[Haskell-cafe] Funny type signature question

[Peter Verswyvelen]

Ah, I did not know this asTypeOf function.
But ScopedTypeVariables also allows you to give inner functions type
signatures that reuse polymorphic type parameters of the parent scope, which
makes code clearer I think.

On Thu, Apr 2, 2009 at 8:54 PM, Michael Snoyman <michael at snoyman.com> wrote:

>
>
> On Thu, Apr 2, 2009 at 9:51 PM, Felipe Lessa <felipe.lessa at gmail.com>wrote:
>
>> On Thu, Apr 02, 2009 at 08:18:27PM +0200, Peter Verswyvelen wrote:
>> > The type inferer seems to struggle to find the type of minBound and
>> > maxBound, and GHC asks to use a type annotation.
>> > To only way I see how to add a type annotation here is to use a GHC
>> > extension:
>> >
>> > {-# LANGUAGE ScopedTypeVariables #-}
>>
>> Just use 'asTypeOf'. It is defined as
>>
>> > asTypeOf :: a -> a -> a
>> > asTypeOf = const
>>
>> so that @asTypeOf x y == x@ but both types are constrained to be
>> equal.  The above function would become
>>
>> > randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a
>> > randomEnum = do
>> >     let min = minBound; max = maxBound
>> >     randVal <- getRandomR (fromEnum min, fromEnum max)
>> >     return $ toEnum randVal `asTypeOf` min `asTypeOf` max
>>
>> Note that I use the fact that 'return' is constrained to the type
>> variable 'a' we want to constrain its argument, and the
>> 'asTypeOf' constrains everything to be of the same type.
>>
>> HTH,
>>
>> --
>> Felipe.
>>
>
> Interesting alternative. However, I think the ScopedTypeVariables looks a
> little bit cleaner. I'll keep the asTypeOf in mind for the future though.
>
> Michael
>
>
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