0005790: Polymorphic field values no longer generalizable with (type t)

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Date Submitted

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0005790

OCaml

OCaml typing

public

2012-10-15 15:13

2012-10-16 12:09

Reporter

lealanko

Assigned To

garrigue

Priority

normal

Severity

minor

Reproducibility

always

Status

resolved

Resolution

fixed

Platform

OS Version

Product Version

4.00.1

Target Version

Fixed in Version

4.00.0

Summary

0005790: Polymorphic field values no longer generalizable with (type t)

Description

Previously, in 3.12, one could use "fun (type t) ->" in front of an expression to keep it general enough for assignment in a polymorphic field:

        Objective Caml version 3.12.1

# type t = { x : 'r . 'r -> 'r };;
type t = { x : 'a. 'a -> 'a; }
# let id x = x;;
val id : 'a -> 'a = <fun>
# { x = id id };;
Error: This field value has type 'a -> 'a which is less general than
         'b. 'b -> 'b
# { x = fun (type r) -> id id };;
- : t = {x = <fun>}

However, in 4.00 this no longer works:

        OCaml version 4.00.1

# type t = { x : 'a . 'a -> 'a };;
type t = { x : 'a. 'a -> 'a; }
# let id x = x;;
val id : 'a -> 'a = <fun>
# { x = fun (type a) -> id id };;
Error: This field value has type 'b -> 'b which is less general than
         'a. 'a -> 'a

In my actual code, the field does not have a function type, so eta-expansion is not an option.

Steps To Reproduce

type t = { x : 'r . 'r -> 'r }
let id x = x
let _ = { x = fun (type r) -> id id }

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Notes
(0008260) garrigue (manager) 2012-10-16 10:39 edited on: 2012-10-16 10:56	Actually, the wrong one here is 3.12.1. This code is clearly unsound. Here is an example accepted by 3.12.1 but refused by 4.00.0 type t = { x : 'r . 'r -> 'r };; let id x = x;; let id' f = let r = ref [] in fun x -> let y = if !r = [] then x else List.hd !r in r := [x]; f y;; let t = { x = fun (type r) -> id' id };; t.x 3;; t.x true;; (Corrected the example) Is it really necessary to fix this in 3.12 too ?

(0008261) ygrek (reporter) 2012-10-16 10:45	ftr, example obviously meant let t = { x = fun (type r) -> id' id };;

(0008264) lealanko (reporter) 2012-10-16 12:09	Ah, right, the value restriction. I've been doing too much Haskell, already forgot about it. :) This poses a bit of a problem for me, but that's the price of imperative features. Sorry about the false alert.

Issue History
Date Modified	Username	Field	Change
2012-10-15 15:13	lealanko	New Issue
2012-10-16 10:39	garrigue	Note Added: 0008260
2012-10-16 10:39	garrigue	Status	new => resolved
2012-10-16 10:39	garrigue	Fixed in Version	=> 4.00.0
2012-10-16 10:39	garrigue	Resolution	open => fixed
2012-10-16 10:39	garrigue	Assigned To	=> garrigue
2012-10-16 10:45	ygrek	Note Added: 0008261
2012-10-16 10:56	garrigue	Note Edited: 0008260	View Revisions
2012-10-16 12:09	lealanko	Note Added: 0008264