
-                    params.len()
-                ))
+                return Err(HashSet::from([
+                    format!(
+                        "unexpected number of type parameters: expected {} but got {}",
+                        type_vars.len(),
+                        params.len()
+                    ),
+                ]))
             }
 
                     let param_ty = params
@@ -401,16 +423,18 @@ pub fn get_type_from_type_annotation_kinds(
                             if ok {
                                 result.insert(*id, p);
                             } else {
-                                return Err(format!(
-                                    "cannot apply type {} to type variable with id {:?}",
-                                    unifier.internal_stringify(
-                                        p,
-                                        &mut |id| format!("class{id}"),
-                                        &mut |id| format!("typevar{id}"),
-                                        &mut None
-                                    ),
-                                    *id
-                                ));
+                                return Err(HashSet::from([
+                                    format!(
+                                        "cannot apply type {} to type variable with id {:?}",
+                                        unifier.internal_stringify(
+                                            p,
+                                            &mut |id| format!("class{id}"),
+                                            &mut |id| format!("typevar{id}"),
+                                            &mut None
+                                        ),
+                                        *id
+                                    )
+                                ]))
                             }
                         }
 
@@ -430,11 +454,13 @@ pub fn get_type_from_type_annotation_kinds(
                             if ok {
                                 result.insert(*id, p);
                             } else {
-                                return Err(format!(
-                                    "cannot apply type {} to type variable {}",
-                                    unifier.stringify(p),
-                                    name.unwrap_or_else(|| format!("typevar{id}").into()),
-                                ))
+                                return Err(HashSet::from([
+                                    format!(
+                                        "cannot apply type {} to type variable {}",
+                                        unifier.stringify(p),
+                                        name.unwrap_or_else(|| format!("typevar{id}").into()),
+                                    ),
+                                ]))
                             }
                         }
 
diff --git a/nac3core/src/typecheck/function_check.rs b/nac3core/src/typecheck/function_check.rs
index 212646c3..a8461f52 100644
--- a/nac3core/src/typecheck/function_check.rs
+++ b/nac3core/src/typecheck/function_check.rs
@@ -6,9 +6,11 @@ use nac3parser::ast::{self, Constant, Expr, ExprKind, Operator::{LShift, RShift}
 use std::{collections::HashSet, iter::once};
 
