Unused var warning when using `map` pattern

[NoahTheDuke]

Map expressions are very helpful, but they lead to "unused-var" warnings which fail builds if not disabled in dune-workspace.

The below raises a warning for editor_status and mode being unused. If I move the map() calls out of the pattern and into normal code (let editor_status = match editor_status with ...), then the warnings go away (as expected).

I'm not sure how to do more debugging.

type mode = GLOBAL | LOCAL

type solution_exa = {
  name : string;
  code : string;
  editor_status : bool;
  mode : mode;
  image : bool Array.t;
}

let read_exa bits =
  match%bitstring bits with
  (* EXAs
    | Field | Type | Value
    ---------------------
    | Lead-in | byte | 0xA |
    | Name | pascal string | two-character name of EXA |
    | Source | pascal string | raw code |
    | Editor display status | bool byte | false: unrolled, true: collapsed |
    | Memory scope | bool byte | false: global, true: local |
    | Bitmap | bool byte[100] | Raw bitmap (row-major), true: set pixel |
    *)
  | {|
      0x0A : 8;
      len : 32 : littleendian;
      name : (Int32.to_int len) * 8 : string;
      len : 32 : littleendian;
      code : (Int32.to_int len) * 8 : string;
      editor_status : 8 : map (fun es -> match es with | 0 -> false | _ -> true);
      mode : 8 : map (fun mode -> match mode with | 0 -> GLOBAL | _ -> LOCAL);
      bitmap : 800 : bitstring;
      rest : -1 : bitstring
    |}
    ->
      let data, off, len = bitmap in
      let image = Array.make 100 false in
      for i = 0 to 99 do
        let c = Bitstring.extract_char_unsigned data (off + (i * 8)) len 8 in
        Array.set image i (c <> 0)
      done;
      ({ name; code; editor_status; mode; image }, rest)
  | {| _ |} -> raise Exit

[xguerin]

#42 should do the trick. Can you check on your end?

[NoahTheDuke]

I'm struggling to use this with dune, so I'm not sure if I can provide feedback that it works in a timely fashion.

[xguerin]

Validated with your example. I'll publish it today on OPAM as version 5.0.2.