[Haskell-cafe] Type families not as useful over functions
Jonathan Cast
jonathanccast at fastmail.fm
Thu Feb 12 18:44:35 EST 2009
On Fri, 2009-02-13 at 10:34 +1100, John Ky wrote:
> Hi Haskell Cafe,
>
> I tried using type families over functions, but when I try it
> complains that the two lines marked conflict with each other.
>
> class Broadcast a where
> type Return a
> broadcast :: a -> Return a
> instance Broadcast [a -> r] where
> type Return [a -> r] = a -> [r] -- Conflict!
> broadcast fs a = []
>
> instance Broadcast [a -> b -> r] where
> type Return [a -> b -> r] = a -> b -> [r] -- Conflict!
> broadcast fs a b = []
>
> Given that in Haskell, every function of n+1 arguments is also a
> function of n arguments, this is likely the cause of the conflict.
This solution is somewhat in-extensible in the ultimate result type (r,
in your code); if the number of types r can take on is limited, it
should work well, though. For expository purposes, I assume that r is
always Int:
-- | Conal Elliot's semantic editor combinator argument
argument :: (a -> a') -> (a -> b) -> (a' -> b)
argument f g = g . f
class Broadcast a where
type Return a
broadcast :: [a] -> Return a
instance Broadcast Int where
type Return Int = [Int]
broadcast ns = ns
instance Broadcast r => Broadcast (a -> r) where
type Return (a -> r) = a -> Return r
broadcast fs x = (map.argument) (const x) fs
jcc
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