I've written this cute ;-) piece of code involving an associated type
synonym,
{-# OPTIONS_GHC -fglasgow-exts #-}
class ZipWithA a where
type Elem a :: *
zipWithA :: [Elem a] -> a
instance ZipWithA [a] where
type Elem [a] = a
zipWithA xs = xs
instance ZipWithA b => ZipWithA ([a] -> b) where
type Elem ([a] -> b) = a -> Elem b
zipWithA fs = zipWithA . zipWith ($) fs
and it behaves as intended:
*Main> zipWithA [2, 3] :: [Int]
[2,3]
*Main> zipWithA [pred, succ] [2, 3] :: [Int]
[1,4]
*Main> zipWithA [(+), (-)] [2, 3] [5, 7] :: [Int]
[7,-4]
However, as soon as the overloading cannot be resolved, the type
checker loops:
*Main> zipWithA [head, last] ["more than", "haskell"] :: String
"ml"
*Main> zipWithA [head, last] ["more than", "haskell"]
<<loops>>
Just to be clear, I do not expect GHC to be able to type this last
example, I would just have thought it would want to tell me in finite
time that it cannot resolve the overloading here.
Should I report this a bug? Or is it perhaps already been taken care
of in the head? Or am I just plain unreasonable here? :-)
Cheers,
Stefan
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