On 11 March 2012 15:57, Peter Hall <peter.hall@memorphic.com> wrote:

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