[Haskell-cafe] Multi-parameter Type Class
pkeir at dcs.gla.ac.uk
Thu Dec 11 09:37:41 EST 2008
I've been trying to refactor my tree conversion code to make
better use of type classes; and I've discovered multi-parameter
type classes and functional dependencies. I have a class with a
function a2b, and I'd like "map" to be used when it's a list of
I've created a simple failing example:
data Foo = Foo Bar deriving(Show)
data Bar = Bar String deriving(Show)
class ZOT a b | a -> b where
zot :: a -> b
instance ZOT Foo Integer where
zot x = 17
instance ZOT Bar String where
zot x = "Eighteen"
instance ZOT [x] [y] where -- This bit
zot xs = map zot xs -- fails
main = do print $ zot $ Foo $ Bar "Blah"
print $ zot $ Bar "Blah"
print $ zot $ [Bar "Blah", Bar "Blah"] -- No map here please
I know this would work if the third instance of zot
explicitly took [Bar] and [String]. Can I not instead generalise
for all the ADTs in my tree in the way I've outlined? Must I
instantiate for the type of each list pair?
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