diff --git a/nac3core/src/typecheck/typedef/mod.rs b/nac3core/src/typecheck/typedef/mod.rs
index 0164641f..33cda9db 100644
--- a/nac3core/src/typecheck/typedef/mod.rs
+++ b/nac3core/src/typecheck/typedef/mod.rs
@@ -204,10 +204,9 @@ impl Unifier {
                     }
                     for v1 in old_range2.iter() {
                         for v2 in range1.iter() {
-                            if !self.shape_match(*v1, *v2) {
-                                continue;
+                            if let Ok(result) = self.shape_match(*v1, *v2){
+                                range2.push(result.unwrap_or(*v2));
                             }
-                            range2.push(*v2);
                         }
                     }
                     if range2.is_empty() {
@@ -648,39 +647,116 @@ impl Unifier {
         Ok(())
     }
 
-    fn shape_match(&mut self, a: Type, b: Type) -> bool {
+    fn shape_match(&mut self, a: Type, b: Type) -> Result<Option<Type>, ()> {
         use TypeEnum::*;
         let x = self.get_ty(a);
         let y = self.get_ty(b);
         match (x.as_ref(), y.as_ref()) {
-            (TVar { id, range, .. }, _) => self.check_var_compatible(*id, b, &range.borrow()).is_ok(),
-            (_, TVar { id, range, .. }) => self.check_var_compatible(*id, a, &range.borrow()).is_ok(),
-            (TTuple { ty: ty1 }, TTuple { ty: ty2 }) => {
-                ty1.len() == ty2.len()
-                    && zip(ty1.iter(), ty2.iter()).all(|(a, b)| self.shape_match(*a, *b))
+            (TVar { range: range1, .. }, TVar { meta, range: range2, .. }) => {
+                // we should restrict range2
+                let range1 = range1.borrow();
+                // new range is the intersection of them
+                // empty range indicates no constraint
+                if !range1.is_empty() {
+                    let range2 = range2.borrow();
+                    let mut range = Vec::new();
+                    if range2.is_empty() {
+                        range.extend_from_slice(&range1);
+                    }
+                    for v1 in range2.iter() {
+                        for v2 in range1.iter() {
+                            let result = self.shape_match(*v1, *v2);
+                            if let Ok(result) = result {
+                                range.push(result.unwrap_or(*v2));
+                            }
+                        }
+                    }
+                    if range.is_empty() {
+                        Err(())
+                    } else {
+                        let id = self.var_id + 1;
+                        self.var_id += 1;
+                        let ty = TVar { id, meta: meta.clone(), range: range.into() };
+                        Ok(Some(self.unification_table.new_key(ty.into())))
+                    }
+                } else {
+                    Ok(Some(b))
+                }
+            }
+            (_, TVar { range, .. }) => {
+                // range should be restricted to the left hand side
+                let range = range.borrow();
+                if range.is_empty() {
+                    Ok(Some(a))
+                } else {
+                    for v in range.iter() {
+                        let result = self.shape_match(a, *v);
+                        if let Ok(result) = result {
+                            return Ok(result.or(Some(a)));
+                        }
+                    }
+                    Err(())
+                }
+            }
+            (TVar { id, range, .. }, _) => {
+                self.check_var_compatible(*id, b, &range.borrow()).or(Err(()))
+            }
+            (TTuple { ty: ty1 }, TTuple { ty: ty2 }) => {
+                if ty1.len() != ty2.len() {
+                    return Err(());
+                }
+                let mut need_new = false;
+                let mut ty = ty1.clone();
+                for (a, b) in zip(ty1.iter(), ty2.iter()) {
+                    let result = self.shape_match(*a, *b)?;
+                    ty.push(result.unwrap_or(*a));
+                    if result.is_some() {
