Hi,
On 11 March 2012 15:57, Peter Hall
Can someone remind me why I can't do this:
data Digit = D0 | D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8 | D9 deriving (Eq, Ord, Show)
instance Num [Digit] where ... -- This isn't allowed
Why is it not allowed? Yes, it needs FlexibleInstances, but this is a fairly common language extension so shouldn't be a big problem for most use cases. The error message generated by ghc should tell you about this. Following is what I get for example: Illegal instance declaration for `Num [Digit]' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Num [Digit]' HTH, Ozgur