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

[TP]

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)
-------