Ocaml: how to pattern match on an arbitrary number of arguments?

Question

Hi all,

is there an OCaml equivalent to Haskell's pattern matching on an arbitrary number of arguments? For example, can I have something resembling:

merge [] lst = lst
merge lst [] = lst
merge l1 @ (n : ns) l2 @ (m : ms) = 
  if n < m then n : merge ns l2 else m : merge l1 ms

(The example's lifted from Developing Applications with Objective Caml :)

Thanks.

Answer 1

You can't match multiple arguments as such, but you can match tuples, so you can do:

let rec merge l1 l2 = match l1, l2 with
| [], lst
| lst, [] -> lst
| (n::ns), (m::ms) -> if n < m then n :: merge ns l2 else m :: merge l1 ms

If you're ok with the function taking its arguments as a tuple you can also use function like this:

let rec merge = function
| [], lst
| lst, [] -> lst
| (n::ns as l1), (m::ms as l2) -> if n < m then n :: merge (ns, l2) else m :: merge (l1, ms)