compiler: correct freeVars to not return duplicates

compiler/tyscheme.sml

@@ -13,13 +13,19 @@ end = struct
   type tyScheme = {sigmas: string list, rhos: (string * Ty.ty) list, tau: Ty.ty}
 
   local open Ty in
-  fun freeVars (Unit | Bool | Int) = []
-    | freeVars (List ty) = freeVars ty
-    | freeVars (Pair (ty1, ty2)) = (freeVars ty1) @ (freeVars ty2)
-    | freeVars (Option ty) = freeVars ty
-    | freeVars (Fun (ty1, ty2)) = (freeVars ty1) @ (freeVars ty2)
-    | freeVars (TyVar a) = [a]
+  fun freeVars' (Unit | Bool | Int) = []
+    | freeVars' (List ty) = freeVars' ty
+    | freeVars' (Pair (ty1, ty2)) = (freeVars' ty1) @ (freeVars' ty2)
+    | freeVars' (Option ty) = freeVars' ty
+    | freeVars' (Fun (ty1, ty2)) = (freeVars' ty1) @ (freeVars' ty2)
+    | freeVars' (TyVar a) = [a]
   end
+  fun dedup [] = []
+    | dedup (x :: xs) =
+        if List.exists (fn x' => x = x') xs
+        then dedup xs
+        else x :: dedup xs
+  val freeVars = dedup o freeVars'
 
   fun generalize ty =
     {sigmas = freeVars ty, rhos = [], tau = ty}