 impl<'a> Inferencer<'a> {
-    fn should_have_value(&mut self, expr: &Expr<Option<Type>>) -> Result<(), String> {
+    fn should_have_value(&mut self, expr: &Expr<Option<Type>>) -> Result<(), HashSet<String>> {
         if matches!(expr.custom, Some(ty) if self.unifier.unioned(ty, self.primitives.none)) {
-            Err(format!("Error at {}: cannot have value none", expr.location))
+            Err(HashSet::from([
+                format!("Error at {}: cannot have value none", expr.location),
+            ]))
         } else {
             Ok(())
         }
@@ -18,10 +20,11 @@ impl<'a> Inferencer<'a> {
         &mut self,
         pattern: &Expr<Option<Type>>,
         defined_identifiers: &mut HashSet<StrRef>,
-    ) -> Result<(), String> {
+    ) -> Result<(), HashSet<String>> {
         match &pattern.node {
-            ExprKind::Name { id, .. } if id == &"none".into() =>
-                Err(format!("cannot assign to a `none` (at {})", pattern.location)),
+            ExprKind::Name { id, .. } if id == &"none".into() => Err(HashSet::from([
+                format!("cannot assign to a `none` (at {})", pattern.location),
+            ])),
             ExprKind::Name { id, .. } => {
                 if !defined_identifiers.contains(id) {
                     defined_identifiers.insert(*id);
@@ -41,15 +44,19 @@ impl<'a> Inferencer<'a> {
                 self.should_have_value(value)?;
                 self.check_expr(slice, defined_identifiers)?;
                 if let TypeEnum::TTuple { .. } = &*self.unifier.get_ty(value.custom.unwrap()) {
-                    return Err(format!(
-                        "Error at {}: cannot assign to tuple element",
-                        value.location
-                    ));
+                    return Err(HashSet::from([
+                        format!(
+                            "Error at {}: cannot assign to tuple element",
+                            value.location
+                        ),
+                    ]))
                 }
                 Ok(())
             }
             ExprKind::Constant { .. } => {
-                Err(format!("cannot assign to a constant (at {})", pattern.location))
+                Err(HashSet::from([
+                    format!("cannot assign to a constant (at {})", pattern.location),
+                ]))
             }
             _ => self.check_expr(pattern, defined_identifiers),
         }
@@ -59,15 +66,17 @@ impl<'a> Inferencer<'a> {
         &mut self,
         expr: &Expr<Option<Type>>,
         defined_identifiers: &mut HashSet<StrRef>,
-    ) -> Result<(), String> {
+    ) -> Result<(), HashSet<String>> {
         // there are some cases where the custom field is None
         if let Some(ty) = &expr.custom {
             if !matches!(&expr.node, ExprKind::Constant { value: Constant::Ellipsis, .. }) && !self.unifier.is_concrete(*ty, &self.function_data.bound_variables) {
-                return Err(format!(
-                    "expected concrete type at {} but got {}",
-                    expr.location,
-                    self.unifier.get_ty(*ty).get_type_name()
-                ));
+                return Err(HashSet::from([
+                    format!(
+                        "expected concrete type at {} but got {}",
+                        expr.location,
+                        self.unifier.get_ty(*ty).get_type_name()
+                    )
+                ]))
             }
         }
         match &expr.node {
@@ -87,10 +96,12 @@ impl<'a> Inferencer<'a> {
                             self.defined_identifiers.insert(*id);
                         }
                         Err(e) => {
-                            return Err(format!(
-                                "type error at identifier `{}` ({}) at {}",
-                                id, e, expr.location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "type error at identifier `{}` ({}) at {}",
+                                    id, e, expr.location
+                                )
+                            ]))
                         }
                     }
                 }
@@ -121,10 +132,12 @@ impl<'a> Inferencer<'a> {
                         };
 
                         if *rhs_val < 0 {
-                            return Err(format!(
-                                "shift count is negative at {}",
-                                right.location
-                            ));
+                            return Err(HashSet::from([
+                                format!(
+                                    "shift count is negative at {}",
+                                    right.location
+                                ),
+                            ]))
                         }
                     }
                 }
@@ -200,7 +213,7 @@ impl<'a> Inferencer<'a> {
         &mut self,
         stmt: &Stmt<Option<Type>>,
         defined_identifiers: &mut HashSet<StrRef>,
-    ) -> Result<bool, String> {
+    ) -> Result<bool, HashSet<String>> {
         match &stmt.node {
             StmtKind::For { target, iter, body, orelse, .. } => {
                 self.check_expr(iter, defined_identifiers)?;
@@ -307,11 +320,11 @@ impl<'a> Inferencer<'a> {
         &mut self,
         block: &[Stmt<Option<Type>>],
         defined_identifiers: &mut HashSet<StrRef>,
-    ) -> Result<bool, String> {
+    ) -> Result<bool, HashSet<String>> {
         let mut ret = false;
         for stmt in block {
             if ret {
-                println!("warning: dead code at {:?}\n", stmt.location);
+                eprintln!("warning: dead code at {:?}\n", stmt.location);
             }
             if self.check_stmt(stmt, defined_identifiers)? {
                 ret = true;
diff --git a/nac3core/src/typecheck/type_inferencer/mod.rs b/nac3core/src/typecheck/type_inferencer/mod.rs
index a7b73de2..7cc5dd9a 100644
--- a/nac3core/src/typecheck/type_inferencer/mod.rs
+++ b/nac3core/src/typecheck/type_inferencer/mod.rs
@@ -64,19 +64,19 @@ pub struct Inferencer<'a> {
 struct NaiveFolder();
 impl Fold<()> for NaiveFolder {
     type TargetU = Option<Type>;
-    type Error = String;
+    type Error = HashSet<String>;
     fn map_user(&mut self, (): ()) -> Result<Self::TargetU, Self::Error> {
         Ok(None)
     }
 }
 
