On Wed, Feb 10, 2010 at 2:29 PM, Andrew Coppin
OK, so I sat down today and tried this, but I can't figure out how.
There are various examples of type-level arithmetic around the place. For example,
http://www.haskell.org/haskellwiki/Type_arithmetic
(This is THE first hit on Google, by the way. Haskell is apparently THAT popular!) But this does type arithmetic using functional dependencies; what I'm trying to figure out is how to do that with associated types.
Any hints?
(I know for a fact that other people have done this - rule 34 requires it.)
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With type families, something like: type family Add m n type instance Add (Succ n) (Succ m) = Succ (Succ (Add n m)) type instance Add Zero (Succ m) = (Succ m) type instance Add (Succ m) Zero = (Succ m) type instance Add Zero Zero = Zero is this what you are after? There's also the tfp library on hackage which has much more type level arithmetic, using type families.