Implement continuation expansion with the OCaml AST mapper

continuum_ppx.ml

@@ -0,0 +1,21 @@
+open Asttypes
+open Parsetree
+
+let expression mapper node =
+  let node = Ast_mapper.default_mapper.expr mapper node in
+  match node.pexp_desc with
+  | Pexp_extension ({ txt = ("continue" | "cps"); loc }, payload) ->
+      begin match payload with
+      | PStr [{ pstr_desc = Pstr_eval
+          ({ pexp_desc = Pexp_let (Nonrecursive, [binding], body); _ }, _); _ }] ->
+          let callback = Ast_helper.Exp.fun_ ~loc:binding.pvb_loc
+              ~attrs:binding.pvb_attributes Nolabel None binding.pvb_pat body in
+          Ast_helper.Exp.apply ~loc:node.pexp_loc ~attrs:node.pexp_attributes
+            binding.pvb_expr [Nolabel, callback]
+      | _ -> Location.raise_errorf ~loc
+          "continuum_ppx: expected let%%continue pattern = expression in body (one nonrecursive binding)"
+      end
+  | _ -> node
+
+let () = Ast_mapper.register "continuum_ppx"
+    (fun _ -> { Ast_mapper.default_mapper with expr = expression })