[Haskell-cafe] overloading show function

[Wolfgang Braun]

"An environment contains local variable bindings, so no subcomputation will ever need to return its environment.
 " - That is not true.  A subcomputation can possible modify an environment except the language forbids such a case.


On 06/30/2011 02:36 PM, Holger Siegel wrote:
> 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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>
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