[Haskell-cafe] Type arithmetic with ATs/TFs

Andrew Coppin andrewcoppin at btinternet.com
Fri Feb 12 14:11:56 EST 2010

Robert Greayer wrote:
> What Ryan said, and here's an example of addition with ATs,
> specifically (not thoroughly tested, but tested a little).  The
> translation to TFs sans ATs is straightforward.
> class Add a b where
>     type SumType a b
> instance Add Zero Zero where
>     type SumType Zero Zero = Zero
> instance Add (Succ a) Zero where
>     type SumType (Succ a) Zero = Succ a
> instance Add Zero (Succ a) where
>     type SumType Zero (Succ a) = Succ a
> instance Add (Succ a) (Succ b) where
>     type SumType (Succ a) (Succ b) = Succ (Succ (SumType a b))

I'm pretty sure this is almost exactly what I wrote in the first place, 
and it didn't work. I'll try again and see if I get anywhere...

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