Ppx_expand command with support for ppx_deriving

CHANGES.md

@@ -8,6 +8,7 @@ merlin NEXT_VERSION
     - Ignore SIGPIPE in the Merlin server process (#1746)
     - Fix lexing of quoted strings in comments (#1754, fixes #1753)
     - Improve cursor position detection in longidents (#1756)
+    - Implement new expand-node command for expanding PPX annotations (#1745)
 
 merlin 4.14
 ===========

src/frontend/ocamlmerlin/new/new_commands.ml

@@ -214,6 +214,21 @@ Otherwise, Merlin looks for the documentation for the entity under the cursor (a
     end
   ;
 
+  command "expand-node"
+    ~doc: "Returns the generated code of a PPX."
+    ~spec: [
+      arg "-position" "<position> Position to complete"
+          (marg_position (fun pos _pos -> pos));
+    ]
+    ~default: `None
+    begin fun buffer pos ->
+      match pos with 
+      | `None -> failwith "-position <pos> is mandatory"
+      | #Msource.position as pos -> 
+        run buffer (Query_protocol.Expand_node pos)
+    end
+  ;
+
   command "enclosing"
     ~spec: [
       arg "-position" "<position> Position to complete"

src/frontend/ocamlmerlin/query_json.ml

@@ -114,6 +114,8 @@ let dump (type a) : a t -> json =
     ]
   | Syntax_document pos ->
     mk "syntax-document" [ ("position", mk_position pos) ]
+  | Expand_node pos ->
+    mk "ppx-expand" [ ("position", mk_position pos) ]
   | Locate (prefix, look_for, pos) ->
     mk "locate" [
       "prefix", (match prefix with
@@ -392,6 +394,19 @@ let json_of_response (type a) (query : a t) (response : a) : json =
         ("url", `String info.documentation);
       ]
     | `No_documentation -> `String "No documentation found")
+  | Expand_node _, resp -> 
+    let str = match resp with
+    | `Found ppx_info -> 
+      `Assoc
+      [
+        ("code", `String ppx_info.code);
+        ("deriver", `Assoc [
+          ("start", Lexing.json_of_position ppx_info.deriver.a_start);
+          ("end", Lexing.json_of_position ppx_info.deriver.a_end);
+        ])
+      ]
+    | `No_deriver -> `String "No PPX deriver/extension node found on this position"
+    in str
   | Locate_type _, resp -> json_of_locate resp
   | Locate _, resp -> json_of_locate resp
   | Jump _, resp ->

src/frontend/query_commands.ml

@@ -512,6 +512,62 @@ let dispatch pipeline (type a) : a Query_protocol.t -> a =
     | Some res -> `Found res
     | None -> `No_documentation)
 
+  | Expand_node pos -> 
+    let pos = Mpipeline.get_lexing_pos pipeline pos in
+    let parsetree = Mpipeline.reader_parsetree pipeline in
+    let ppx_parsetree = Mpipeline.ppx_parsetree pipeline in
+    let nodes = Mtyper.node_at_p parsetree pos in
+    let ppx_expansion ~ppx ~a_start ~a_end = 
+      `Found ({
+        code = ppx;
+        deriver = {
+          a_start;
+          a_end
+        }
+      })
+    in
+    let check_at_pos (loc:Warnings.loc) = 
+      Location_aux.compare_pos pos loc = 0
+    in
+    let has_ppx_deriver (attr:Parsetree.attribute) =
+        check_at_pos attr.attr_loc &&
+        attr.attr_name.txt = "deriving"
+    in
+    let has_ppx_extension (node:Browse_raw_p.node) = 
+      match node with
+      | Expression {pexp_desc = Pexp_extension _; pexp_loc = loc; _} -> 
+        check_at_pos loc 
+      | _ -> false
+    in
+    let extension = List.find_opt ~f:has_ppx_extension nodes in
+    begin match extension with
+    | Some (Expression ({pexp_desc = Pexp_extension _; _} as exp)) ->
+        let nodes = Mtyper.node_at_p ppx_parsetree pos in
+        let derived_nodes =
+          Mbrowse_p.get_ext_children ~cursor_pos:pos ~expression_loc:(exp.pexp_loc) nodes in
+        ppx_expansion 
+          ~ppx:(Mbrowse_p.pprint_deriver_nodes () derived_nodes)
+          ~a_start:(exp.pexp_loc.loc_start)
+            ~a_end:(exp.pexp_loc.loc_end)
+    | Some _ | None ->
+      let nodes = Mtyper.node_at_p parsetree pos in
+      let has_deriving_attribute = 
+        List.find_opt ~f:has_ppx_deriver 
+        (List.concat_map ~f:(fun node -> 
+          Browse_raw_p.node_attributes node) nodes)
+      in
+      (match has_deriving_attribute with
+        | Some attribute ->
+          let nodes = Mtyper.node_at_p ppx_parsetree pos in
+          let derived_nodes = Mbrowse_p.get_children ~cursor_pos:pos nodes in
+          ppx_expansion 
+          ~ppx:(Mbrowse_p.pprint_deriver_nodes () derived_nodes)
+          ~a_start:(attribute.attr_loc.loc_start)
+          ~a_end:(attribute.attr_loc.loc_end)
+        | None ->
+          `No_deriver)
+    end
+
   | Locate (patho, ml_or_mli, pos) ->
     let typer = Mpipeline.typer_result pipeline in
     let local_defs = Mtyper.get_typedtree typer in

src/frontend/query_protocol.ml

@@ -103,6 +103,18 @@ type syntax_doc_result =
     documentation : string 
 }
 
+type ppx_deriver_pos =
+{ 
+  a_start : Lexing.position;
+  a_end : Lexing.position;
+}
+
+type ppx_expand_result = 
+{
+  code : string;
+  deriver : ppx_deriver_pos
+}
+
 type is_tail_position = [`No | `Tail_position | `Tail_call]
 
 type _ _bool = bool
@@ -145,6 +157,11 @@ type _ t =
     -> [ `Found of syntax_doc_result
        | `No_documentation
        ] t
+  | Expand_node
+    : Msource.position
+    -> [ `Found of ppx_expand_result
+       | `No_deriver 
+       ] t
   | Locate_type
     : Msource.position
       -> [ `Found of string option * Lexing.position