Add a ppx facility to test expressions (#403)

* embed compiler-libs/pprintast into the grading environment

* add a ppx shortcut to avoid code duplication for printable_fun (cf. issue #379)

* add a ppx shortcut [%code e] to build the tuple (Code.(e), Solution.(e), [%expr e])

src/grader/dune

@@ -146,7 +146,8 @@
         %{ocaml-config:standard_library}/compiler-libs/ast_mapper.cmi
         %{ocaml-config:standard_library}/compiler-libs/parsetree.cmi
         %{ocaml-config:standard_library}/compiler-libs/location.cmi
-        %{ocaml-config:standard_library}/compiler-libs/parse.cmi)
+        %{ocaml-config:standard_library}/compiler-libs/parse.cmi
+        %{ocaml-config:standard_library}/compiler-libs/pprintast.cmi)
        (:generated-cmis
         ../ppx-metaquot/.ty.objs/byte/ty.cmi
         ../ppx-metaquot/.fun_ty.objs/byte/fun_ty.cmi

src/ppx-metaquot/ppx_metaquot.ml

@@ -274,6 +274,22 @@ module Main : sig val expander: string list -> Ast_mapper.mapper end = struct
          fun_ty_next
          (Typ.constr fun_ty_id [glob_cty_ty; ucty; ret])
     | _ -> invalid_arg "fun_ty_of: not an arrow type"
+
+  let printable_of this e =
+    (* [%printable e] is a shortcut for
+       [Test_lib.printable_fun e (Pprintast.string_of_expression [%expr e])] *)
+    app (evar "Test_lib.printable_fun")
+      [app (evar "Pprintast.string_of_expression")
+        [(exp_lifter !loc this) # lift_Parsetree_expression e]; e]
+
+  let code_of this e =
+    (* [%code e] is a shortcut for [(Code.(e), Solution.(e), [%expr e])] *)
+    let open_module name e =
+      Exp.open_ (Opn.mk (Mod.ident (lid name))) e
+    in
+    tuple [open_module "Code" e;
+           open_module "Solution" e;
+           (exp_lifter !loc this) # lift_Parsetree_expression e]
   (* ------ </edited for learn-ocaml> ------ *)
 
   let expander _args =
@@ -303,6 +319,10 @@ module Main : sig val expander: string list -> Ast_mapper.mapper end = struct
               ty_of this ty
            | Pexp_extension({txt="funty";loc=l}, e) ->
               fun_ty_of this l e
+           | Pexp_extension({txt="printable";loc=l}, e) ->
+              printable_of this (get_exp l e)
+           | Pexp_extension({txt="code";loc=l}, e) ->
+              code_of this (get_exp l e)
 (* ------ </edited for learn-ocaml> ------ *)
            | _ ->
                super.expr this e