recursion - Please walk me through this "Erlang Programming" recursive sample

Question

From page 90 of Erlang Programming by Cesarini and Thomson, there is an example that has no detailed discussion. I'm quite the newbie to functional programming and recursive thinking, so I'm not familiar in solving problems this way.

"For example, the following function merges two lists (of the same length) by interleaving their values: "

merge(Xs,Ys) -> lists:reverse(mergeL(Xs,Ys,[])).

mergeL([X|Xs],Ys,Zs) ->  mergeR(Xs,Ys,[X|Zs]);
mergeL([],[],Zs) ->  Zs.

mergeR(Xs,[Y|Ys],Zs) ->  mergeL(Xs,Ys,[Y|Zs]);
mergeR([],[],Zs) ->  Zs.

How does this work? Thanks!

Answer 1

step through it

merge([1,2],[3,4])
reverse(mergeL([1,2],[3,4],[]))
reverse(mergeR([2],[3,4],[1]))
reverse(mergeL([2],[4],[3,1]))
reverse(mergeR([], [4], [2,3,1]))
reverse(mergeL([], [], [4,2,3,1]))
reverse([4,2,3,1])
[1,3,2,4]

It's always good to work these functions by hand on a piece of paper with a small input where you're trying to figure it. You'll quickly see how it works.