[Haskell-cafe] using type variables in type declarations inside function
migmit at gmail.com
Thu Apr 12 20:57:33 UTC 2018
It could still be useful in local definitions. Like
something :: a -> ...
let somethinElse :: forall b. b -> a -> ...
Az iPademről küldve
2018. ápr. 12. dátummal, 22:43 időpontban Brandon Allbery <allbery.b at gmail.com> írta:
> 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:
>> 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?
>>> 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 = ...
>>>> 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
> Haskell-Cafe mailing list
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