-fn report_error<T>(msg: &str, location: Location) -> Result<T, String> {
-    Err(format!("{msg} at {location}"))
+fn report_error<T>(msg: &str, location: Location) -> Result<T, HashSet<String>> {
+    Err(HashSet::from([format!("{msg} at {location}")]))
 }
 
 impl<'a> Fold<()> for Inferencer<'a> {
     type TargetU = Option<Type>;
-    type Error = String;
+    type Error = HashSet<String>;
 
     fn map_user(&mut self, (): ()) -> Result<Self::TargetU, Self::Error> {
         Ok(None)
@@ -159,9 +159,9 @@ impl<'a> Fold<()> for Inferencer<'a> {
                                 }
                                 if let Some(old_typ) = self.variable_mapping.insert(name, typ) {
                                     let loc = handler.location;
-                                    self.unifier.unify(old_typ, typ).map_err(|e| {
-                                        e.at(Some(loc)).to_display(self.unifier).to_string()
-                                    })?;
+                                    self.unifier.unify(old_typ, typ).map_err(|e| HashSet::from([
+                                        e.at(Some(loc)).to_display(self.unifier).to_string(),
+                                    ]))?;
                                 }
                             }
                             let mut type_ = naive_folder.fold_expr(*type_)?;
@@ -274,7 +274,7 @@ impl<'a> Fold<()> for Inferencer<'a> {
                         .collect();
                     let loc = node.location;
                     let targets = targets
-                        .map_err(|e| e.at(Some(loc)).to_display(self.unifier).to_string())?;
+                        .map_err(|e| HashSet::from([e.at(Some(loc)).to_display(self.unifier).to_string()]))?;
                     return Ok(Located {
                         location: node.location,
                         node: ast::StmtKind::Assign {
@@ -528,22 +528,24 @@ impl<'a> Fold<()> for Inferencer<'a> {
     }
 }
 
-type InferenceResult = Result<Type, String>;
+type InferenceResult = Result<Type, HashSet<String>>;
 
 impl<'a> Inferencer<'a> {
     /// Constrain a <: b
     /// Currently implemented as unification
-    fn constrain(&mut self, a: Type, b: Type, location: &Location) -> Result<(), String> {
+    fn constrain(&mut self, a: Type, b: Type, location: &Location) -> Result<(), HashSet<String>> {
         self.unify(a, b, location)
     }
 
-    fn unify(&mut self, a: Type, b: Type, location: &Location) -> Result<(), String> {
+    fn unify(&mut self, a: Type, b: Type, location: &Location) -> Result<(), HashSet<String>> {
         self.unifier
             .unify(a, b)
-            .map_err(|e| e.at(Some(*location)).to_display(self.unifier).to_string())
+            .map_err(|e| HashSet::from([
+                e.at(Some(*location)).to_display(self.unifier).to_string(),
+            ]))
     }
 
