Isn't it possible to constrain instances?
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
return i = TE (\c -> do print c; putChar ':'; print i; putChar '\n'
return (c+1,i))
m >>= f = TE (\c -> do (c',v) <- (unTE m) c
(unTE (f v)) c')
Neither GHC, nor hugs likes this. Hugs report:
ERROR "MonadProblem.hs" (line 9): Cannot justify constraints in
instance member binding
*** Expression : return
*** Type : Monad TracingEv => a -> TracingEv a
*** Given context : Monad TracingEv
*** Constraints : Show a
?? "Cannot justify constraints??" Isn't the case here rather that some
constraints are lacking?
GHC offers a little more explanation, but it remains cryptic:
MonadProblem.hs:9:
Could not deduce `Show a' from the context: ()
Probable cause: missing `Show a' in type signature for `return'
arising from use of `TE' at MonadProblem.hs:9
In the right-hand side of an equation for `return':
TE (\ c -> do
print c
putChar ':'
print i
putChar '\n'
return (c + 1, i))
Sure there's a missing Show a, but I can't change the signature of
return and it seems to me that Show a should follow from the
constraint on TracingEv
Is this a fundamental limitation? If so, why?
Regards,
Tommy
