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module types and type constraints #5143

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vicuna opened this issue Sep 3, 2010 · 5 comments
Closed

module types and type constraints #5143

vicuna opened this issue Sep 3, 2010 · 5 comments
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@garrigue
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duplicate feature-wish typing

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@vicuna
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vicuna commented Sep 3, 2010

Original bug ID: 5143
Reporter: montagu
Assigned to: @garrigue
Status: closed (set by @mshinwell on 2017-03-10T08:48:47Z)
Resolution: duplicate
Priority: normal
Severity: feature
Version: 3.12.0
Target version: later
Category: typing

Bug description

Type constraints are not stable under equivalence of module types.

Additional information

module type S1 = sig
type t
type u = t
end

module type S2 = sig
type u
type t = u
end

(* S1 and S2 are equivalent signatures, ie. S1 is included in S2 and S2 is included in S1 *)

module type S1' = S1 with type t = int
(* ok *)

module type S2' = S2 with type t = int
(* fails *)
@vicuna
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vicuna commented Sep 5, 2010

Comment author: @garrigue

I'm afraid this is not a bug.
Namely, two module types can mutually convertible without
being equal.
The specification of constraints is to replace the current
manifest type with a new one. If the current manifest type
was necessary, this is bound to break something.
You can always write:
module type S2' = S2 with type t = int and type u = int

@vicuna
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vicuna commented Sep 5, 2010

Comment author: montagu

No: in 3.12,
module type S2' = S2 with type t = int and type u = int

fails, whereas
module type S2' = S2 with type u = int and type t = int

is accepted.

@vicuna
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vicuna commented Apr 23, 2013

Comment author: @garrigue

Indeed, but I would rather say that the problem is that the order of with constraints is relevant.
Changing this behavior would require changing the algorithm for with constraints, which I believe is part of the specification.

@vicuna
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vicuna commented Mar 10, 2017

Comment author: @mshinwell

It seems like this is now a feature wish for better documentation?

@vicuna
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vicuna commented Mar 10, 2017

Comment author: @mshinwell

This is in fact a duplicate of #6360

@vicuna vicuna closed this as completed Mar 10, 2017
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