I presume the point was to allow the writing of functions that accept either list type, such as sort :: List a evenorodd -> List a evenorodd or similar. David On Wed, Apr 23, 2008 at 7:55 PM, Iavor DiIatchki wrote:

Hello, I am not sure of the use case here but you could also do the following:

data EvenList a = Nil | ConsE a (OddList a)

data OddList a = ConsO a (EvenList a)

This does not use any type system extensions.

-Iavor

On Wed, Apr 23, 2008 at 4:46 PM, David Roundy wrote:

...

2008/4/23 Martijn Schrage :

...

It depends a bit on what you want to use these lists for, but the following encoding works for your examples and doesn't need the type class.

data E data O

type Even = (E,O) type Odd = (O,E)

That's a nice little trick! I like how you achieve type signatures relating two distinct types just by sticking them in a tuple. :)

David _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe