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More relaxed scoping of type variables and polymorphic type annotations #6569

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vicuna opened this issue Sep 18, 2014 · 2 comments
Closed

More relaxed scoping of type variables and polymorphic type annotations #6569

vicuna opened this issue Sep 18, 2014 · 2 comments
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@garrigue
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feature-wish typing

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@vicuna
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vicuna commented Sep 18, 2014

Original bug ID: 6569
Reporter: @lpw25
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2016-12-07T10:36:58Z)
Resolution: not a bug
Priority: normal
Severity: feature
Version: 4.02.0
Category: typing
Related to: #6673
Monitored by: @hcarty

Bug description

The following definition fails:

# let foo : 'a. 'a list -> unit =
    fun (x : 'a list) -> ();;
  Characters 36-59:
      fun (x : 'a list) -> ();;
      ^^^^^^^^^^^^^^^^^^^^^^^
Error: This definition has type 'a list -> unit which is less general than
         'a0. 'a0 list -> unit

This is because the 'a in (x : 'a list) is scoped at the level of the let expression, whilst the : 'a . 'a list -> unit is considered to be just inside the let expression.

This means that 'a cannot be generalized, so the definition is less general than the type constraint.

It would be nice to relax this slightly to allow what is a perfectly reasonable coding style.
@vicuna
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vicuna commented Sep 19, 2014

Comment author: @garrigue

There is a special syntax for such cases:

let foo : type a. a list -> unit =
fun (x : a list) -> ();;

The scoping works correctly here because a is locally abstract type
rather than a type variable. There are also other advantages.
(There is a drawback though, since currently we cannot use
locally abstract types for row variables. This is under consideration.)

@vicuna
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vicuna commented Sep 19, 2014

Comment author: @lpw25

I don't think this request should be marked as resolved. Whilst you can use a different syntax to accomplish this, the feature request is to be able to use the (more visually appealing) 'a. ... syntax.

Also, as you note, we cannot use locally abstract types with rows. Even when we are able to I suspect that it will be a bit verbose, so it would be good to be able to use 'a. ... instead.

@vicuna vicuna closed this as completed Dec 7, 2016
@vicuna vicuna added the typing label Mar 14, 2019
@vicuna vicuna mentioned this issue Mar 14, 2019
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