[Haskell-cafe] overloading show function

[Steffen Schuldenzucker]

Hi Philipp,

On 06/29/2011 11:50 PM, Philipp Schneider wrote:
> Hi cafe,
>
> in my program i use a monad of the following type
>
> newtype M a = M (State ->  (a, State))

btw., it looks like you just rebuilt the State monad.

>
> ...
>
> 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

This is a well-known issue. The problem is as follows: Your second 
instance declares an instance Show (M a) for any type a. If a is of the 
Form (b, c), we can derive a tuple instance from that. This however 
conflicts with the tuple instance declared above.

If you want GHC to choose the most specific instance (which would be 
your first one for tuples), use the

{-# LANGUAGE OverlappingInstances #-}

pragma. Be careful with this however, as it might lead to unexpected 
results. For a similar problem, you may want to consult the haskell wiki[1].

-- Steffen

[1] http://haskell.org/haskellwiki/GHC/AdvancedOverlap