Introduce the type [ground_sort] and the functions [ground] and [unground].

src/SortUnification.ml

@@ -44,6 +44,9 @@ type sort = term =
   | TVar of int
   | TNode of sort structure
 
+type ground_sort =
+  | GArrow of ground_sort list
+
 (* -------------------------------------------------------------------------- *)
 
 (* Sort constructors. *)
@@ -68,6 +71,21 @@ let domain (x : variable) : variable list option =
 
 (* -------------------------------------------------------------------------- *)
 
+(* Converting between sorts and ground sorts. *)
+
+let rec ground (s : sort) : ground_sort =
+  match s with
+  | TVar _ ->
+      (* All variables are replaced with [*]. *)
+      GArrow []
+  | TNode (Arrow ss) ->
+      GArrow (List.map ground ss)
+
+let rec unground (GArrow ss : ground_sort) : sort =
+  TNode (Arrow (List.map unground ss))
+
+(* -------------------------------------------------------------------------- *)
+
 (* A name generator for unification variables. *)
 
 let make_gensym () : unit -> string =

src/SortUnification.mli

@@ -16,6 +16,9 @@ type sort =
   | TVar of int
   | TNode of sort structure
 
+type ground_sort =
+  | GArrow of ground_sort list
+
 (* -------------------------------------------------------------------------- *)
 
 (* Sort unification. *)
@@ -39,6 +42,11 @@ val unify: variable -> variable -> unit
 
 val decode: variable -> sort
 
+(* Grounding a sort replaces all sort variables with the sort [*]. *)
+
+val ground: sort -> ground_sort
+val unground: ground_sort -> sort
+
 (* -------------------------------------------------------------------------- *)
 
 (* A sort can be printed. *)