Would this work for you?
data BiList a b
= Empty
| a :# (BiList b a)
infixr 5 :#
blah :: BiList Char Int
blah = 'a' :# 1 :# 'a' :# Empty
On Wed, Apr 2, 2014 at 9:52 PM, Jacek Dudek
-- As an exercise I wanted to define a datatype that is an alternating list of elements of two different types. The best that I could do are the type definitions below:
module BiList ( BiList (..) , AList (EmptyA) , BList (EmptyB) ) where
data BiList a b = Empty | BA (AList a b) | BB (BList a b)
data AList a b = EmptyA | AL a (BList a b)
data BList a b = EmptyB | BL b (AList a b)
(<#) :: a -> (BList a b) -> (AList a b) a <# bs = AL a bs
(<@) :: b -> (AList a b) -> (BList a b) b <@ as = BL b as
infixr 5 <# infixr 5 <@
example :: BiList Int Char example = BA $ 1 <# 'a' <@ 2 <# 'b' <@ 3 <# 'c' <@ 4 <# 'd' <@ EmptyA
-- There are two things that I don't like about this implementation.
-- (1) The BA and BB constructors that must be applied on top of instances of (AList a b) and (BList a b) to lift them to be of type (BiList a b).
-- (2) Having three different constructors for an empty list: Empty, EmptyA, EmptyB, where ideally I would just have one.
-- Is it possible to get around either of these annoyances with some type theory gymnastics? Maybe something like the function fromIntegral (the mechanics of which I don't really understand at this point)? _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners