Multiple Statements In Haskell

Question

How do you have multiple statements in haskell?

Here's what I'm trying to do: given a list such as [a,b,c,d], return every other element, so you get [a,c]. I can see the solution, and here's what I have so far:

fact (xs)   | length( xs ) `mod` 2  == 1 = head( xs )
    | otherwise = fact(tail( xs ))

This works fine the first time around, but then it quits. What I want to be able to say is return the head, and then call fact(tail(xs)) How do I do that?

Answer 1

The function you specified returns only a single element. You'd need to change it to something like:

fact [] = [] -- can't call tail on a list of length 0!
fact (xs)   | length( xs ) `mod` 2  == 1 = head( xs ) : fact(tail(xs))
            | otherwise = fact(tail( xs ))

You may find it helpful to write out type signatures to help figure out thinkos like this:

fact :: [a] -> [a] -- convert a list of anything to another (shorter) list

However note that this is very slow - O(n^2) in fact, since it's taking length at each step. A much more haskelly solution would use pattern matching to process two elements at a time:

fact :: [a] -> [a]
-- Take the first element of each two-element pair...
fact (x:_:xs) = x:fact xs
-- If we have only one element left, we had an odd-length list.
-- So grab the last element too.
fact [x]      = [x]
-- Return nothing if we have an empty list
fact _        = []