Question

So I was looking at the following bit of code

let rec zip list list' =
  match list, list' with
  | [], _ -> []
  | _, [] -> []
  | h::t, h'::t' -> (h, h')::(zip t t')

when I noticed that it is not a syntactic error to replace

match list, list' with

by

  match list list' with

being the only error shown

This expression was expected to have type
    'a -> 'b list * 'c list    
but here has type
    'b list    

on zip t t' (over t).

My question is if makes any sense, at all, to have match list list' with instead of match list list' with. Shouldn’t the targets of pattern matching be always comma separated?

Answer 1

Yes, but if you write match list list' with there is only a single target, said target being what you get from applying the function list to the argument list'. Since list isn’t actually a function, you get the type error you quote.