CODEWITHSUNDEEP
ocaml
prosseek
prosseek

Reputation: 191149

What's it for ';;' in OCaml?

This is my simple OCaml code to print out a merged list.

let rec merge cmp x y = match x, y with
    | [], l -> l
    | l, [] -> l
    | hx::tx, hy::ty ->
        if cmp hx hy
            then hx :: merge cmp tx (hy :: ty) 
            else hy :: merge cmp (hx :: tx) ty

let rec print_list = function 
    | [] -> ()
    | e::l -> print_int e ; print_string " " ; print_list l 
    ;;


print_list (merge ( <= ) [1;2;3] [4;5;6]) ;;

I copied the print_list function from Print a List in OCaml, but I had to add ';;' after the function's implementation in order to avoid this error message.

File "merge.ml", line 11, characters 47-57:
Error: This function has type int list -> unit
       It is applied to too many arguments; maybe you forgot a `;'.

My question is why ';;' is needed for print_list while merge is not?

Upvotes: 0

Views: 174

Answers (2)

didierc
didierc

Reputation: 14750

This is an expansion of Jeffrey's answer.

As you know, when doing language parsing, a program has to break the flow in manageable lexical elements, and expect these so called lexemes (or tokens) to follow certain syntactic rules, allowing to regroup lexemes in larger units of meaning.

In many languages, the largest syntactic element is the statement, which diversifies in instructions and definitions. In these same languages, the structure of the grammar requires a special lexeme to indicate the end of some of these units, usually either instructions or statements. Others use instead a lexeme to separate instructions (rather than end each of them), but it's basically the same idea.

In OCaml, the grammar follows patterns which, within the algorithm used to parse the language, permits to elide such instruction terminator (or separator) in various circumstances. The keyword let for instance, is always necessary to introduce a new definition. This can be used to detect the end of the preceding statement at the outermost level of program statements (the top level).

However, you can easily see the problem it induces: the interactive version of Ocaml always need the beginning of a new statement to be able to figure out the previous one, and a user would never be able to provide statements one by one! The ;; special token was thus defined(*) to indicate the end of a top level statement.

(*): I actually seem to recall that the token was originally in OCaml ancestors, but then made optional when OCaml was devised; however, don't quote me on that.

Upvotes: 1

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66823

The ;; is, in essence, a way of marking the end of an expression. It's not necessary in source code (though you can use it if you like). It is useful in the toplevel (the OCaml REPL) to cause the evaluation of what you've typed so far. Without such a symbol, the toplevel has no way to know whether you're going to type more of the expression later.

In your case, it marks the end of the expression representing the body of the function print_list. Without this marker, the following symbols look like they're part of the same expression, which leads to a type error.

For top-level expressions in OCaml files, I prefer to write something like the following:

let () = print_list (merge ( <= ) [1;2;3] [4;5;6])

If you code this way you don't need to use the ;; token in your source files.

Upvotes: 4

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