0005924: Adding expressiveness to polymorphic variants - MantisBT

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ID

Project

Category

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Date Submitted

Last Update

0005924

OCaml

OCaml typing

public

2013-02-17 16:48

2013-02-18 01:22

Reporter

jpdeplaix

Assigned To

garrigue

Priority

normal

Severity

minor

Reproducibility

always

Status

resolved

Resolution

won't fix

Platform

OS

OS Version

Product Version

4.00.1

Target Version

Fixed in Version

Summary

0005924: Adding expressiveness to polymorphic variants

Description

Hi,

I'd like to be able to do the kind of functions described here:
http://caml.inria.fr/pub/docs/manual-ocaml/manual006.html [^]

This function:
  let f = function `A -> `C | `B -> `D | x -> x
is typed as:
  val f : ([> `A | `B | `C | `D ] as 'a) -> 'a
but a more fine type would be:
  val f : [ `A | `B | 'r ] -> [ `C | `D | 'r ]

gasche already told me that it is choice made by the caml developers, but I'm very interested in making such functions. So, if someone can add this expressiveness to the language or explain me why it is really not possible to add this, it would be very helpful.

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Notes
(0008854) garrigue (manager) 2013-02-18 01:22	This is not strictly impossible to do that, but this requires huge changes in the type system. * since we would have explicit row variables, you need to introduce a notion of type variable sort * one would need to add exclusion information on row variables. For instance, with your example, we would the information that 'r cannot contain any of `A, `B, `C, `D. This is implicit in the type of f, but it can become hidden in more complex functions, so it would have to be explicit. * typing of pattern-matching would have to compute remaining cases, which is not trivial in the general case. For all these reasons there is no plan to introduce something like that at this point. If you are interested in this subject, you can read papers by Didier Rémy and, more recently, Matthias Blume.

Issue History
Date Modified	Username	Field	Change
2013-02-17 16:48	jpdeplaix	New Issue
2013-02-18 01:22	garrigue	Note Added: 0008854
2013-02-18 01:22	garrigue	Status	new => resolved
2013-02-18 01:22	garrigue	Resolution	open => won't fix
2013-02-18 01:22	garrigue	Assigned To	=> garrigue

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