[Haskell-cafe] Overcome type restrictions?

[Tobias Brandt]

The problem is, that the type parameter 'a' of the functions in the list is
fixed. You can work around that with RankNTypes:

newtype Wrap = Wrap { unwrap :: forall a. [a] -> a }

unwrap $ (\(h:t) -> (unwrap h) t) [Wrap head, Wrap last]

This specializes the type of the functions at every point of use separately.


On 7 March 2014 09:14, Kolář Dušan <kolar at fit.vutbr.cz> wrote:

> Well my fault, the example should have been like this:
>
> We have and error of infinite type for
> Prelude> :t ((\(h:t) -> h t) [head, last, head, last, head, last])
>
> Of course, head and tail are incompatible on type level...
>
> Dušan
>
>
>
>  On Fri, 07 Mar 2014 07:57:09 +0100, Kolář Dušan <kolar at fit.vutbr.cz>
>> wrote:
>>
>> :
>> :
>>
>>> But we have an error of infinite type construction for
>>>
>>> Prelude> :t ((\(h:t) -> h t) [head,tail, head, tail, head, tail])
>>>
>>> Well I can overcome this by encoding functions into data types and then
>>> performing "conversion" back and forth, nevertheless, is there any way how
>>> to overcome this?
>>>
>>
>> It seems like you need heterogenous collections[0]
>>
>> Regards,
>> Henk-Jan van Tuyl
>>
>>
>> [0] http://www.haskell.org/haskellwiki/Heterogenous_collections
>>
>>
>> --
>> Folding at home
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>>
>>
>> http://Van.Tuyl.eu/
>> http://members.chello.nl/hjgtuyl/tourdemonad.html
>> Haskell programming
>> --
>>
>>
>
>
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