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

3 Apr 2014


      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