-pp flag ignored?

[progman1]

I have a pre-processor that emits AST and works fine via ocamlc -pp ... In a .merlin file I have "FLG -pp ./yaccy"

Invoking merlin in order to get feedback I do: cat demo.ml |ocamlmerlin single errors "-protocol" "sexp" "-log-file" "-" "-filename" ./demo.ml

merlin picks it up: "flags_applied": [ { "cwd": "./ppocaml", "flags": [ "-pp", "./yaccy" ] } ], "extensions": [],

but doesn't appear to honour it:

{ "start": { "line": 25, "col": 5 }, "end": { "line": 25, "col": 6 }, "type": "parser", "sub": [], "valid": true, "message": "Syntax error, expecting :'" } ]((assoc) (class . "return") (value ((assoc) (start (assoc) (line . 17) (col . 14)) (end (assoc) (line . 17) (col . 47)) (type . "warning") (sub) (valid . true) (message . "Warning 22: Using modified grammar")) ((assoc) (start (assoc) (line . 25) (col . 5)) (end (assoc) (line . 25) (col . 6)) (type . "parser") (sub) (valid . true) (message . "Syntax error, expecting:'"))) (notifications) (timing (assoc) (total . 227.522) (query . 8.578) (reader . 196.908) (ppx . 0.265) (typer . 21.625) (error . 0.146)))

Am I doing something wrong?

[progman1]

I see back in 2016 the intention was to drop -pp and then in 2018 it was re-introduced but only for source-to-source transformations. why is an AST outputting pre-processor not accepted?

[aalekseyev]

In Dune to support future-syntax we also use a -pp preprocessor that returns an AST.

The problem we observe is different though: merlin does honor the -pp flag, but it tries to parse the result as an OCaml source file, and reports errors consistent with that.

The relevant -pp flag:

$ cat .merlin | grep -- '-pp'
FLG -pp '.../dune/_boot/install/default/bin/ocaml-syntax-shims -dump-ast'

The error messages:

$ echo "haha" | ocamlmerlin single errors -filename c.ml  | jq
{
  "class": "return",
  "value": [
    {
      "start": {
        "line": 1,
        "col": 0
      },
      "end": {
        "line": 1,
        "col": 16
      },
      "type": "typer",
      "sub": [],
      "valid": true,
      "message": "Unbound constructor Caml1999M023����"
    },
    {
      "start": {
        "line": 1,
        "col": 0
      },
      "end": {
        "line": 1,
        "col": 60
      },
      "type": "typer",
      "sub": [],
      "valid": true,
      "message": "This expression has type int but an expression was expected of type\n         unit\n       because it is in the left-hand side of a sequence"
    },
    {
      "start": {
        "line": 1,
        "col": 16
      },
      "end": {
        "line": 1,
        "col": 17
      },
      "type": "lexer",
      "sub": [],
      "valid": true,
      "message": "Illegal character (\\000)"
    },
....

[progman1]

As I followed up with, -pp was re-introduced with source file parsing, not AST. so to those that know, why does merlin not accept an AST?