Name binding in pattern matching in OCaml-CodePudding

What does the name binding with the as- keyword do in this exercise: https://ocaml.org/problems#17

# let split list n =
    let rec aux i acc = function
      | [] -> List.rev acc, []
      | h :: t as l -> if i = 0 then List.rev acc, l
                       else aux (i - 1) (h :: acc) t 
    in
      aux n [] list;;

Does it allow us to choose in the if / else clause whether the tail t or the list l is used in the function pattern matching? What is the common use of name binding in functional Programming? Thank you very much.

CodePudding user response：

The as keyword allows you to bind a name to larger and smaller parts of a pattern. In the example you provided:

let split list n =
  let rec aux i acc = function
  | [] -> List.rev acc, []
  | h::t as l -> if i = 0 then List.rev acc, l
                 else aux (i - 1) (h :: acc) t 
  in
  aux n [] list

l is bound to the entire list, and h is bound to the head of the list and t to the tail.

Without as you might write:

let split list n =
  let rec aux i acc = function
  | [] -> List.rev acc, []
  | h::t -> if i = 0 then List.rev acc, (h::t)
            else aux (i - 1) (h :: acc) t 
  in
  aux n [] list

Keep in mind that as is "greedy" to its left, and parentheses may be needed to disambiguate precedence.

E.g.

h::h'::t as lst

The entire list is bound to lst.

If you want to to bind the tail, but also the second element in a list:

h::(h'::_ as tl)