5.3 effect syntax

astlib/migrate_502_503.ml

@@ -325,6 +325,10 @@ and copy_pattern_desc :
         (copy_loc (fun x -> Option.map (fun x -> x) x) x0)
   | Ast_502.Parsetree.Ppat_exception x0 ->
       Ast_503.Parsetree.Ppat_exception (copy_pattern x0)
+  | Ast_502.Parsetree.Ppat_extension
+      ( { txt = "ppxlib.migration.ppat_effect"; _ },
+        PPat ({ ppat_desc = Ppat_tuple [ e; c ]; _ }, None) ) ->
+      Ast_503.Parsetree.Ppat_effect (copy_pattern e, copy_pattern c)
   | Ast_502.Parsetree.Ppat_extension x0 ->
       Ast_503.Parsetree.Ppat_extension (copy_extension x0)
   | Ast_502.Parsetree.Ppat_open (x0, x1) ->

astlib/migrate_503_502.ml

@@ -2,10 +2,6 @@ open Stdlib0
 module From = Ast_503
 module To = Ast_502
 
-let migration_error loc missing_feature =
-  Location.raise_errorf ~loc
-    "migration error: %s is not supported before OCaml 5.03" missing_feature
-
 let rec copy_toplevel_phrase :
     Ast_503.Parsetree.toplevel_phrase -> Ast_502.Parsetree.toplevel_phrase =
   function
@@ -330,7 +326,17 @@ and copy_pattern_desc loc :
         (copy_loc (fun x -> Option.map (fun x -> x) x) x0)
   | Ast_503.Parsetree.Ppat_exception x0 ->
       Ast_502.Parsetree.Ppat_exception (copy_pattern x0)
-  | Ast_503.Parsetree.Ppat_effect _ -> migration_error loc "effect pattern"
+  | Ast_503.Parsetree.Ppat_effect (e, c) ->
+      Ast_502.Parsetree.Ppat_extension
+        ( Location.{ txt = "ppxlib.migration.ppat_effect"; loc },
+          Ast_502.Parsetree.PPat
+            ( {
+                ppat_desc = Ppat_tuple [ copy_pattern e; copy_pattern c ];
+                ppat_attributes = [];
+                ppat_loc_stack = [];
+                ppat_loc = loc;
+              },
+              None ) )
   | Ast_503.Parsetree.Ppat_extension x0 ->
       Ast_502.Parsetree.Ppat_extension (copy_extension x0)
   | Ast_503.Parsetree.Ppat_open (x0, x1) ->

test/503_migrations/preserve-effect-downward/driver.ml

@@ -0,0 +1,18 @@
+module To_before_503 =
+  Ppxlib_ast.Convert (Ppxlib_ast.Js) (Ppxlib_ast__.Versions.OCaml_502)
+
+module From_before_503 =
+  Ppxlib_ast.Convert
+    (Ppxlib_ast__.Versions.OCaml_502)
+    (Ppxlib_ast.Compiler_version)
+
+let impl _ctxt str =
+  (* This manual migration is here to ensure the test still works even once our
+     internal AST has been bumped past 5.3 *)
+  let before_503_ast = To_before_503.copy_structure str in
+  let roundtrip = From_before_503.copy_structure before_503_ast in
+  Ocaml_common.Pprintast.structure Format.std_formatter roundtrip;
+  str
+
+let () = Ppxlib.Driver.V2.register_transformation ~impl "503-downward-roundtrip"
+let () = Ppxlib.Driver.standalone ()

test/503_migrations/preserve-effect-downward/dune

@@ -0,0 +1,10 @@
+(executable
+ (name driver)
+ (enabled_if
+  (>= %{ocaml_version} "5.3"))
+ (libraries ppxlib ocaml-compiler-libs.common compiler-libs.common))
+
+(cram
+ (enabled_if
+  (>= %{ocaml_version} "5.3"))
+ (deps driver.exe))

test/503_migrations/preserve-effect-downward/run.t

@@ -0,0 +1,24 @@
+We should be able to migrate the effect syntax downward and back. This allows users
+to run ppx-es on 5.3 on source files that use the effect syntax until we update our
+internal AST to fully support it.
+
+We have a custom driver that will force migration of the AST down to 5.2 and back to
+the compiler's version and print it as source code using the compiler's printer,
+regardless of ppxlib's internal AST version.
+
+If we run the driver on the following source file:
+
+  $ cat > test.ml << EOF
+  > let handler f =
+  > match f () with
+  > | x -> x
+  > | effect Random_bits, k -> Effect.Deep.continue k (Random.bits ())
+  > EOF
+
+it should successfully roundtrip to 5.2 and print the source code unchanged:
+
+  $ ./driver.exe test.ml -o test.pp.ml
+  let handler f =
+    match f () with
+    | x -> x
+    | effect Random_bits,  k -> Effect.Deep.continue k (Random.bits ())