[Haskell-cafe] Type families not as useful over functions
Miguel Mitrofanov
miguelimo38 at yandex.ru
Thu Feb 12 18:42:43 EST 2009
What do you need that for?
Can you live with
infixl |$|
(|$|) :: [a -> r] -> a -> [r]
fs |$| x = map ($ x) fs
and, instead of "broadcast fs a b" use
fs |$| a |$| b
?
On 13 Feb 2009, at 02:34, 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.
>
> In this case, currying is not my friend.
>
> Unfortunately this means I'm stuck with numbered function names:
>
> bc0 :: [r] -> [r]
> bc0 rs = rs
>
> bc1 :: [a -> r] -> a -> [r]
> bc1 [] a = []
> bc1 (r:rs) a = (r a):bc1 rs a
>
> bc2 rs a b = rs `bc1` a `bc1` b
>
> bc3 rs a b c = rs `bc1` a `bc1` b `bc1` c
>
> -- etc
>
> Cheers,
>
> -John
>
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