Dear Hasekellers, I am trying to build some abstractions on top of the module Control.Concurrent and run into a problem with the type system that I do not understand. In particular, I want to define two classes 'Channel' and 'Port' (see below) that define a common interface for several concrete implementations with different synchronization characteristics. The following code is a simplified excerpt that demonstrates the problem:
module Main where import Control.Concurrent.MVar
class Channel c where port :: Port p => c a -> IO (p a)
class Port p where take :: p a -> IO a put :: p a -> a -> IO ()
The problem arises when I instantiate the 'Channel' class and implement the 'port' function.
data C a = C (P a) instance Channel C where port (C p) = return p
Couldn't match expected type `p' (a rigid variable) against inferred type `P' `p' is bound by the type signature for `port' Expected type: p a Inferred type: P a In the first argument of `return', namely `p' In the expression: return p I am quite new to Haskell and my knowledge of how the type system works is fairly limited. Any help on this particular problem will be greatly appreciated, but any pointers to reading material that brings me closer to enlightenment are also very welcome. Thanks, Henrik
newC = do p <- newP return (C p)
data P a = P (MVar a)
newP = do v <- newEmptyMVar return (P v)
instance Port P where take (P mv) = takeMVar mv
main = do c <- newC p <- port c put p 1