Hello,
in the following example, where I define a
state transformer monad "St" parametrized with
the state "c", the ghc-2.08 typechecker reports
the following error message in the type definition of
function "foo" which uses the instantiated "St":
`St' should have no arguments, but has been given 1 .
It works if I comment out the type definition. It also
works in hugs-1.3 with the type definition. Is there a
way to get it being typechecked by given a special compiler option?
Thanks in advance
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Christoph Herrmann
E-mail: [EMAIL PROTECTED]
WWW: http://brahms.fmi.uni-passau.de/cl/staff/herrmann.html
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data State c a = State (c -> (a,c))
unState :: State c a -> (c -> (a,c))
unState (State x) = x
unitState :: a -> State c a
unitState a = State (\s0 -> (a,s0))
bindState :: State c a -> (a -> State c b) -> State c b
bindState m k = State (\s0 -> let (a,s1) = (unState m) s0
(b,s2) = (unState (k a)) s1
in (b,s2))
instance Eq c => Monad (State c) where
return = unitState
(>>=) = bindState
data TS = TS { vs::Int } deriving (Show,Eq)
type St = State TS
foo :: Int -> St Int -- it works if this line is not given
foo x = return x
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END OF MESSAGE