-    fn infer_pattern(&mut self, pattern: &ast::Expr<()>) -> Result<(), String> {
+    fn infer_pattern(&mut self, pattern: &ast::Expr<()>) -> Result<(), HashSet<String>> {
         match &pattern.node {
             ExprKind::Name { id, .. } => {
                 if !self.defined_identifiers.contains(id) {
@@ -592,9 +594,9 @@ impl<'a> Inferencer<'a> {
                                 .map(|v| v.name)
                                 .rev()
                                 .collect();
-                            self.unifier.unify_call(&call, ty, sign, &required).map_err(|e| {
-                                e.at(Some(location)).to_display(self.unifier).to_string()
-                            })?;
+                            self.unifier.unify_call(&call, ty, sign, &required).map_err(|e| HashSet::from([
+                                e.at(Some(location)).to_display(self.unifier).to_string(),
+                            ]))?;
                             return Ok(sign.ret);
                         }
                     }
@@ -623,7 +625,7 @@ impl<'a> Inferencer<'a> {
         location: Location,
         args: Arguments,
         body: ast::Expr<()>,
-    ) -> Result<ast::Expr<Option<Type>>, String> {
+    ) -> Result<ast::Expr<Option<Type>>, HashSet<String>> {
         if !args.posonlyargs.is_empty()
             || args.vararg.is_some()
             || !args.kwonlyargs.is_empty()
@@ -692,7 +694,7 @@ impl<'a> Inferencer<'a> {
         location: Location,
         elt: ast::Expr<()>,
         mut generators: Vec<Comprehension>,
-    ) -> Result<ast::Expr<Option<Type>>, String> {
+    ) -> Result<ast::Expr<Option<Type>>, HashSet<String>> {
         if generators.len() != 1 {
             return report_error(
                 "Only 1 generator statement for list comprehension is supported",
@@ -765,7 +767,7 @@ impl<'a> Inferencer<'a> {
         func: ast::Expr<()>,
         mut args: Vec<ast::Expr<()>>,
         keywords: Vec<Located<ast::KeywordData>>,
-    ) -> Result<ast::Expr<Option<Type>>, String> {
+    ) -> Result<ast::Expr<Option<Type>>, HashSet<String>> {
         let func =
             if let Located { location: func_location, custom, node: ExprKind::Name { id, ctx } } =
                 func
@@ -899,7 +901,9 @@ impl<'a> Inferencer<'a> {
                     .collect();
                 self.unifier
                     .unify_call(&call, func.custom.unwrap(), sign, &required)
-                    .map_err(|e| e.at(Some(location)).to_display(self.unifier).to_string())?;
+                    .map_err(|e| HashSet::from([
+                        e.at(Some(location)).to_display(self.unifier).to_string(),
+                    ]))?;
                 return Ok(Located {
                     location,
                     custom: Some(sign.ret),
@@ -1073,8 +1077,11 @@ impl<'a> Inferencer<'a> {
     ) -> InferenceResult {
         let boolean = self.primitives.bool;
         for (a, b, c) in izip!(once(left).chain(comparators), comparators, ops) {
-            let method =
-                comparison_name(c).ok_or_else(|| "unsupported comparator".to_string())?.into();
+            let method = comparison_name(c)
+                .ok_or_else(|| HashSet::from([
+                    "unsupported comparator".to_string()
+                ]))?
+                .into();
             self.build_method_call(
                 a.location,
                 method,
@@ -1105,7 +1112,7 @@ impl<'a> Inferencer<'a> {
             ExprKind::Constant { value: ast::Constant::Int(val), .. } => {
                 // the index is a constant, so value can be a sequence.
                 let ind: Option<i32> = (*val).try_into().ok();
-                let ind = ind.ok_or_else(|| "Index must be int32".to_string())?;
+                let ind = ind.ok_or_else(|| HashSet::from(["Index must be int32".to_string()]))?;
                 let map = once((
                     ind.into(),
                     RecordField::new(ty, ctx == &ExprContext::Store, Some(value.location)),
diff --git a/nac3core/src/typecheck/type_inferencer/test.rs b/nac3core/src/typecheck/type_inferencer/test.rs
index cc18715c..4589c839 100644
--- a/nac3core/src/typecheck/type_inferencer/test.rs
+++ b/nac3core/src/typecheck/type_inferencer/test.rs
@@ -43,8 +43,9 @@ impl SymbolResolver for Resolver {
         unimplemented!()
     }
 
-    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, String> {
-        self.id_to_def.get(&id).cloned().ok_or_else(|| "Unknown identifier".to_string())
+    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, HashSet<String>> {
+        self.id_to_def.get(&id).cloned()
+            .ok_or_else(|| HashSet::from(["Unknown identifier".to_string()]))
     }
 
     fn get_string_id(&self, _: &str) -> i32 {
diff --git a/nac3standalone/src/basic_symbol_resolver.rs b/nac3standalone/src/basic_symbol_resolver.rs
index 3108847f..cdc3575c 100644
--- a/nac3standalone/src/basic_symbol_resolver.rs
+++ b/nac3standalone/src/basic_symbol_resolver.rs
@@ -10,6 +10,7 @@ use nac3core::{
 use nac3parser::ast::{self, StrRef};
 use parking_lot::{Mutex, RwLock};
 use std::{collections::HashMap, sync::Arc};
+use std::collections::HashSet;
 
 pub struct ResolverInternal {
     pub id_to_type: Mutex<HashMap<StrRef, Type>>,
@@ -61,8 +62,9 @@ impl SymbolResolver for Resolver {
         unimplemented!()
     }
 
