Re: [Haskell-beginners] Data type definition for a list of elements of alternating types?

Thanks David, that's really clever! I was trying to do it without any auxiliary data types, but couldn't see how I could use an instance of (BiList a b) in the constructor expression for (BiList a b) without losing the property that the list elements alternate from one type to the other with each new element. But now I see that when you write (BiList b a) in the constructor expression, that's written in the context provided by the (data BiList a b) line, so having the type variables in the opposite order makes all the difference. Brent, David's definition actually solved both (1) and (2), try it out! On 4/3/14, Denis Kasak wrote:

On 3 April 2014 22:58, Brent Yorgey wrote:

...

data BiList a b = BA (AltList a b) | BB (AltList b a)

data AltList a b = Empty | Cons a (AltList b a)

So this addresses (2) but not (1). I don't think there is any way around the need for (1). (Note, however, that you do still have two distinct representations of the empty list: BA Empty and BB Empty. I can't see any way around that either.)

You could move the Empty constructor to BiList while making AltList a non-empty list, i.e.

data BiList a b = Empty | BA (AltList a b) | BB (AltList b a)

data AltList a b = Elem a | Cons a (AltList b a)

-- Denis Kasak _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners