start statement check, fix some error message

Cargo.lock

@@ -387,6 +387,7 @@ dependencies = [
  "num-bigint 0.3.2",
  "num-traits",
  "rustpython-parser",
+ "test-case",
 ]
 
 [[package]]
@@ -825,6 +826,19 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "test-case"
+version = "1.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b114ece25254e97bf48dd4bfc2a12bad0647adacfe4cae1247a9ca6ad302cec"
+dependencies = [
+ "cfg-if",
+ "proc-macro2",
+ "quote",
+ "syn",
+ "version_check",
+]
+
 [[package]]
 name = "tiny-keccak"
 version = "2.0.2"

nac3core/Cargo.toml

@@ -11,3 +11,6 @@ inkwell = { git = "https://github.com/TheDan64/inkwell", branch = "master", feat
 rustpython-parser = { git = "https://github.com/RustPython/RustPython", branch = "master" }
 indoc = "1.0"
 
+[dev-dependencies]
+test-case = "1.2.0"
+indoc = "1.0"

nac3core/src/lib.rs

@@ -1,6 +1,9 @@
 #![warn(clippy::all)]
 #![allow(clippy::clone_double_ref)]
 
+#[cfg(test)]
+extern crate test_case;
+
 extern crate num_bigint;
 extern crate inkwell;
 extern crate rustpython_parser;

nac3core/src/typecheck/context/inference_context.rs

@@ -6,12 +6,12 @@ use rustpython_parser::ast;
 use std::boxed::Box;
 use std::collections::HashMap;
 
-struct ContextStack<'a> {
+pub struct ContextStack {
     /// stack level, starts from 0
     level: u32,
     /// stack of symbol definitions containing (name, level) where `level` is the smallest level
     /// where the name is assigned a value
-    sym_def: Vec<(&'a str, u32)>,
+    sym_def: Vec<(String, u32)>,
 }
 
 pub struct InferenceContext<'a> {
@@ -22,12 +22,12 @@ pub struct InferenceContext<'a> {
     /// File ID
     file: FileID,
 
-    /// identifier to (type, readable) mapping.
+    /// identifier to (type, readable, location) mapping.
     /// an identifier might be defined earlier but has no value (for some code path), thus not
     /// readable.
-    sym_table: HashMap<&'a str, (Type, bool, Location)>,
+    sym_table: HashMap<String, (Type, bool, Location)>,
     /// stack
-    stack: ContextStack<'a>,
+    stack: ContextStack,
 }
 
 // non-trivial implementations here
@@ -52,19 +52,28 @@ impl<'a> InferenceContext<'a> {
     /// execute the function with new scope.
     /// variable assignment would be limited within the scope (not readable outside), and type
     /// returns the list of variables assigned within the scope, and the result of the function
-    pub fn with_scope<F, R>(&mut self, f: F) -> (Vec<(&'a str, Type, Location)>, R)
+    pub fn with_scope<F, R>(&mut self, f: F) -> (Vec<(String, Type, Location)>, R)
     where
         F: FnOnce(&mut Self) -> R,
     {
-        self.stack.level += 1;
+        self.start_scope();
         let result = f(self);
+        let poped_names = self.end_scope();
+        (poped_names, result)
+    }
+
+    pub fn start_scope(&mut self) {
+        self.stack.level += 1;
+    }
+
+    pub fn end_scope(&mut self) -> Vec<(String, Type, Location)> {
         self.stack.level -= 1;
         let mut poped_names = Vec::new();
         while !self.stack.sym_def.is_empty() {
             let (_, level) = self.stack.sym_def.last().unwrap();
             if *level > self.stack.level {
                 let (name, _) = self.stack.sym_def.pop().unwrap();
-                let (t, b, l) = self.sym_table.get_mut(name).unwrap();
+                let (t, b, l) = self.sym_table.get_mut(&name).unwrap();
                 // set it to be unreadable
                 *b = false;
                 poped_names.push((name, t.clone(), *l));
@@ -72,13 +81,13 @@ impl<'a> InferenceContext<'a> {
                 break;
             }
         }
-        (poped_names, result)
+        poped_names
     }
 
     /// assign a type to an identifier.
     /// may return error if the identifier was defined but with different type
-    pub fn assign(&mut self, name: &'a str, ty: Type, loc: ast::Location) -> Result<Type, String> {
-        if let Some((t, x, _)) = self.sym_table.get_mut(name) {
+    pub fn assign(&mut self, name: String, ty: Type, loc: ast::Location) -> Result<Type, String> {
+        if let Some((t, x, _)) = self.sym_table.get_mut(&name) {
             if t == &ty {
                 if !*x {
                     self.stack.sym_def.push((name, self.stack.level));
@@ -89,7 +98,7 @@ impl<'a> InferenceContext<'a> {
                 Err("different types".into())
             }
         } else {
-            self.stack.sym_def.push((name, self.stack.level));
+            self.stack.sym_def.push((name.clone(), self.stack.level));
             self.sym_table.insert(
                 name,
                 (ty.clone(), true, Location::CodeRange(self.file, loc)),
@@ -124,6 +133,11 @@ impl<'a> InferenceContext<'a> {
             self.resolver.get_symbol_location(name)
         }
     }
+
+    /// check if an identifier is already defined
+    pub fn defined(&self, name: &String) -> bool {
+        self.sym_table.get(name).is_some()
+    }
 }
 
 // trivial getters:

nac3core/src/typecheck/inference_core.rs

@@ -173,7 +173,8 @@ mod tests {
     use std::rc::Rc;
 
     fn get_inference_context(ctx: GlobalContext) -> InferenceContext {
-        InferenceContext::new(ctx, Box::new(|_| Err("unbounded identifier".into())))
+        // InferenceContext::new(ctx, Box::new(|_| Err("unbounded identifier".into())))
+        crate::typecheck::type_check::test::new_ctx().ctx
     }
 
     #[test]

nac3core/src/typecheck/location.rs

@@ -2,7 +2,7 @@ use rustpython_parser::ast;
 use std::vec::Vec;
 
 #[derive(Clone, Copy, PartialEq)]
-pub struct FileID(u32);
+pub struct FileID(pub u32);
 
 #[derive(Clone, Copy, PartialEq)]
 pub enum Location {

nac3core/src/typecheck/mod.rs

@@ -5,3 +5,4 @@ pub mod magic_methods;
 pub mod primitives;
 pub mod symbol_resolver;
 pub mod typedef;
+pub mod type_check;

nac3core/src/typecheck/type_check.rs

@@ -0,0 +1,752 @@
+use std::convert::TryInto;
+
+use crate::typecheck::context::InferenceContext;
+use crate::typecheck::inference_core;
+use crate::typecheck::magic_methods;
+use crate::typecheck::typedef::{Type, TypeEnum};
+use crate::typecheck::primitives;
+use rustpython_parser::ast;
+
+struct NaiveFolder;
+impl ast::fold::Fold<()> for NaiveFolder {
+    type TargetU = Option<Type>;
+    type Error = String;
+    fn map_user(&mut self, _user: ()) -> Result<Self::TargetU, Self::Error> {
+        Ok(None)
+    }
+}
+
+pub struct TypeInferencer<'a> {
+    pub ctx: InferenceContext<'a>,
+    pub error_stack: Vec<(String, ast::Location)>
+}
+
+impl<'a> ast::fold::Fold<()> for TypeInferencer<'a> {
+    type TargetU = Option<Type>;
+    type Error = String;
+
+    fn map_user(&mut self, _user: ()) -> Result<Self::TargetU, Self::Error> {
+        Ok(None)
+    }
+
+    fn fold_expr(&mut self, node: ast::Expr<()>) -> Result<ast::Expr<Self::TargetU>, Self::Error> {
+        
+        self.error_stack.push(("Checking ".to_string() + node.node.name(), node.location));
+
+        let expr = match &node.node {
+            ast::ExprKind::ListComp { .. } => return self.fold_listcomp(node),
+            _ => rustpython_parser::ast::fold::fold_expr(self, node)?
+        };
+
+        let ret = Ok(ast::Expr {
+            // compute type info and store in the custom field
+            custom: match &expr.node {
+                ast::ExprKind::Constant {value, kind: _} => self.infer_constant(value),
+                ast::ExprKind::Name {id, ctx: _} => Ok(Some(self.ctx.resolve(id)?)),
+                ast::ExprKind::List {elts, ctx: _} => self.infer_list(elts),
+                ast::ExprKind::Tuple {elts, ctx: _} => self.infer_tuple(elts),
+                ast::ExprKind::Attribute {value, attr, ctx: _} => self.infer_attribute(value, attr),
+                ast::ExprKind::BoolOp {op: _, values} => self.infer_bool_ops(values),
+                ast::ExprKind::BinOp {left, op, right} => self.infer_bin_ops(left, op, right),
+                ast::ExprKind::UnaryOp {op, operand} => self.infer_unary_ops(op, operand),
+                ast::ExprKind::Compare {left, ops, comparators} => self.infer_compare(left, ops, comparators),
+                ast::ExprKind::Call {func, args, keywords} => self.infer_call(func, args, keywords),
+                ast::ExprKind::Subscript {value, slice, ctx: _} => self.infer_subscript(value, slice),
+                ast::ExprKind::IfExp {test, body, orelse} => self.infer_if_expr(test, body, orelse),
+                ast::ExprKind::ListComp {elt: _, generators: _} => unreachable!("should not earch here, the list comp should have been folded before"), // already folded
+                ast::ExprKind::Slice { .. } => Ok(None), // special handling for slice, which is supported
+                _ => Err("not supported yet".into())
+            }?,
+            location: expr.location,
+            node: expr.node
+        });
+        self.error_stack.pop();
+        ret
+    }
+
+    fn fold_stmt(&mut self, node: ast::Stmt<()>) -> Result<ast::Stmt<Self::TargetU>, Self::Error> {
+        let stmt = match node.node {
+            ast::StmtKind::AnnAssign {target, annotation, value, simple} => {
+                let target_folded = Box::new(self.fold_expr( *target)?);
+                let value = if let Some(v) = value {
+                    let value_folded = Box::new(self.fold_expr(*v)?);
+                    if target_folded.custom == value_folded.custom {
+                        Some(value_folded)
+                    } else {
+                        return Err("Assignment LHF does not have the same type as RHS".into())
+                    }
+                } else {
+                    None
+                };
+                // TODO check consistency with type annotation
+                ast::Located {
+                    location: node.location,
+                    custom: None,
+                    node: ast::StmtKind::AnnAssign {
+                        target: target_folded, 
+                        annotation: Box::new(NaiveFolder.fold_expr(*annotation)?), 
+                        value, 
+                        simple
+                    },
+                }
+            }
+            _ => ast::fold::fold_stmt(self, node)?
+        };
+
+        match &stmt.node {
+            ast::StmtKind::For { target, iter, .. } => {
+                if let Some(TypeEnum::ParametricType(primitives::LIST_TYPE, ls)) = iter.custom.as_deref() {
+                    unimplemented!()
+                    // TODO
+                } else {
+                    return Err("can only iterate over list".into())
+                }
+            }
+            ast::StmtKind::If { test, .. } | ast::StmtKind::While { test, .. } => {
+                if test.custom != Some(self.ctx.get_primitive(primitives::BOOL_TYPE)) {
+                    return Err("Test should be bool".into());
+                }
+            }
+            ast::StmtKind::Assign { targets, value, .. } => {
+                unimplemented!();
+                // TODO
+            }
+            ast::StmtKind::AnnAssign { .. } | ast::StmtKind::Expr { .. } => {}
+            ast::StmtKind::Break | ast::StmtKind::Continue => {}
+            ast::StmtKind::Return { value } => {
+                unimplemented!()
+                // TODO
+            }
+            _ => return Err("Unsupported statement type".to_string()),
+        }
+
+        Ok(stmt)
+    }
+}
+
+impl<'a> TypeInferencer<'a> {
+    fn infer_constant(&self, constant: &ast::Constant) -> Result<Option<Type>, String> {
+        match constant {
+            ast::Constant::Bool(_) => 
+                Ok(Some(self.ctx.get_primitive(primitives::BOOL_TYPE))),
+            
+            ast::Constant::Int(val) => {
+                let int32: Result<i32, _> = val.try_into();
+                let int64: Result<i64, _> = val.try_into();
+                
+                if int32.is_ok() {
+                    Ok(Some(self.ctx.get_primitive(primitives::INT32_TYPE)))
+                } else if int64.is_ok() {
+                    Ok(Some(self.ctx.get_primitive(primitives::INT64_TYPE)))
+                } else {
+                    Err("Integer out of bound".into())
+                }
+            },
+    
+            ast::Constant::Float(_) => 
+                Ok(Some(self.ctx.get_primitive(primitives::FLOAT_TYPE))),
+
+            ast::Constant::Tuple(vals) => {
+                let result = vals
+                    .iter()
+                    .map(|x| self.infer_constant(x))
+                    .collect::<Vec<_>>();
+                
+                if result.iter().all(|x| x.is_ok()) {
+                    Ok(Some(TypeEnum::ParametricType(
+                        primitives::TUPLE_TYPE, 
+                        result
+                            .into_iter()
+                            .map(|x| x.unwrap().unwrap())
+                            .collect::<Vec<_>>(),
+                    ).into()))
+                } else {
+                    Err("Some elements in tuple cannot be typed".into())
+                }
+            }
+
+            _ => Err("not supported".into())
+        }
+    }
+
+    fn infer_list(&self, elts: &[ast::Expr<Option<Type>>]) -> Result<Option<Type>, String> {
+        if elts.is_empty() {
+            Ok(Some(TypeEnum::ParametricType(primitives::LIST_TYPE, vec![TypeEnum::BotType.into()]).into()))
+        } else {
+            let types = elts
+                .iter()
+                .map(|x| &x.custom)
+                .collect::<Vec<_>>();
+            
+            if types.iter().all(|x| x.is_some()) {
+                let head = types.iter().next().unwrap(); // here unwrap alone should be fine after the previous check
+                if types.iter().all(|x| x.eq(head)) {
+                    Ok(Some(TypeEnum::ParametricType(primitives::LIST_TYPE, vec![(*head).clone().unwrap()]).into()))
+                } else {
+                    Err("inhomogeneous list is not allowed".into())
+                }
+            } else {
+                Err("list elements must have some type".into())
+            }
+        }
+    }
+
+    fn infer_tuple(&self, elts: &[ast::Expr<Option<Type>>]) -> Result<Option<Type>, String> {
+        let types = elts
+            .iter()
+            .map(|x| (x.custom).clone())
+            .collect::<Vec<_>>();
+
+        if types.iter().all(|x| x.is_some()) {
+            Ok(Some(TypeEnum::ParametricType(
+                primitives::TUPLE_TYPE, 
+                types.into_iter().map(|x| x.unwrap()).collect()).into())) // unwrap alone should be fine after the previous check
+        } else {
+            Err("tuple elements must have some type".into())
+        }
+    }
+
+    fn infer_attribute(&self, value: &ast::Expr<Option<Type>>, attr: &str) -> Result<Option<Type>, String> {
+        let ty = value.custom.clone().ok_or_else(|| "no value".to_string())?;
+        if let TypeEnum::TypeVariable(id) = ty.as_ref() {
+            let v = self.ctx.get_variable_def(*id);
+            if v.bound.is_empty() {
+                return Err("no fields on unbounded type variable".into());
+            }
+            let ty = v.bound[0].get_base(&self.ctx).and_then(|v| v.fields.get(attr));
+            if ty.is_none() {
+                return Err("unknown field".into());
+            }
+            for x in v.bound[1..].iter() {
+                let ty1 = x.get_base(&self.ctx).and_then(|v| v.fields.get(attr));
+                if ty1 != ty {
+                    return Err("unknown field (type mismatch between variants)".into());
+                }
+            }
+            return Ok(Some(ty.unwrap().clone()));
+        }
+        
+        match ty.get_base(&self.ctx) {
+            Some(b) => match b.fields.get(attr) {
+                Some(t) => Ok(Some(t.clone())),
+                None => Err("no such field".into()),
+            },
+            None => Err("this object has no fields".into()),
+        }
+    }
+
+    fn infer_bool_ops(&self, values: &[ast::Expr<Option<Type>>]) -> Result<Option<Type>, String> {
+        assert_eq!(values.len(), 2); 
+        let left = values[0].custom.clone().ok_or_else(|| "no value".to_string())?;
+        let right = values[1].custom.clone().ok_or_else(|| "no value".to_string())?;
+        let b = self.ctx.get_primitive(primitives::BOOL_TYPE);
+        if left == b && right == b {
+            Ok(Some(b))
+        } else {
