Here is something else that I don't quite understand...
Original version compiles:
push :: Show b => State b -> Dispatcher b a -> (ScriptState a b) ()
push state dispatcher =
do w <- get
trace 95 $ "push: Pushing " ++ show state ++ " onto the stack"
let s = stack w
putStrict $ w { stack = (state, dispatcher):s }
data State a
= Start
| Stop
| (Show a, Eq a) => State a
instance Eq a => Eq (State a) where
(State a) == (State b) = a == b
Start == Start = True
Stop == Stop = True
_ == _ = False
instance Show a => Show (State a) where
show (State a) = show a
show Start = "Start"
show Stop = "Stop"
This version does not. Why does it require Eq in the ++ context? And
why doesn't the other version?
data (Show a, Eq a) => State a
= Start
| Stop
| State a
deriving (Eq, Show)
Could not deduce (Eq b) from the context (Show b)
arising from use of `show' at ./Script/Engine.hs:86:38-41
Probable fix: add (Eq b) to the type signature(s) for `push'
In the first argument of `(++)', namely `show state'
In the second argument of `(++)', namely `(show state) ++ " onto the
stack"'
--
http://wagerlabs.com/
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
