Jim Apple wrote:
Does anyone have examples of these? This one scares the foo out of me:
* It's not even safe in general to add a signature giving the same type that the compiler would infer anyway
Here's an example: > len :: [a] -> Int > > len xs = let ?accum = 0 in len' xs > > len' [] = ?accum > len' (x:xs) = let ?accum = ?accum + (1::Int) in len' xs *Main> :t len' len' :: forall a. (?accum :: Int) => [a] -> Int *Main> len "hello" 0 > len :: [a] -> Int > > len xs = let ?accum = 0 in len' xs > > len' :: forall a. (?accum :: Int) => [a] -> Int > > len' [] = ?accum > len' (x:xs) = let ?accum = ?accum + (1::Int) in len' xs *Main> :t len' len' :: forall a. (?accum :: Int) => [a] -> Int *Main> len "hello" 5 This happens as a side effect of the way that type inference currently works on recursive binding groups. It happens with typeclass dictionaries too, but it isn't observable because they can't be rebound in a local scope. -- Ben