+                        need_new = true;
+                    }
+                }
+                if need_new {
+                    Ok(Some(self.add_ty(TTuple { ty })))
+                } else {
+                    Ok(None)
+                }
+            }
+            (TList { ty: ty1 }, TList { ty: ty2 }) => {
+                Ok(self.shape_match(*ty1, *ty2)?.map(|ty| self.add_ty(TList { ty })))
+            }
+            (TVirtual { ty: ty1 }, TVirtual { ty: ty2 }) => {
+                Ok(self.shape_match(*ty1, *ty2)?.map(|ty| self.add_ty(TVirtual { ty })))
+            }
+            (TObj { obj_id: id1, .. }, TObj { obj_id: id2, .. }) => {
+                if id1 == id2 {
+                    Ok(None)
+                } else {
+                    Err(())
+                }
             }
-            (TList { ty: ty1 }, TList { ty: ty2 })
-            | (TVirtual { ty: ty1 }, TVirtual { ty: ty2 }) => self.shape_match(*ty1, *ty2),
-            (TObj { obj_id: id1, .. }, TObj { obj_id: id2, .. }) => id1 == id2,
             // don't deal with function shape for now
-            _ => false,
+            _ => Err(()),
         }
     }
 
-    fn check_var_compatible(&mut self, id: u32, b: Type, range: &[Type]) -> Result<(), String> {
-        let mut in_range = range.is_empty();
+    fn check_var_compatible(
+        &mut self,
+        id: u32,
+        b: Type,
+        range: &[Type],
+    ) -> Result<Option<Type>, String> {
+        if range.is_empty() {
+            return Ok(None);
+        }
         for t in range.iter() {
-            if self.shape_match(*t, b) {
-                in_range = true;
+            let result = self.shape_match(*t, b);
+            if let Ok(result) = result {
+                return Ok(result);
             }
         }
-        if !in_range {
-            return Err(format!(
-                "Cannot unify type variable {} with {} due to incompatible value range",
-                id,
-                self.get_ty(b).get_type_name()
-            ));
-        }
-        Ok(())
+        return Err(format!(
+            "Cannot unify type variable {} with {} due to incompatible value range",
+            id,
+            self.get_ty(b).get_type_name()
+        ));
     }
 }
diff --git a/nac3core/src/typecheck/typedef/test.rs b/nac3core/src/typecheck/typedef/test.rs
index f7e09084..993e7333 100644
--- a/nac3core/src/typecheck/typedef/test.rs
+++ b/nac3core/src/typecheck/typedef/test.rs
@@ -403,4 +403,40 @@ fn test_typevar_range() {
         env.unifier.unify(float_list, v),
         Err("Cannot unify type variable 8 with TList due to incompatible value range".to_string())
     );
+
+    let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
+    let b = env.unifier.get_fresh_var_with_range(&[boolean, float]).0;
+    env.unifier.unify(a, b).unwrap();
+    env.unifier.unify(a, float).unwrap();
+
+    let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
+    let b = env.unifier.get_fresh_var_with_range(&[boolean, float]).0;
+    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())
+    );
+
+    let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
+    let b = env.unifier.get_fresh_var_with_range(&[boolean, float]).0;
+    let a_list = env.unifier.add_ty(TypeEnum::TList { ty: a});
+    let a_list = env.unifier.get_fresh_var_with_range(&[a_list]).0;
+    let b_list = env.unifier.add_ty(TypeEnum::TList { ty: b});
+    let b_list = env.unifier.get_fresh_var_with_range(&[b_list]).0;
+    env.unifier.unify(a_list, b_list).unwrap();
+    let float_list = env.unifier.add_ty(TypeEnum::TList { ty: float});
+    env.unifier.unify(a_list, float_list).unwrap();
+    // previous unifications should not affect a and b
+    env.unifier.unify(a, int).unwrap();
+
+    let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
+    let b = env.unifier.get_fresh_var_with_range(&[boolean, float]).0;
+    let a_list = env.unifier.add_ty(TypeEnum::TList { ty: a});
+    let b_list = env.unifier.add_ty(TypeEnum::TList { ty: b});
+    env.unifier.unify(a_list, b_list).unwrap();
+    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())
+    );
 }