[Haskell-cafe] overloading show function

[Holger Siegel]

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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