a try

lib/inference.ml

@@ -35,14 +35,21 @@ let typeof t =
           | None, _ -> Some (Type.Arrow (Type.Var id, ty_body)), env')
        | _ -> None, env)
     | Term.App (t1, t2) ->
-      (* TODO: When a variable is a function *)
+      (* Infer the type of t1 and t2 *)
       (match infer env t1, infer env t2 with
        | (Some (Type.Arrow (ty_param, ty_fn)), _), (Some ty_args, _) ->
-         (* Check if the function type matches the arguments *)
+         (* Check if the argument type matches the parameter type *)
          (match Unification.unify ty_param ty_args with
           | Some _ -> Some ty_fn, env
           | None -> None, env)
-       | _, _ -> None, env)
+       | (Some _ (* (Type.Var _ as ty1) *), _), _ ->
+         (* On this case we may have a function represented as the variable *)
+         (* let ty2 = Type.Arrow (ty1, ty1) in
+            (match Unification.unify ty1 ty2 with
+            | Some env' -> Some ty2, env'
+            | _ -> None, env) *)
+         None, env
+       | _ -> None, env)
   in
   fst (infer TypeSubstitution.empty t)
 ;;