Re: [Haskell-cafe] Deepest polymorphic functor

Ryan Ingram wrote:

The problem is this:

...

instance Num a => Num [a] where ...

test = deep_fmap (+1) [[[ 1, 2, 3 :: Int ]]]

What (+1) should be used?

(+1) :: Int -> Int (+1) :: [Int] -> [Int] (+1) :: [[Int]] -> [[Int]] (+1) :: [[[Int]]] -> [[[Int]]]

They could all be type-correct, so the snippet is ambiguous.

But why then the following snippet doesn't cause ambiguity: deep_fmap (++"a") "b" // -> "ba" deep_fmap (++"a") ["b"] // -> ["ba"] deep_fmap (++"a") [["b"]] // -> [["ba"]] -- View this message in context: http://www.nabble.com/Deepest-polymorphic-functor-tp24709303p24751663.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.