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

18 May 2014


      On Sun, May 18, 2014 at 5:14 PM, Claude Heiland-Allen
 wrote:
...
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
Alternatively, you can turn on ScopedTypeVariables. That way, the 'a'
variable from the class header is in scope in the code below it.

Regards,

Erik

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

Erik Hesselink