Keean Schupke wrote:
Hmm... actually they can be used on the LHS...
{-# OPTIONS -fglasgow-exts #-}
module Main where
class Test a b | a -> b where
test :: a -> b
newtype I = I (forall a . Integral a => a)
newtype S = S (forall a . Show a => a)
instance Test I Int where
test _ = 7
instance Test S String where
test _ = "TEST"
main = do
putStrLn $ show $ test (I undefined)
putStrLn $ show $ test (S undefined)
Not sure how you would use them, but you can wrap a higher ranked
type in a newtype and use it as a label.
It is actually the head of the type (I or S) that selects the instance, the
rest is not useful. This raises the question of how you get your
higher ranked type into the right wrapper... the answer is (I think)
you can't... it must already be in the wrapper to differentiate it. This
reduces to the same situation as before and you might as well have
I and S as first class labels and drop the higher-ranked type.
Keean.
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