diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index eaf1a881f..4c3ffe4a2 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -56,7 +56,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
             // we cannot have other types, virtual type should be handled by function calls
             _ => unreachable!(),
         };
-        let def = &self.top_level.definitions[obj_id.0];
+        let def = &self.top_level.definitions.read()[obj_id.0];
         let index = if let TopLevelDef::Class { fields, .. } = &*def.read() {
             fields.iter().find_position(|x| x.0 == attr).unwrap().0
         } else {
@@ -104,7 +104,8 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
         ret: Type,
     ) -> Option<BasicValueEnum<'ctx>> {
         let key = self.get_subst_key(obj.map(|(a, _)| a), fun.0);
-        let definition = self.top_level.definitions.get(fun.1 .0).unwrap();
+        let top_level_defs = self.top_level.definitions.read();
+        let definition = top_level_defs.get(fun.1 .0).unwrap();
         let val = if let TopLevelDef::Function { instance_to_symbol, .. } = &*definition.read() {
             let symbol = instance_to_symbol.get(&key).unwrap_or_else(|| {
                 // TODO: codegen for function that are not yet generated
diff --git a/nac3core/src/codegen/mod.rs b/nac3core/src/codegen/mod.rs
index ccd73ec84..b3c59e7f0 100644
--- a/nac3core/src/codegen/mod.rs
+++ b/nac3core/src/codegen/mod.rs
@@ -206,7 +206,8 @@ fn get_llvm_type<'ctx>(
         match &*unifier.get_ty(ty) {
             TObj { obj_id, fields, .. } => {
                 // a struct with fields in the order of declaration
-                let definition = top_level.definitions.get(obj_id.0).unwrap();
+                let top_level_defs = top_level.definitions.read();
+                let definition = top_level_defs.get(obj_id.0).unwrap();
                 let ty = if let TopLevelDef::Class { fields: fields_list, .. } = &*definition.read()
                 {
                     let fields = fields.borrow();
diff --git a/nac3core/src/top_level.rs b/nac3core/src/top_level.rs
index 5e6a63605..052a7f931 100644
--- a/nac3core/src/top_level.rs
+++ b/nac3core/src/top_level.rs
@@ -339,7 +339,7 @@ pub enum TopLevelDef {
 }
 
 pub struct TopLevelContext {
-    pub definitions: Arc<Vec<Arc<RwLock<TopLevelDef>>>>,
+    pub definitions: Arc<RwLock<Vec<Arc<RwLock<TopLevelDef>>>>>,
     pub unifiers: Arc<RwLock<Vec<(SharedUnifier, PrimitiveStore)>>>,
 }
 
@@ -357,21 +357,15 @@ pub struct TopLevelComposer {
     pub keyword_list: Vec<String>,
 }
 
-impl TopLevelContext {
-    pub fn read_top_level_def_list(&self) -> &[Arc<RwLock<TopLevelDef>>] {
-        self.definitions.as_slice()
-    }
-}
-
 impl TopLevelComposer {
     pub fn make_top_level_context(self) -> TopLevelContext {
         TopLevelContext {
-            definitions: self
+            definitions: RwLock::new(self
                 .definition_ast_list
                 .into_iter()
                 .map(|(x, ..)| x)
                 .collect::<Vec<_>>()
-                .into(),
+            ).into(),
             // FIXME: all the big unifier or?
             unifiers: Default::default(),
         }
@@ -924,7 +918,8 @@ impl TopLevelComposer {
             fields,
             methods,
             resolver,
-            type_vars
+            type_vars,
+            ..
         } = class_def.deref_mut() {
             if let ast::StmtKind::ClassDef { name, bases, body, .. } = &class_ast {
                 (object_id, name.clone(), bases, body, ancestors, fields, methods, type_vars, resolver)
diff --git a/nac3core/src/typecheck/type_inferencer/mod.rs b/nac3core/src/typecheck/type_inferencer/mod.rs
index a4ccbe17c..d6989c5df 100644
--- a/nac3core/src/typecheck/type_inferencer/mod.rs
+++ b/nac3core/src/typecheck/type_inferencer/mod.rs
@@ -82,8 +82,9 @@ impl<'a> fold::Fold<()> for Inferencer<'a> {
                 } else {
                     None
                 };
+                let top_level_defs = self.top_level.definitions.read();
                 let annotation_type = self.function_data.resolver.parse_type_annotation(
-                    self.top_level.read_top_level_def_list(),
+                    top_level_defs.as_slice(),
                     self.unifier,
                     &self.primitives,
                     annotation.as_ref(),
@@ -344,8 +345,9 @@ impl<'a> Inferencer<'a> {
                     }
                     let arg0 = self.fold_expr(args.remove(0))?;
                     let ty = if let Some(arg) = args.pop() {
+                        let top_level_defs = self.top_level.definitions.read();
                         self.function_data.resolver.parse_type_annotation(
-                            self.top_level.read_top_level_def_list(),
+                            top_level_defs.as_slice(),
                             self.unifier,
                             self.primitives,
                             &arg,
diff --git a/nac3core/src/typecheck/type_inferencer/test.rs b/nac3core/src/typecheck/type_inferencer/test.rs
index 88dfc2181..71c8d715d 100644
--- a/nac3core/src/typecheck/type_inferencer/test.rs
+++ b/nac3core/src/typecheck/type_inferencer/test.rs
@@ -269,7 +269,7 @@ impl TestEnvironment {
         .collect();
 
