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
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