relation references as a term kind, written #Name

2 files changed, 24 insertions, 0 deletions

diff --git a/lib/term.ml b/lib/term.ml
index a0dc760..fcef6f9 100644
--- a/lib/term.ml
+++ b/lib/term.ml
@@ -2,6 +2,7 @@ type t =
   | Integer of Z.t
   | Rational of Q.t
   | String of string
+  | Reference of string
   | Nil
   | Record of t Label.Map.t
 
@@ -9,6 +10,16 @@ let integer z = Integer z
 let string s = String s
 let nil = Nil
 
+let reference name =
+  let is_letter c = (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') in
+  let is_body c = is_letter c || (c >= '0' && c <= '9') || c = '_' in
+  if
+    String.length name > 0
+    && is_letter name.[0]
+    && String.for_all is_body name
+  then Ok (Reference name)
+  else Error (`Invalid_reference name)
+
 let rational ~num ~den =
   if Z.equal den Z.zero then Error `Zero_denominator
   else
@@ -52,6 +63,7 @@ let rec compare a b =
   | Integer x, Integer y -> Z.compare x y
   | Rational x, Rational y -> Q.compare x y
   | String x, String y -> String.compare x y
+  | Reference x, Reference y -> String.compare x y
   | Nil, Nil -> 0
   | Record x, Record y -> Label.Map.compare compare x y
   | Integer _, _ -> -1
@@ -60,6 +72,8 @@ let rec compare a b =
   | _, Rational _ -> 1
   | String _, _ -> -1
   | _, String _ -> 1
+  | Reference _, _ -> -1
+  | _, Reference _ -> 1
   | Nil, _ -> -1
   | _, Nil -> 1
 
@@ -81,6 +95,7 @@ let rec pp fmt t =
   | Integer z -> Z.pp_print fmt z
   | Rational q -> Q.pp_print fmt q
   | String s -> Format.fprintf fmt "%S" s
+  | Reference name -> Format.fprintf fmt "#%s" name
   | Nil -> Format.pp_print_string fmt "#()"
   | Record map -> (
       match as_cell t with
diff --git a/lib/term.mli b/lib/term.mli
index cf79d89..8dc201e 100644
--- a/lib/term.mli
+++ b/lib/term.mli
@@ -17,6 +17,11 @@ type t = private
           its minimal shape, so integer membership inclusion holds
           literally. *)
   | String of string
+  | Reference of string
+      (** A relation reference, written [#Name]: a first-class
+          value denoting a relation by name, resolved through the
+          catalog of a state. Distinct from {!String} on purpose;
+          identity is not representation. *)
   | Nil  (** The empty sequence, written [#()]. *)
   | Record of t Label.Map.t
       (** A nonempty labeled tuple. Attribute order is canonical
@@ -30,6 +35,10 @@ val nil : t
 (** The empty sequence, [#()]. The distinguished base every
     {!sequence} chain ends in. *)
 
+val reference : string -> (t, [ `Invalid_reference of string ]) result
+(** A relation reference. Validates the name: a letter, then
+    letters, digits, or underscores. *)
+
 val rational : num:Z.t -> den:Z.t -> (t, [ `Zero_denominator ]) result
 (** Normalizes on entry: reduces by the gcd, moves the sign to the
     numerator, and collapses to {!Integer} when the reduced