Why is there a let in OCaml's List.map?

Question

In OCaml 3.12.1, List.map is written as follows:

let rec map f = function
    [] -> []
  | a::l -> let r = f a in r :: map f l

I'd expect that last line to be written as | a::l -> f a :: map f l, but instead, there is a seemingly useless let binding. Why?

Answer 1

I believe it is there to guarantee an order of function application for the map. The order of evaluation of simple expressions in OCaml is unspecified, so without the let the order of applications of f to the elements of the list would be unspecified. Since OCaml is not a pure language, you really would like the order to be specified (f is called on the head of the list first, and so on recursively).