0004745: inferred type with a weak type variable that maybe shouldn't be

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Date Submitted

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0004745

OCaml

OCaml general

public

2009-03-15 19:53

2009-03-16 05:25

Reporter

silroquen

Assigned To

Priority

normal

Severity

minor

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

3.11.0

Target Version

Fixed in Version

3.11.1+dev

Summary

0004745: inferred type with a weak type variable that maybe shouldn't be

Description

Here is a simple linked list type:

type 'a linked_list = 'a node option
and 'a node = {
    value : 'a;
    next : 'a linked_list;
}

and three apparently equivalent functions to find the linked list's last node:

let rec find_end_a = function
    | None -> None
    | Some {next = None} as li -> li
    | Some node -> find_end_a node.next

let rec find_end_b = function
    | None -> None
    | Some {next = None} as li -> li
    | Some {next = n} -> find_end_b n

let rec find_end_c = function
    | None -> None
    | Some {next = None} as li -> li
    | Some {next = Some n} -> find_end_c (Some n)

Note that the inferred type for find_end_a and find_end_b is
    '_a linked_list -> '_a node option
but, for find_end_c:
    'a node option -> 'a node option

It seems like all three functions should have the type of find_end_c.

Additional Information

Note that if the linked_list type is eliminated, so instead

type 'a node = {
    value : 'a;
    next : 'a node option
}

then the functions find_end_* all have type
    'a node option -> 'a node option
as expected.

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Notes
(0004855) garrigue (manager) 2009-03-16 05:25	Thanks for pointing this subtle bug, that appears only in the toplevel. This was due to side-effects to types during compilation... Solved by taking clean copies when needed.

Issue History
Date Modified	Username	Field	Change
2009-03-15 19:53	silroquen	New Issue
2009-03-16 05:25	garrigue	Note Added: 0004855
2009-03-16 05:25	garrigue	Status	new => closed
2009-03-16 05:25	garrigue	Resolution	open => fixed
2009-03-16 05:25	garrigue	Fixed in Version	=> 3.11.1+dev