Am 29.06.2011 um 23:50 schrieb Philipp Schneider:
Hi cafe,
in my program i use a monad of the following type
newtype M a = M (State -> (a, State))
i use the monad in two different ways. The type variable "a" can be a pair as in
interp :: Term -> Environment -> M (Value,Environment)
and it can be just a value as in
type Environment = [(Name, Either Value (M Value))]
Simple rule: Never return an environment! An environment contains local variable bindings, so no subcomputation will ever need to return its environment. I don't know anything about the language your program interprets, but I'm sure that you can rewrite function interp as interp :: Term -> Environment -> M Value The structure of the interpreter will become clearer and your problem will vanish.
now in any case when i print the monad, i just want to print the value and never the environment.
More specific i want to use somthing like the following
instance (Show a,Show b) => Show (M (a,b)) where show (M f) = let ((v,_), s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s
instance Show a => Show (M a) where show (M f) = let (v, s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s
however this gives me the following error message:
Overlapping instances for Show (M (Value, Environment)) arising from a use of `print' Matching instances: instance (Show a, Show b) => Show (M (a, b)) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:78:10-42 instance Show a => Show (M a) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:82:10-29 In a stmt of an interactive GHCi command: print it
Any ideas how to fix it? Thanks! Philipp
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