-    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, String> {
-        self.0.id_to_def.lock().get(&id).copied().ok_or_else(|| "Undefined identifier".to_string())
+    fn get_identifier_def(&self, id: StrRef) -> Result<DefinitionId, HashSet<String>> {
+        self.0.id_to_def.lock().get(&id).copied()
+            .ok_or_else(|| HashSet::from(["Undefined identifier".to_string()]))
     }
 
     fn get_string_id(&self, s: &str) -> i32 {
diff --git a/nac3standalone/src/main.rs b/nac3standalone/src/main.rs
index d29a44a9..3ea7238d 100644
--- a/nac3standalone/src/main.rs
+++ b/nac3standalone/src/main.rs
@@ -8,6 +8,7 @@ use inkwell::{
 };
 use parking_lot::{Mutex, RwLock};
 use std::{collections::HashMap, fs, path::Path, sync::Arc};
+use std::collections::HashSet;
 
 use nac3core::{
     codegen::{
@@ -74,24 +75,28 @@ fn handle_typevar_definition(
     def_list: &[Arc<RwLock<TopLevelDef>>],
     unifier: &mut Unifier,
     primitives: &PrimitiveStore,
-) -> Result<Type, String> {
+) -> Result<Type, HashSet<String>> {
     let ExprKind::Call { func, args, .. } = &var.node else {
-        return Err(format!(
-            "expression {var:?} cannot be handled as a generic parameter in global scope"
-        ))
+        return Err(HashSet::from([
+            format!(
+                "expression {var:?} cannot be handled as a generic parameter in global scope"
+            ),
+        ]))
     };
 
     match &func.node {
         ExprKind::Name { id, .. } if id == &"TypeVar".into() => {
             let ExprKind::Constant { value: Constant::Str(ty_name), .. } = &args[0].node else {
-                return Err(format!("Expected string constant for first parameter of `TypeVar`, got {:?}", &args[0].node))
+                return Err(HashSet::from([
+                    format!("Expected string constant for first parameter of `TypeVar`, got {:?}", &args[0].node),
+                ]))
             };
             let generic_name: StrRef = ty_name.to_string().into();
 
             let constraints = args
                 .iter()
                 .skip(1)
-                .map(|x| -> Result<Type, String> {
+                .map(|x| -> Result<Type, HashSet<String>> {
                     let ty = parse_ast_to_type_annotation_kinds(
                         resolver,
                         def_list,
@@ -109,7 +114,9 @@ fn handle_typevar_definition(
             let loc = func.location;
 
             if constraints.len() == 1 {
-                return Err(format!("A single constraint is not allowed (at {loc})"))
+                return Err(HashSet::from([
+                    format!("A single constraint is not allowed (at {loc})"),
+                ]))
             }
 
             Ok(unifier.get_fresh_var_with_range(&constraints, Some(generic_name), Some(loc)).0)
@@ -117,14 +124,18 @@ fn handle_typevar_definition(
 
         ExprKind::Name { id, .. } if id == &"ConstGeneric".into() => {
             if args.len() != 2 {
-                return Err(format!("Expected 2 arguments for `ConstGeneric`, got {}", args.len()))
+                return Err(HashSet::from([
+                    format!("Expected 2 arguments for `ConstGeneric`, got {}", args.len()),
+                ]))
             }
 
             let ExprKind::Constant { value: Constant::Str(ty_name), .. } = &args[0].node else {
-                return Err(format!(
-                    "Expected string constant for first parameter of `ConstGeneric`, got {:?}",
-                    &args[0].node
-                ))
+                return Err(HashSet::from([
+                    format!(
+                        "Expected string constant for first parameter of `ConstGeneric`, got {:?}",
+                        &args[0].node
+                    ),
+                ]))
             };
             let generic_name: StrRef = ty_name.to_string().into();
 
@@ -145,9 +156,11 @@ fn handle_typevar_definition(
             Ok(unifier.get_fresh_const_generic_var(constraint, Some(generic_name), Some(loc)).0)
         }
 
-        _ => Err(format!(
-            "expression {var:?} cannot be handled as a generic parameter in global scope"
-        ))
+        _ => Err(HashSet::from([
+            format!(
+                "expression {var:?} cannot be handled as a generic parameter in global scope"
+            ),
+        ]))
     }
 }