[Haskell-beginners] Writer and mappend

[Isaac Dupree]

On 03/15/11 16:18, Britt Anderson wrote:
> Or can I do something like
>
> instance Monoid MyDat where
> mempty = Mydat [] 0
> mappend a b = (Mydat (mappend (list1 a) (list1 b)) (item1 a))
>
> without defining an instance for Float?

Yes, you can do this.  Try it!  Does it typecheck?  Then there's a large 
chance it's correct!  (In addition to more-philosophical reasons.)

-Isaac