Re: [Haskell-beginners] multi-parameter typeclass with default implementation

20 Aug 2013


      Ben Gamari wrote:
...
As far as I know, you'd need to do something like this to accomplish
what you are after,
{-# LANGUAGE MultiParamTypeClasses, DefaultSignatures #-}
class Bar a where
        bar :: a -> Int
class FooBar a b where
        foobar :: a -> b -> Int
        default foobar :: Bar a => a -> b -> Int
        foobar avalue bvalue = bar avalue
instance Bar Int where
        bar i = 5
    instance FooBar Int Int
main = do
        print $ bar (4::Int)
        print $ foobar (5::Int) (2::Int)
It seems that the "DefaultSignatures" extension is not necessary, the 
following version works correctly on my computer (ghc 7.6.2):

-------
{-# LANGUAGE MultiParamTypeClasses #-}

class Bar a where
    bar :: a -> Int

class FooBar a b where
    foobar :: Bar a => a -> b -> Int
    foobar avalue bvalue = bar avalue

instance Bar Int where
    bar i = 5
instance FooBar Int Int

main = do
    print $ bar (4::Int)
    print $ foobar (5::Int) (2::Int)
-------

Re: [Haskell-beginners] multi-parameter typeclass with default implementation

TP