         let top_level = TopLevelContext {
-            definitions: Arc::new(top_level_defs),
+            definitions: Arc::new(top_level_defs.into()),
             unifiers: Default::default(),
         };
 
diff --git a/nac3core/src/typecheck/typedef/test.rs b/nac3core/src/typecheck/typedef/test.rs
index 2972b3f9f..ee795056f 100644
--- a/nac3core/src/typecheck/typedef/test.rs
+++ b/nac3core/src/typecheck/typedef/test.rs
@@ -263,7 +263,7 @@ fn test_unify(
         ("v1", "Tuple[int]"),
         ("v2", "List[int]"),
     ],
-    (("v1", "v2"), "Cannot unify TList with TTuple")
+    (("v1", "v2"), "Cannot unify list[0] with tuple[0]")
     ; "type mismatch"
 )]
 #[test_case(2,
@@ -271,7 +271,7 @@ fn test_unify(
         ("v1", "Tuple[int]"),
         ("v2", "Tuple[float]"),
     ],
-    (("v1", "v2"), "Cannot unify objects with ID 0 and 1")
+    (("v1", "v2"), "Cannot unify 0 with 1")
     ; "tuple parameter mismatch"
 )]
 #[test_case(2,
@@ -377,7 +377,7 @@ fn test_typevar_range() {
     let v = env.unifier.get_fresh_var_with_range(&[int, boolean]).0;
     assert_eq!(
         env.unifier.unify(int_list, v),
-        Err("Cannot unify type variable 3 with TList due to incompatible value range".to_string())
+        Err("Cannot unify variable 3 with list[0] due to incompatible value range".to_string())
     );
 
     // unification between v and float
@@ -385,7 +385,7 @@ fn test_typevar_range() {
     let v = env.unifier.get_fresh_var_with_range(&[int, boolean]).0;
     assert_eq!(
         env.unifier.unify(float, v),
-        Err("Cannot unify type variable 4 with TObj due to incompatible value range".to_string())
+        Err("Cannot unify variable 4 with 1 due to incompatible value range".to_string())
     );
 
     let v1 = env.unifier.get_fresh_var_with_range(&[int, boolean]).0;
@@ -405,7 +405,7 @@ fn test_typevar_range() {
     // where v in (int, List[v1]), v1 in (int, bool)
     assert_eq!(
         env.unifier.unify(float_list, v),
-        Err("Cannot unify type variable 8 with TList due to incompatible value range".to_string())
+        Err("Cannot unify variable 8 with list[1] due to incompatible value range".to_string())
     );
 
     let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
@@ -418,7 +418,7 @@ fn test_typevar_range() {
     env.unifier.unify(a, b).unwrap();
     assert_eq!(
         env.unifier.unify(a, int),
-        Err("Cannot unify type variable 12 with TObj due to incompatible value range".into())
+        Err("Cannot unify variable 12 with 0 due to incompatible value range".into())
     );
 
     let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
@@ -441,7 +441,7 @@ fn test_typevar_range() {
     let int_list = env.unifier.add_ty(TypeEnum::TList { ty: int });
     assert_eq!(
         env.unifier.unify(a_list, int_list),
-        Err("Cannot unify type variable 19 with TObj due to incompatible value range".into())
+        Err("Cannot unify variable 19 with 0 due to incompatible value range".into())
     );
 
     let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
@@ -452,7 +452,7 @@ fn test_typevar_range() {
     env.unifier.unify(a_list, b_list).unwrap();
     assert_eq!(
         env.unifier.unify(b, boolean),
-        Err("Cannot unify type variable 21 with TObj due to incompatible value range".into())
+        Err("Cannot unify variable 21 with 2 due to incompatible value range".into())
     );
 }
 
@@ -467,11 +467,11 @@ fn test_rigid_var() {
     let int = env.parse("int", &HashMap::new());
     let list_int = env.parse("List[int]", &HashMap::new());
 
-    assert_eq!(env.unifier.unify(a, b), Err("Cannot unify TRigidVar with TRigidVar".to_string()));
+    assert_eq!(env.unifier.unify(a, b), Err("Cannot unify var3 with var2".to_string()));
     env.unifier.unify(list_a, list_x).unwrap();
     assert_eq!(
         env.unifier.unify(list_x, list_int),
-        Err("Cannot unify TObj with TRigidVar".to_string())
+        Err("Cannot unify 0 with var2".to_string())
     );
 
     env.unifier.replace_rigid_var(a, int);
diff --git a/nac3standalone/src/main.rs b/nac3standalone/src/main.rs
index 4bbc1eb13..666b9a742 100644
--- a/nac3standalone/src/main.rs
+++ b/nac3standalone/src/main.rs
@@ -52,8 +52,8 @@ fn main() {
 
     let (_, composer) = TopLevelComposer::new();
     let mut unifier = composer.unifier.clone();
-    let primitives = composer.primitives.clone();
-    let top_level = Arc::new(composer.to_top_level_context());
+    let primitives = composer.primitives_ty.clone();
+    let top_level = Arc::new(composer.make_top_level_context());
     unifier.top_level = Some(top_level.clone());
     let fun = unifier.add_ty(TypeEnum::TFunc(RefCell::new(FunSignature {
         args: vec![FuncArg {