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Why is there a space in OCaml List.map?

In OCaml 3.12.1, it is List.mapwritten as follows:

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

I expect the last line to be written as | a::l -> f a :: map f l, but there seems to be a useless binding instead let. Why?

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I believe that it should guarantee the order in which the function is applied to the card. The order of evaluation of simple expressions is not specified in OCaml, therefore, without the letorder of applications fto the list items, it will not be indicated . Since OCaml is not a pure language, you really need the order to be specified ( ffirst called at the top of the list, and so on recursively).

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