+            Err("bool operands must be bool".to_string())
+        }
+    }
+
+    fn infer_bin_ops(&self, left: &ast::Expr<Option<Type>>, op: &ast::Operator, right: &ast::Expr<Option<Type>>) -> Result<Option<Type>, String> {
+        inference_core::resolve_call(
+            &self.ctx, 
+            Some(left.custom.clone().ok_or_else(|| "no value".to_string())?), 
+            magic_methods::binop_name(op), 
+            &[right.custom.clone().ok_or_else(|| "no value".to_string())?])
+            .map_err(|_| "unsupported binary operator between the oprands".to_string())
+    }
+
+    fn infer_unary_ops(&self, op: &ast::Unaryop, operand: &ast::Expr<Option<Type>>) -> Result<Option<Type>, String> {
+        if let ast::Unaryop::Not = op {
+            if operand.custom == Some(self.ctx.get_primitive(primitives::BOOL_TYPE)) {
+                Ok(Some(self.ctx.get_primitive(primitives::BOOL_TYPE)))
+            } else {
+                Err("logical not must be applied to bool".into())
+            }
+        } else {
+            inference_core::resolve_call(&self.ctx, operand.custom.clone(), magic_methods::unaryop_name(op), &[])
+            .map_err(|_| "unsupported unary operator".to_string())
+        }
+    }
+
+    fn infer_compare(&self, left: &ast::Expr<Option<Type>>, ops: &[ast::Cmpop], comparators: &[ast::Expr<Option<Type>>]) -> Result<Option<Type>, String> {
+        if left.custom.is_none() || (!comparators.iter().all(|x| x.custom.is_some())) {
+            Err("comparison operands must have type".into())
+        } else {
+            let bool_type = Some(self.ctx.get_primitive(primitives::BOOL_TYPE));
+            let ty_first = inference_core::resolve_call(
+                &self.ctx, 
+                Some(left.custom.clone().ok_or_else(|| "comparator must be able to be typed".to_string())?), 
+                magic_methods::comparison_name(&ops[0]).ok_or_else(|| "unsupported comparison".to_string())?, 
+                &[comparators[0].custom.clone().ok_or_else(|| "comparator must be able to be typed".to_string())?])
+                .map_err(|_| "Comparison between the comparators are not supportes".to_string())?;
+            if ty_first != bool_type {
+                return Err("comparison result must be boolean".into());
+            }
+
+            for ((a, b), op) 
+            in comparators[..(comparators.len() - 1)]
+                .iter()
+                .zip(comparators[1..].iter())
+                .zip(ops[1..].iter()) {
+                    let ty = inference_core::resolve_call(
+                        &self.ctx, 
+                        Some(a.custom.clone().ok_or_else(|| "comparator must be able to be typed".to_string())?.clone()), 
+                        magic_methods::comparison_name(op).ok_or_else(|| "unsupported comparison".to_string())?,
+                        &[b.custom.clone().ok_or_else(|| "comparator must be able to be typed".to_string())?.clone()])
+                        .map_err(|_| "Comparison between the comparators are not supportes".to_string())?;
+                    if ty != bool_type {
+                        return Err("comparison result must be boolean".into());
+                    }
+                }
+            Ok(bool_type)
+        }
+    }
+
+    fn infer_call(&self, func: &ast::Expr<Option<Type>>, args: &[ast::Expr<Option<Type>>], _keywords: &[ast::Keyword<Option<Type>>]) -> Result<Option<Type>, String> {
+        if args.iter().all(|x| x.custom.is_some()) {
+            match &func.node {
+                ast::ExprKind::Name {id, ctx: _} 
+                    => inference_core::resolve_call(
+                        &self.ctx, 
+                        None, 
+                        id, 
+                        &args.iter().map(|x| x.custom.clone().unwrap()).collect::<Vec<_>>()),
+                
+                ast::ExprKind::Attribute {value, attr, ctx: _}
+                    => inference_core::resolve_call(
+                        &self.ctx, 
+                        Some(value.custom.clone().ok_or_else(|| "no value".to_string())?), 
+                        &attr, 
+                        &args.iter().map(|x| x.custom.clone().unwrap()).collect::<Vec<_>>()),
+                
+                _ => Err("not supported".into())
+            }
+        } else {
+            Err("function params must have type".into())
+        }
+    }
+
+    fn infer_subscript(&self, value: &ast::Expr<Option<Type>>, slice: &ast::Expr<Option<Type>>) -> Result<Option<Type>, String> {
+        let val_type = value.custom.as_ref().ok_or_else(|| "no value".to_string())?.as_ref();
+        if let TypeEnum::ParametricType(primitives::LIST_TYPE, ls) = val_type {
+            if let ast::ExprKind::Slice {lower, upper, step} = &slice.node {
+                let int32_type = self.ctx.get_primitive(primitives::INT32_TYPE);
+                let l = lower.as_ref().map_or(
+                    Ok(&int32_type),
+                    |x| x.custom.as_ref().ok_or_else(|| "lower bound cannot be typped".to_string()))?;
+                let u = upper.as_ref().map_or(
+                    Ok(&int32_type),
+                    |x| x.custom.as_ref().ok_or_else(|| "upper bound cannot be typped".to_string()))?;
+                let s = step.as_ref().map_or(
+                    Ok(&int32_type),
