0005337: variance not inferred for package type - MantisBT

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ID

Project

Category

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Date Submitted

Last Update

0005337

OCaml

OCaml general

public

2011-08-15 19:22

2012-07-10 13:59

Reporter

sweeks

Assigned To

garrigue

Priority

normal

Severity

feature

Reproducibility

always

Status

acknowledged

Resolution

open

Platform

OS

OS Version

Product Version

3.12.0

Target Version

Fixed in Version

Summary

0005337: variance not inferred for package type

Description

OCaml doesn't accept the following program, because it does not infer that type [t] is
covariant.

  module type S = sig
    type t
    val x : t
  end

  type +'a t = (module S with type t = 'a)

It would be nice if OCaml's variance inference would look inside package types.

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Notes
(0006090) yminsky (reporter) 2011-08-15 19:54	Note that this also fails in 3.12.1 # module type S = sig type t val x : t end;; module type S = sig type t val x : t end # type +'a t = (module S with type t = 'a);; Error: In this definition, expected parameter variances are not satisfied. The 1st type parameter was expected to be covariant, but it is invariant

(0006220) garrigue (manager) 2011-12-10 04:16	Unfortunately, there is no cheap solution. Computing the variance of a module would require going through all syntactic constructs, which probably means a huge number of lines of code. An easy workaround is to use objects in place of first-class modules, at least when you need variance :-)

Issue History
Date Modified	Username	Field	Change
2011-08-15 19:22	sweeks	New Issue
2011-08-15 19:54	yminsky	Note Added: 0006090
2011-12-10 04:13	garrigue	Assigned To	=> garrigue
2011-12-10 04:13	garrigue	Status	new => assigned
2011-12-10 04:16	garrigue	Note Added: 0006220
2011-12-10 04:16	garrigue	Status	assigned => acknowledged
2012-07-10 13:59	doligez	Severity	minor => feature

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