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

20 Aug 2013

      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

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

Twan van Laarhoven