[Haskell-cafe] Type families not as useful over functions

John Ky newhoggy at gmail.com
Thu Feb 12 18:34:42 EST 2009

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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20090213/fa43a6c1/attachment-0001.htm

More information about the Haskell-Cafe mailing list