Add new simple output mode for drivers

With -pp-simple drivers can print a simplified AST. This is useful
for debugging purposes. It is similar to the ppxlib-pp-ast tool
packaged with ppx-tools, but it is not configurable.

Signed-off-by: Patrick Ferris <[email protected]>

src/driver.ml

@@ -913,6 +913,7 @@ let load_source_file fn =
 
 type output_mode =
   | Pretty_print
+  | Pp_simple
   | Dump_ast
   | Dparsetree
   | Reconcile of Reconcile.mode
@@ -1149,6 +1150,14 @@ let process_file (kind : Kind.t) fn ~input_name ~relocate ~output_mode
           Ast_io.write oc
             { input_name; input_version; ast }
             ~add_ppx_context:true)
+  | Pp_simple ->
+      with_output output ~binary:false ~f:(fun oc ->
+          let ppf = Stdlib.Format.formatter_of_out_channel oc in
+          let ast = add_cookies ast in
+          (match ast with
+          | Intf ast -> Pp_ast.signature ppf ast
+          | Impl ast -> Pp_ast.structure ppf ast);
+          Stdlib.Format.pp_print_newline ppf ())
   | Dparsetree ->
       with_output output ~binary:false ~f:(fun oc ->
           let ppf = Stdlib.Format.formatter_of_out_channel oc in
@@ -1191,11 +1200,12 @@ let set_output_mode mode =
   match (!output_mode, mode) with
   | Pretty_print, _ -> output_mode := mode
   | _, Pretty_print -> assert false
-  | Dump_ast, Dump_ast | Dparsetree, Dparsetree -> ()
+  | Dump_ast, Dump_ast | Pp_simple, Pp_simple | Dparsetree, Dparsetree -> ()
   | Reconcile a, Reconcile b when Poly.equal a b -> ()
   | x, y ->
       let arg_of_output_mode = function
         | Pretty_print -> assert false
+        | Pp_simple -> "-pp-simple"
         | Dump_ast -> "-dump-ast"
         | Dparsetree -> "-dparsetree"
         | Reconcile Using_line_directives -> "-reconcile"
@@ -1343,6 +1353,14 @@ let standalone_args =
       Arg.Unit (fun () -> set_output_mode Dump_ast),
       " Dump the marshaled ast to the output file instead of pretty-printing it"
     );
+    ( "-pp-simple",
+      Arg.Unit (fun () -> set_output_mode Pp_simple),
+      " Pretty-print a simple version of the AST using the internal Pp_ast \
+       module. This is useful for comparing ASTs. For more configuration see \
+       the ppxlib-tools package." );
+    ( "--pp-simple",
+      Arg.Unit (fun () -> set_output_mode Pp_simple),
+      " Same as -pp-simple" );
     ( "--dump-ast",
       Arg.Unit (fun () -> set_output_mode Dump_ast),
       " Same as -dump-ast" );