[Haskell-cafe] using type variables in type declarations inside function

[Brandon Allbery]

It's just compatibility with the standard: there might be existing code
that depends on Haskell 98 not extending the scope of a type variable, so
you need to be explicit about which type variables' scope to extend.
"forall" is already there for other extensions, and is otherwise a no-op in
this situation, so it's a safe way to specify the extended scope.

On Thu, Apr 12, 2018 at 4:41 PM, Ryan Reich <ryan.reich at gmail.com> wrote:

> I too am curious about the forall in ScopedTypeVariables. It seems
> formally unnecessary, so I assume it is designed to avert some kind of
> inconsistency with standard behavior? Thinking about other extensions, e.g.
> FlexibleInstances or MultiParamTypeClasses, it seems like they all give
> meaning to constructs that are forbidden by the standard, while this one
> actually changes the standard behavior (for the better, imho) and so
> requires protection by some nonstandard signifier, i.e. forall.
>
> On Apr 12, 2018 13:27, "Dennis Raddle" <dennis.raddle at gmail.com> wrote:
>
> Thanks!
>
> By the way, why do I sometimes have to use forall, and sometimes not?
>
> I'm also learning Purescript, and I noticed that the examples use 'forall'
> in every case. Why would it be different with Purescript?
> D
>
> On Thu, Apr 12, 2018 at 12:51 PM, Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>> On Thu, Apr 12, 2018 at 3:47 PM, Dennis Raddle <dennis.raddle at gmail.com>
>> wrote:
>>
>>> myFunc :: a -> b -> c
>>> myFunc x y z = ...
>>>   where
>>>     helper :: a -> [b]
>>>     helper xx = ...
>>>
>>> Notice that I'm attempting to declare 'helper' using my type variables.
>>> I've noticed that this results in an error.
>>> Is this actually possible, and how?
>>>
>>
>> You need the ScopedTypeVariables extension, *and* to "declare" the type
>> variables whose scope is to be extended with an explicit "forall" in the
>> signature.
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>>
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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