[Haskell-cafe] Constants in Class definitions - could not deduce

[martin]

Thanks, typeOf did the trick

Am 05/18/2014 05:14 PM, schrieb Claude Heiland-Allen:
> Hi Martin,
> 
> On 18/05/14 15:54, martin wrote:
>> Hello all,
>>
>> in my attempt to solve a Calendar related problem, I wanted to define Months and Weekdays such that I can increment a
>> value. Unlike the "succ" Funktion in Enum my "inc" function shall circle endlessly.
>>
>> I came up with the following
>>
>> class (Ix a, Enum a) => Incrementable a where
>>          from :: a
>>          to  :: a
>>          inc :: a -> a
>>          inc x = [from .. to] !! ((1 + index (from, to) x) `mod` rangeSize (from,to))
>>
>> but GHC complains:
>>
>> /home/martin/projects/haskell/euler/p19.hs:39:65:
>>      Could not deduce (Ix a0) arising from a use of `rangeSize'
>>      from the context (Incrementable a)
>>
>> I suppose this is because GHC does not know that I want "to" and "from" to be taken from the same instance as x. If I
>> replace the signature of "from" by "a -> a" and implement it by using a function which throws aways its argument, the
>> types become clear and GHC no longer complains.
>>
>> Alternatively I can omit the implementation of "inc" in the class and leave that to the instances. I can easily fix the
>> types in the instances, but this leads to some code duplication.
>>
>> I tried (from::a) to fix the type, but this had no effect. How is this done correctly?
> 
> You could use `asTypeOf` (which is `const` with a specialized type):
> 
> 
> class (Ix a, Enum a) => Incrementable a where
>         from :: a
>         to  :: a
>         inc :: a -> a
>         inc x = [f .. t] !! ((1 + index (f, t) x) `mod` rangeSize (f,t))
>           where f = from `asTypeOf` x ; t = to `asTypeOf` x
> 
> 
> hth,
> 
> 
> Claude