diff --git a/nac3core/src/typecheck/typedef/mod.rs b/nac3core/src/typecheck/typedef/mod.rs
index c97926da..6980f00c 100644
--- a/nac3core/src/typecheck/typedef/mod.rs
+++ b/nac3core/src/typecheck/typedef/mod.rs
@@ -47,6 +47,9 @@ pub enum TypeVarMeta {
 
 #[derive(Clone)]
 pub enum TypeEnum {
+    TRigidVar {
+        id: u32,
+    },
     TVar {
         id: u32,
         meta: TypeVarMeta,
@@ -74,6 +77,7 @@ pub enum TypeEnum {
 impl TypeEnum {
     pub fn get_type_name(&self) -> &'static str {
         match self {
+            TypeEnum::TRigidVar { .. } => "TRigidVar",
             TypeEnum::TVar { .. } => "TVar",
             TypeEnum::TTuple { .. } => "TTuple",
             TypeEnum::TList { .. } => "TList",
@@ -127,6 +131,12 @@ impl Unifier {
         self.unification_table.probe_value(a).clone()
     }
 
+    pub fn get_fresh_rigid_var(&mut self) -> (Type, u32) {
+        let id = self.var_id + 1;
+        self.var_id += 1;
+        (self.add_ty(TypeEnum::TRigidVar { id }), id)
+    }
+
     pub fn get_fresh_var(&mut self) -> (Type, u32) {
         self.get_fresh_var_with_range(&[])
     }
@@ -139,9 +149,17 @@ impl Unifier {
         (self.add_ty(TypeEnum::TVar { id, range, meta: TypeVarMeta::Generic }), id)
     }
 
+    /// Unification would not unify rigid variables with other types, but we want to do this for
+    /// function instantiations, so we make it explicit.
+    pub fn replace_rigid_var(&mut self, rigid: Type, b: Type) {
+        assert!(matches!(&*self.get_ty(rigid), TypeEnum::TRigidVar { .. }));
+        self.set_a_to_b(rigid, b);
+    }
+
     pub fn is_concrete(&mut self, a: Type, allowed_typevars: &[Type]) -> bool {
         use TypeEnum::*;
         match &*self.get_ty(a) {
+            TRigidVar { .. } => true,
             TVar { .. } => allowed_typevars.iter().any(|b| self.unification_table.unioned(a, *b)),
             TCall { .. } => false,
             TList { ty } => self.is_concrete(*ty, allowed_typevars),
@@ -435,6 +453,7 @@ impl Unifier {
         use TypeVarMeta::*;
         let ty = self.unification_table.probe_value(ty).clone();
         match ty.as_ref() {
+            TypeEnum::TRigidVar { id } => var_to_name(*id),
             TypeEnum::TVar { id, meta: Generic, .. } => var_to_name(*id),
             TypeEnum::TVar { meta: Sequence(map), .. } => {
                 let fields = map
@@ -544,6 +563,7 @@ impl Unifier {
         // variables, i.e. things like TRecord, TCall should not occur, and we
         // should be safe to not implement the substitution for those variants.
         match &*ty {
+            TypeEnum::TRigidVar { .. } => None,
             TypeEnum::TVar { id, meta: Generic, .. } => mapping.get(&id).cloned(),
             TypeEnum::TTuple { ty } => {
                 let mut new_ty = Cow::from(ty);
@@ -634,7 +654,7 @@ impl Unifier {
         let ty = self.unification_table.probe_value(b).clone();
 
         match ty.as_ref() {
-            TypeEnum::TVar { meta: Generic, .. } => {}
+            TypeEnum::TRigidVar { .. } | TypeEnum::TVar { meta: Generic, .. } => {}
             TypeEnum::TVar { meta: Sequence(map), .. } => {
                 for t in map.borrow().values() {
                     self.occur_check(a, *t)?;
diff --git a/nac3core/src/typecheck/typedef/test.rs b/nac3core/src/typecheck/typedef/test.rs
index be7401f0..cf0cc9c0 100644
--- a/nac3core/src/typecheck/typedef/test.rs
+++ b/nac3core/src/typecheck/typedef/test.rs
@@ -419,22 +419,22 @@ fn test_typevar_range() {
 
     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.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.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});
+    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});
+    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});
+    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())
@@ -442,12 +442,34 @@ fn test_typevar_range() {
 
     let a = env.unifier.get_fresh_var_with_range(&[int, float]).0;
     let b = env.unifier.get_fresh_var().0;
-    let a_list = env.unifier.add_ty(TypeEnum::TList { ty: a});
+    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.add_ty(TypeEnum::TList { ty: b });
     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())
     );
 }
+
+#[test]
+fn test_rigid_var() {
+    let mut env = TestEnvironment::new();
+    let a = env.unifier.get_fresh_rigid_var().0;
+    let b = env.unifier.get_fresh_rigid_var().0;
+    let x = env.unifier.get_fresh_var().0;
+    let list_a = env.unifier.add_ty(TypeEnum::TList { ty: a });
+    let list_x = env.unifier.add_ty(TypeEnum::TList { ty: x });
+    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()));
+    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())
+    );
+
+    env.unifier.replace_rigid_var(a, int);
+    env.unifier.unify(list_x, list_int).unwrap();
+}