+                    |x| x.custom.as_ref().ok_or_else(|| "step cannot be typped".to_string()))?;
+                
+                if l == &int32_type && u == &int32_type && s == &int32_type {
+                    Ok(value.custom.clone())
+                } else {
+                    Err("slice must be int32 type".into())
+                }
+            } else if slice.custom == Some(self.ctx.get_primitive(primitives::INT32_TYPE)) {
+                Ok(Some(ls[0].clone()))
+            } else {
+                Err("slice or index must be int32 type".into())
+            }
+        } else if let TypeEnum::ParametricType(primitives::TUPLE_TYPE, ls) = val_type {
+            if let ast::ExprKind::Constant {kind: _, value: ast::Constant::Int(val)} = &slice.node {
+                let ind: Result<usize, _> = val.try_into();
+                if ind.is_ok() && ind.unwrap() < ls.len() {
+                    Ok(Some(ls[ind.unwrap()].clone()))
+                } else {
+                    Err("tuple constant index out of range".into())
+                }
+            } else {
+                Err("tuple index can only be constant".into())
+            }
+        } else {
+            Err("subscript is not supported for types other than list or tuple".into())
+        }
+    }
+
+    fn infer_if_expr(&self, test: &ast::Expr<Option<Type>>, body: &ast::Expr<Option<Type>>, orelse: &ast::Expr<Option<Type>>) -> Result<Option<Type>, String> {
+        if test.custom != Some(self.ctx.get_primitive(primitives::BOOL_TYPE)) {
+            Err("test should be bool".into())
+        } else if body.custom == orelse.custom {
+            Ok(body.custom.clone())
+        } else {
+            Err("divergent type at if expression".into())
+        }
+    }
+
+    fn _infer_list_comprehesion(&self, elt: &ast::Expr<Option<Type>>, generators: &[ast::Comprehension<Option<Type>>]) -> Result<Option<Type>, String> {
+        if generators[0]
+            .ifs
+            .iter()
+            .all(|x| x.custom == Some(self.ctx.get_primitive(primitives::BOOL_TYPE))) {
+                Ok(Some(TypeEnum::ParametricType(
+                    primitives::LIST_TYPE, 
+                    vec![elt.custom.clone().ok_or_else(|| "elements should have value".to_string())?]).into()))
+            } else {
+                Err("test must be bool".into())
+            }
+    }
+
+    // some pre-folds need special handling
+    fn fold_listcomp(&mut self, expr: ast::Expr<()>) -> Result<ast::Expr<Option<Type>>, String> {
+        
+        self.error_stack.push(("list comprehension at ".into(), expr.location));
+
+        if let ast::Expr {
+            location, 
+            custom: _, 
+            node: ast::ExprKind::ListComp {
+                elt, 
+                mut generators}} = expr {
+            // if is list comprehension, need special pre-fold
+            if generators.len() != 1 {
+                return Err("only 1 generator statement is supported".into());
+            }
+            let gen = generators.remove(0);
+            if gen.is_async {
+                return Err("async is not supported".into());
+            }
+
+            let ast::Comprehension {iter, 
+                target, 
+                ifs, 
+                is_async} = gen;
+            let iter_folded = Box::new(self.fold_expr(*iter)?);
+
+            let ret = if let TypeEnum::ParametricType(
+                primitives::LIST_TYPE, 
+                ls) = iter_folded
+                    .custom
+                    .as_ref()
+                    .ok_or_else(|| "no value".to_string())?
+                    .as_ref()
+                    .clone() {
+
+                let result: Result<ast::Expr<Option<Type>>, String>;
+                self.ctx.start_scope();
+                {
+                    self.infer_simple_binding(&target, ls[0].clone())?;
+                    let elt_folded = Box::new(self.fold_expr(*elt)?);
+                    let target_folded = Box::new(self.fold_expr(*target)?);
+                    let ifs_folded = ifs
+                        .into_iter()
+                        .map(|x| self.fold_expr(x))
+                        .collect::<Result<Vec<ast::Expr<Option<Type>>>, _>>()?;
+                    
+                    result = 
+                        if ifs_folded
+                        .iter()
+                        .all(|x| x.custom == Some(self.ctx.get_primitive(primitives::BOOL_TYPE))) {
+                        // only pop the error stack when return Ok(..)
+                        self.error_stack.pop();
+                        
+                        Ok(ast::Expr {
+                            location,
+                            custom: Some(TypeEnum::ParametricType(
+                                primitives::LIST_TYPE, 
+                                vec![elt_folded
+                                    .custom
+                                    .clone()
+                                    .ok_or_else(|| "elements cannot be typped".to_string())?]).into()),
+                            node: ast::ExprKind::ListComp {
+                                elt: elt_folded,
+                                generators: vec![ast::Comprehension {
+                                    target: target_folded,
