[Haskell-cafe] overloading show function

[Philipp Schneider]

Thank you very much, this worked.

On 06/30/2011 12:03 AM, aditya siram wrote:
> Try enabling OverlappingInstances extension by adding this to the top
> of the file:
> {-# LANGUAGE OverlappingInstances #-}
>
> -deech
>
> On Wed, Jun 29, 2011 at 4:50 PM, Philipp Schneider
> <philipp.schneider5 at gmx.net> wrote:
>> 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))]
>>
>> 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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