On 26.07.2010 23:55, Ozgur Akgun wrote:
I think it is pretty cool as well. But I think there is a problem with
viewing it as a wildcard.
let's say we define the following:
(??) = flip
foo :: a -> b -> c
foo ?? x :: a -> c
Perfect!
But saying ?? can be used as a wildcard might in the following wrong
perception:
foo x ?? :: b -> c -- WRONG
This looks interesting. I played around with this for a bit:
{-# LANGUAGE MultiParamTypeClasses
, FunctionalDependencies
, FlexibleInstances
#-}
class Wildcard f v r | f -> v r where
(??) :: f -> v -> r
instance Wildcard (a -> b -> c) b (a -> c) where
(??) = flip
instance Wildcard (b -> c) b c where
(??) = id
f :: String -> Int -> String
f s i = s ++ show i
a :: String -> String
a = (f ?? 5)
b :: Int -> String
b = (f "Int: " ??)
Sadly, this won't typecheck:
pattern.hs:19:0:
Couldn't match expected type `Int' against inferred type `[Char]'
Expected type: Int
Inferred type: String
When using functional dependencies to combine
Wildcard (b -> c) b c,
arising from the dependency `f -> a r'
in the instance declaration at pattern.hs:12:9
Wildcard (String -> Int -> String) Int (String -> String),
arising from a use of `??' at pattern.hs:19:5-10
When generalising the type(s) for `a'
Ideas anyone? :)
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