Original bug ID: 3123
Reporter: administrator
Status: closed
Resolution: not a bug
Priority: normal
Severity: minor
Category: ~DO NOT USE (was: OCaml general)

Bug description

briand@aracnet.com wrote:

I went back through the manual and really couldn't find anything which
explained the difference between ; and ;;

Yeah, this is a bug in section 1.5 "Imperative Features" of the manual. (IMHO,
the whole Part I is not what it claims to be and the Ocaml team should stop
calling it an introduction into the language, but I've said that before
http://caml.inria.fr/archives/200210/msg00451.html)

Now, if you want to be anal, you might say that all that needs to be said is
indeed said in the manual, in section 6.7.2 "Control Structures"

| The expression expr1 ; expr2 evaluates expr1 first, then expr2, and returns
| the value of expr2.

together with section 6.11.2 "Structures":

| For compatibility with toplevel phrases (chapter 9) and with Caml Light, an
| optional ;; is allowed after each definition in a structure. The ;; has no
| semantic meaning. Also for compatibility, ;; expr is allowed as a component
| of a structure, meaning let _ = expr, i.e. evaluate expr for its
| side-effects.

But to find this section (and to see why it is indeed the answer to your
question) requires some ingenuity, since you are not dealing with a structure
proper, but with a compilation unit, and section 6.12 doesn't mention this
peculiarity. You have to guess its relevance from the "behaves roughly as".

With this knoledge, you can see let your version with a single semicolon

let w = area#misc#realize ();
area#misc#window;;

is equivalent to the definition:

let w = let _ = area#misc#realize () in area#misc#window

while the version with the double semicolon is

let w = area.misc.realize ()
let _ = area#misc#window

which binds w to unit and ignores the value of area#misc#window.

Yours, Florian