0004382: Strange typing restriction with polymorphic record fields

View Issue Details [ Jump to Notes ]

[ Issue History ] [ Print ]

Project

Category

View Status

Date Submitted

Last Update

0004382

OCaml

OCaml general

public

2007-09-05 20:44

2007-09-06 04:21

Reporter

mottl

Assigned To

Priority

normal

Severity

minor

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

3.10+dev

Target Version

Fixed in Version

3.10+dev

Summary

0004382: Strange typing restriction with polymorphic record fields

Description

In the following example I match + bind a record with a polymorphic field. The bound function is fully polymorphic and works without problems:

----------------------------------------
type t = { id : 'a. 'a -> 'a }

let mk_id () = { id = fun x -> x }

let { id = id } = mk_id ()

let () =
  let x = id 0 in
  let y = id 0. in
  Printf.printf "%d %f\n" x y
----------------------------------------

But if I replace the line with the pattern match on the record field by the following:

  let { id = id } as t = mk_id ()

then I get a type error:

  File "foo.ml", line 9, characters 13-15:
  This expression has type float but is here used with type int

It seems strange that adding the "as t" binding, which is not even used, would suddenly make "id" not fully polymorphic anymore, especially since the following, which should be semantically equivalent, works again:

  let t = mk_id ()
  let { id = id } = t

It is worth pointing out that OCaml 3.09.3 doesn't compile any but the most recent example.

It would be great if type inference in the above cases could be made more predictable and/or general.

Tags

No tags attached.

Attached Files

Relationships

Notes
(0004144) garrigue (manager) 2007-09-06 04:21	Fixed by avoid polymorphic fields in Typecore.build_as_type.

Issue History
Date Modified	Username	Field	Change
2007-09-05 20:44	mottl	New Issue
2007-09-06 04:21	garrigue	Status	new => closed
2007-09-06 04:21	garrigue	Note Added: 0004144
2007-09-06 04:21	garrigue	Resolution	open => fixed
2007-09-06 04:21	garrigue	Fixed in Version	=> 3.10+dev