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
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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