[Haskell-cafe] Overcome type restrictions?

[Dušan Kolář]

Well,

   for the case, we even do not need RankNTypes, as the type of the list 
is known, so

newtype Wrap = Wrap { unwrap :: [Wrap] -> Wrap }

wself (h:t) = unwr $ unwr h t

will work, which is not too far from encoding function into data :-) 
This solution saves space, of course.

Best regards,

Dušan

On 03/07/2014 10:35 AM, Tobias Brandt wrote:
> 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 
> <mailto: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 <mailto: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
>
>
>         -- 
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>
>
>
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