+                                    ifs: ifs_folded,
+                                    iter: iter_folded,
+                                    is_async
+                                }]
+                            }
+                        })
+                    } else {
+                        Err("test must be bool".into())
+                    };
+                }
+                self.ctx.end_scope();                
+                result
+            } else {
+                Err("iteration is supported for list only".into())
+            };
+            ret
+        } else {
+            panic!("this function is for list comprehensions only!");
+        }
+    }
+
+    fn infer_simple_binding<T>(&mut self, name: &ast::Expr<T>, ty: Type) -> Result<(), String> {
+        self.error_stack.push(("resolving list comprehension variables".into(), name.location));
+        let ret = match &name.node {
+            ast::ExprKind::Name {id, ctx: _} => {
+                if id == "_" {
+                    self.error_stack.pop();
+                    Ok(())
+                } else if self.ctx.defined(id) {
+                    Err("duplicated naming".into())
+                } else {
+                    self.ctx.assign(id.clone(), ty, name.location)?;
+                    self.error_stack.pop();
+                    Ok(())
+                }
+            }
+    
+            ast::ExprKind::Tuple {elts, ctx: _} => {
+                if let TypeEnum::ParametricType(primitives::TUPLE_TYPE, ls) = ty.as_ref() {
+                    if elts.len() == ls.len() {
+                        for (a, b) in elts.iter().zip(ls.iter()) {
+                            self.infer_simple_binding(a, b.clone())?;
+                        }
+                        self.error_stack.pop();
+                        Ok(())
+                    } else {
+                        Err("different length".into())
+                    }
+                } else {
+                    Err("not supported".into())
+                }
+            }
+            _ => Err("not supported".into())
+        };
+        
+        ret
+    }
+
+    fn fold_expr(&mut self, node: ast::Expr<()>) -> Result<ast::Expr<Option<Type>>, String> {
+        let result = <Self as ast::fold::Fold<()>>::fold_expr(self, node);
+        if result.is_err() {
+            println!("{:?}", result);
+            println!("{:?}", self.error_stack.pop().unwrap());
+        }
+        result
+    }
+}
+
+#[cfg(test)]
+pub mod test {
+    use crate::typecheck::{symbol_resolver::SymbolResolver, symbol_resolver::*, location::*};
+    use rustpython_parser::ast::Expr;
+    use super::*;
+    
+    #[cfg(test)]
+    use test_case::test_case;
+    
+    pub fn new_ctx<'a>() -> TypeInferencer<'a> {
+        struct S;
+        impl SymbolResolver for S {
+            fn get_symbol_location(&self, _str: &str) -> Option<Location> { None }
+            fn get_symbol_type(&self, _str: &str) -> Option<SymbolType> { None }
+            fn get_symbol_value(&self, _str: &str) -> Option<SymbolValue> { None }
+        }
+
+        TypeInferencer {
+            ctx: InferenceContext::new(primitives::basic_ctx(), Box::new(S{}), FileID(3)),
+            error_stack: Vec::new()
+        }
+    }
+
+    #[test]
+    fn test_i32() {
+        let mut inferencer = new_ctx();
+        let ast: Expr = Expr {
+            location: ast::Location::new(0, 0),
+            custom: (),
+            node: ast::ExprKind::Constant {
+                value: ast::Constant::Int(123.into()),
+                kind: None
+            }
+        };
+
+        let new_ast = inferencer.fold_expr(ast);
+        assert_eq!(
+            new_ast,
+            Ok(ast::Expr {
+                location: ast::Location::new(0, 0),
+                custom: Some(inferencer.ctx.get_primitive(primitives::INT32_TYPE)),
+                node: ast::ExprKind::Constant {
+                    value: ast::Constant::Int(123.into()),
+                    kind: None
+                }
+            })
+        );
+    }
+
+    #[test]
+    fn test_i64() {
+        let mut inferencer = new_ctx();
+
+        let location = ast::Location::new(0, 0);
+        let num: i64 = 99999999999;
+
+        let ast: Expr = Expr {
+            location,
+            custom: (),
+            node: ast::ExprKind::Constant {
+                value: ast::Constant::Int(num.into()),
+                kind: None,
+            }
+        };
+        
+        let new_ast = inferencer.fold_expr(ast).unwrap();
+
+        assert_eq!(
+            new_ast, 
+            Expr {
+                location,
+                custom: Some(inferencer.ctx.get_primitive(primitives::INT64_TYPE)),
+                node: ast::ExprKind::Constant {
+                    value: ast::Constant::Int(num.into()),
+                    kind: None,
+                }
+            }
+        );
+    }
+
+    #[test]
+    fn test_tuple() {
+        let mut inferencer = new_ctx();
+        let i32_t = inferencer.ctx.get_primitive(primitives::INT32_TYPE);
+        let float_t = inferencer.ctx.get_primitive(primitives::FLOAT_TYPE);
+        let ast = rustpython_parser::parser::parse_expression("(123, 123.123, 999999999)").unwrap();
+        let loc = ast.location;
