Question

I’d like to know how to swap every second element of a list in Haskell.

Example output should look like this:

swap [1,2,3,4,5]  
[2,1,4,3,5]

What I have so far is

swap :: [a] -> [a]  
swap [] = []  
swap (x:xs) = head xs : [x]

but this only swaps the first two elements and any attempt I make to make the function recursive causes errors when I try to load the file that contains the function. How to make it recursive?

Answer 1

You need to grab out 2 elements at a time:

swap [] = []
swap (x:y:rest) = y:x:(swap rest)
swap [x] = [x]

The last line is needed to allow odd-length lists — it matches a list having length exactly 1, so it doesn’t overlap either of the other 2 cases (length 0, and length 2 or more).