tyscheme if necessary

LUCA-2/tyscheme.sml

+structure TyScheme: sig
+
+  type tyScheme = {sigmas: string list, rhos: (string * Ty.ty) list, tau: Ty.ty}
+
+  val generalize: Ty.ty -> tyScheme
+
+  val instantiate: tyScheme -> Ty.ty
+
+  val untyScheme: tyScheme -> string
+
+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]
+  end
+
+  fun generalize ty =
+    {sigmas = freeVars ty, rhos = [], tau = ty}
+
+  fun instantiate {sigmas = sigmas, rhos = _, tau = ty} =
+    foldl (fn (a, inner) => Unify.subst (Ty.freshTyVar ()) a inner) ty sigmas
+
+  local open Utils in
+  fun untyScheme {sigmas = sigmas, rhos = rhos, tau = ty} =
+    cat (
+      (map (fn a => cat ["\\/", a, "."]) sigmas) @
+      (map (fn (x, t) => (paren o spc) [x, "::", Ty.unty t]) rhos) @
+      [Ty.unty ty]
+    )
+  end
+
+end