CODEWITHSUNDEEP
ocaml
MK.
MK.

Reputation: 4027

writing main() to call a function

I have an OCaml function that converts a string to an array. What is the canonical way of writing a "main" function to call this and print the array.

let createArray pattern patArray  =
    (* some unimportant way of setting all the elements in the array patArray
       based on the string pattern *)


let main () =
    let pattern =  "Pattern" in
        let patArray = Array.create (String.length pattern) 0 in
            let res = createArray pattern patArray in
                Array.iter ~f:(printf "%d ") patArray;;  <------------------

main ()

1) In the above, if I leave out the ';;' , it does not work. What is the significance of that?

2) Instead of using a dummy binding "res" , can I somehow just write two statements to be executed sequentially , like so:

createArray pattern patArray
Array.iter ~f:(printf "%d ") patArray

Upvotes: 0

Views: 363

Answers (1)

Reimer Behrends
Reimer Behrends

Reputation: 8730

Without the ;;, the parser cannot know that the main () call following that line is supposed to be a stand-alone expression (whitespace is not significant here).

You can use the following idiom instead:

let main () = ...
let () = main ()

The let () = expr idiom will evaluate an expression of type unit at that point. The initial let informs the parser that a new top-level let construct begins. Using ;; is an alternative way to tell the parser about the end of a top-level construct, but is primarily intended for interactive use.

In order to evaluate two expressions sequentially, separate them with a semicolon (use parentheses or begin ... end if you're unsure about precedence rules). For example:

let patArray = Array.create (String.length pattern) 0 in
  createArray pattern patArray;
  Array.iter ~f:(printf "%d ") patArray

Or, using begin and end to make precedence clearer:

let patArray = Array.create (String.length pattern) 0 in begin
  createArray pattern patArray;
  Array.iter ~f:(printf "%d ") patArray
end

Without the ;, the parser would not know whether Array.iter on the next line is supposed to an additional argument to the createArray call.

Upvotes: 2

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