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?