+        let folded = inferencer.fold_expr(ast).unwrap();
+        assert_eq!(
+            folded,
+            ast::Expr {
+                location: loc,
+                custom: Some(TypeEnum::ParametricType(primitives::TUPLE_TYPE, vec![i32_t.clone(), float_t.clone(), i32_t.clone()]).into()),
+                node: ast::ExprKind::Tuple {
+                    ctx: ast::ExprContext::Load, 
+                    elts: vec![
+                        ast::Expr {
+                            location: ast::Location::new(1, 2),
+                            custom: Some(i32_t.clone()),
+                            node: ast::ExprKind::Constant {
+                                value: ast::Constant::Int(123.into()),
+                                kind: None
+                            }
+                        },
+                        ast::Expr {
+                            location: ast::Location::new(1, 7),
+                            custom: Some(float_t),
+                            node: ast::ExprKind::Constant {
+                                value: ast::Constant::Float(123.123),
+                                kind: None
+                            }
+                        },
+                        ast::Expr {
+                            location: ast::Location::new(1, 16),
+                            custom: Some(i32_t),
+                            node: ast::ExprKind::Constant {
+                                value: ast::Constant::Int(999999999.into()),
+                                kind: None
+                            }
+                        },
+                    ]
+                }
+            }
+        );
+
+    }
+
+    #[test]
+    fn test_list() {
+        let mut inferencer = new_ctx();
+        let location = ast::Location::new(0, 0);
+
+        let ast: Expr = Expr {
+            location,
+            custom: (),
+            node: ast::ExprKind::List {
+                ctx: ast::ExprContext::Load,
+                elts: vec![
+                    Expr {
+                        location,
+                        custom: (),
+                        node: ast::ExprKind::Constant {
+                            value: ast::Constant::Int(1.into()),
+                            kind: None,
+                        },
+                    },
+
+                    Expr {
+                        location,
+                        custom: (),
+                        node: ast::ExprKind::Constant {
+                            value: ast::Constant::Int(2.into()),
+                            kind: None,
+                        },
+                    },
+                ],
+            }
+        };
+
+        let new_ast = inferencer.fold_expr(ast).unwrap();
+        assert_eq!(
+            new_ast,
+            Expr {
+                location,
+                custom: Some(TypeEnum::ParametricType(primitives::LIST_TYPE, vec![inferencer.ctx.get_primitive(primitives::INT32_TYPE)]).into()),
+                node: ast::ExprKind::List {
+                    ctx: ast::ExprContext::Load,
+                    elts: vec![
+                        Expr {
+                            location,
+                            custom: Some(inferencer.ctx.get_primitive(primitives::INT32_TYPE)),
+                            node: ast::ExprKind::Constant {
+                                value: ast::Constant::Int(1.into()),
+                                kind: None,
+                            },
+                        },
+
+                        Expr {
+                            location,
+                            custom: Some(inferencer.ctx.get_primitive(primitives::INT32_TYPE)),
+                            // custom: None,
+                            node: ast::ExprKind::Constant {
+                                value: ast::Constant::Int(2.into()),
+                                kind: None,
+                            },
+                        },
+                    ],
+                }
+            }
+        );
+    }
+
+    #[test_case("False == [True or True, False][0]")]
+    #[test_case("1 < 2 < 3")]
+    #[test_case("1 + [123, 1232][0]")]
+    #[test_case("not True")]
+    #[test_case("[[1]][0][0]")]
+    #[test_case("[[1]][0]")]
+    #[test_case("[[(1, 2), (2, 3), (3, 4)], [(2, 4), (4, 6)]][0]")]
+    #[test_case("[1, 2, 3, 4, 5][1: 2]")]
+    #[test_case("4 if False and True else 8")]
+    #[test_case("(1, 2, 3, 4)[1]")]
+    #[test_case("(1, True, 3, False)[1]")]
+    fn test_mix(prog: &'static str) {
+        let mut inf = new_ctx();
+        let ast = rustpython_parser::parser::parse_expression(prog).unwrap();
+        let folded = inf.fold_expr(ast).unwrap();
+        // println!("{:?}\n", folded.custom);
+    }
+
+    #[test_case("[1, True, 2]")]
+    #[test_case("True if 1 else False")]
+    #[test_case("1 if True else False")]
+    #[test_case("1 and 2")]
+    #[test_case("False or 1")]
+    #[test_case("1 + False")]
+    #[test_case("1 < 2 > False")]
+    #[test_case("not 2")]
+    #[test_case("-True")]
+    fn test_err_msg(prog: &'static str) {
+        let mut inf = new_ctx();
+        let ast = rustpython_parser::parser::parse_expression(prog).unwrap();
+        let _folded = inf.fold_expr(ast);
+        println!("")
+    }
+}