[Haskell-cafe] Type class for sequences with dependent types

[Markus Läll]

Hi

This has been discussed here[1] and here[2], and the answer is that
its not possible because of making type inference generally
impossible. It would be a useful feature though, and could be
implemented by requiring the user give additional type signatures to
help the inference algorithm out. But that is AFAIK not implemented
yet.

[1] http://www.haskell.org/pipermail/haskell-cafe/2011-July/093790.html
[2] http://comments.gmane.org/gmane.comp.lang.haskell.glasgow.user/21017

On Thu, Jan 5, 2012 at 3:12 PM, Robbert Krebbers
<mailinglists at robbertkrebbers.nl> wrote:
> Hello,
>
> in a Haskell development I have to represent paths that have elements of
> alternating types. GADTs allow me to do this easily:
>
> data AltList :: * -> * -> * where
>  Nil   :: AltList a a
>  ICons :: Int -> AltList Int b -> AltList () b
>  UCons :: () -> AltList () b -> AltList Int b
>
> Since I have various kinds of these data structures I defined a type class
> for common operations on such data structures.
>
> class DepSequence l where
>  nil :: l a a
>  app :: l a b -> l b c -> l a c
>
> The instance for AltList is as follows:
>
> instance DepSequence AltList where
>  nil = Nil
>  Nil       `app` k = k
>  ICons i l `app` k = ICons i (l `app` k)
>  UCons i l `app` k = UCons i (l `app` k)
>
> This all works nicely, however, I also want ordinary lists to be an instance
> of this class too. I tried the following:
>
> type IntListAlt a b = [Int]
> instance DepSequence IntListAlt where
>  nil = []
>  app = (++)
>
> But GHC does not like this, it yields:
>
>  Type synonym `IntListAlt' should have 2 arguments, but has been
>  given none In the instance declaration for `DepList IntListAlt'
>
> The following alternative works, but it is quite ugly
>
> newtype IntList a b = IntList { getList :: [Int] }
> instance DepSequence IntList where
>  nil     = IntList []
>  app l k = IntList (getList l ++ getList k)
>
> and also does not give me a nil of type [Int] and an app of type [Int] ->
> [Int] -> [Int].
>
> Does anyone know whether Haskell allows me to do this in a better way?
>
> Best,
>
> Robbert
>
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-- 
Markus Läll