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

[Twan van Laarhoven]

On 20/08/13 12:13, TP wrote:
> {-# 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)

It might be better to make Bar a superclass of FooBar,

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

Then the compiler knows that every instance of FooBar also requires an instance 
of Bar.


Twan