[Haskell-cafe] Constants in Class definitions - could not deduce
hesselink at gmail.com
Sun May 18 17:45:28 UTC 2014
On Sun, May 18, 2014 at 5:14 PM, Claude Heiland-Allen
<claude at mathr.co.uk> 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
>> 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
>> but GHC complains:
>> 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.
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