At 13:22 2000-04-29 +0200, you wrote:
>I tried the following where the idea is to implement a simple monad,
>but one which can only pass and return values that are showable.
>
> newtype Show a => TracingEv a = TE (Int -> IO (Int,a))
> unTE :: Show a => TracingEv a -> Int -> IO (Int,a)
> unTE (TE x) = x
>
> instance Monad TracingEv where
To be a Monad, the type constructor TracingEv needs to work with _all_ types.
-----------------
You can work around this, with overlapping instances.
class MightPrint a where
pr :: Int -> a -> IO ()
instance MightPrint a where
pr i c = return ()
instance (Show a) => MightPrint a where
pr i c = do print c; putChar ':'; print i; putChar '\n'
newtype TracingEv a = TE (Int -> IO (Int,a))
unTE :: TracingEv a -> Int -> IO (Int,a)
unTE (TE x) = x
instance Monad TracingEv where
return i = TE (\c -> do pr c i
return (c+1,i))
m >>= f = TE (\c -> do (c',v) <- (unTE m) c
(unTE (f v)) c')
But Hugs is not happy with the overlap unless you use -98 +m,
and the +m is an experimental feature.
--
Scott Turner
[EMAIL PROTECTED] http://www.ma.ultranet.com/~pkturner
