Re: [Haskell-cafe] Type Mismatch

7 Jan 2008


      Cetin Sert wrote:
...
class Streamable a where
  to :: a -> Stream a
The type of to looks wrong for me. a -> Stream a means to takes a single 
element into a stream of such elements, but you probably want to convert 
between different representations of streams of elements.
...
toStream :: [a] -> Stream a
instance Streamable [a] where
  to = toStream                       -- ERROR: see below
Here to :: [a] -> Stream [a] and toStream :: [a] -> Stream a. These 
types are different, as ghc complained.

Possible solutions include multi parameter type classes

   class Streamable s a where
     to :: s -> Stream a

   instance Streamable [a] a where
     to = toStream

and constructor classes

   class Streamable s where
     to :: s a -> Stream a

   instance Streamable [] where
     to = toStream

Tillmann

Re: [Haskell-cafe] Type Mismatch

Tillmann Rendel