[Haskell-cafe] Re: Scrap your rolls/unrolls

Forgot to reply to list On 22 October 2010 12:14, Dan Doel wrote:

Another solution, though, is SHE. With it, you can write:

 data ListF a r = NilF | ConsF a r  newtype List a = Roll (ListF a (List a))

 pattern Nil       = Roll NilF  pattern Cons x xs = Roll (ConsF x xs)

And not worry about Rolls anymore.

Ah yes, pattern synonyms. This solution is somewhat unsatisfying because you will also need some smart constructors: """ nil = Roll NilF cons x xs = Roll (ConsF x xs) """ Now the names of the smart constructors for building the data type are not the same as the things you pattern match on, which is a slightly annoying asymmetry. It's certainly less complicated than the TypeFamilies based solution though! Cheers, Max