1) Ignore indentation (I intentionally wrote it to deceive, but similar examples could easily appear in real code). 2) `match` cases always bind to the innermost `match` statement. 3) Statement separators (`;`) are tricky.

The correct indentation of the above code is:

  if a > 0 then
    print_endline "a > 0" ;
  match a with
    | 0 -> print_endline "impossible"
    | 2 ->
      match b with
        | 0 -> print_endline "2, 0"
        | _ -> print_endline "2, not 0"
        | 3 -> print_endline "3, something"
        | _ -> print_endline "something, something"
            ;
            print_endline "done"

To have the same meaning as the indentation implies, you would need to add `(...)` or `begin ... end` at the appropriate places.

There are two things to remember. The first is the match binding thing that tomp mentions in his reply.

The second one is that in OCaml, semicolon is a separator, and not a terminator. In contrast, in C/C++, semicolon is a terminator. If you have an expression, you end it with a ";" just because.

This is not the case for OCaml. In OCaml, semicolon is used to separate two sequential expressions where only one expression is expected. Thus,

    <expr1>;<expr2> 

is evaluated in sequence, and can be used in a place where only one is expected. For example, if statements have the following syntax:

    if <expr1> then <expr2> else <expr3>

Now if you wanted to do two things (instead of one) in the "then" block, you would simply write

    if <expr1> then <expr2.1>;<expr2.2> else <expr3>

Notice that under these rules

    if <expr1> then <expr2>; else <expr3> 

makes no sense. Separators are not terminators. We are used to thinking of ";" as terminators because of C/C++.