This 'begin' might be unmatched

Learning
1

Hello,

I have problems with begin and end:
How can I write this correctly?

if true then begin
  if false then begin
    print_int 2;
    print_newline();
  end;
  else begin
    print_int 3;
    print_newline();
  end;
end;

1 | if true then begin
This ‘begin’ might be unmatched

6 | else begin
** ^^^^**
Error: Syntax error: ‘end’ expected

1 Like
2

Hi @Daoued,

You’re being a little too liberal with semicolons. Only 2 out of 7 in your example are actually necessary:

if true then begin
  if false then begin
    print_int 2;
    print_newline ()
  end
  else begin
    print_int 3;
    print_newline ()
  end
end

In OCaml, we typically use ; only when sequencing two expressions when the first one has type unit. In your case, we need them to split the adjacent print_* applications. We could also do this with let bindings (see explanation here), but it’s a nice bit of “syntax sugar”.

The OCaml parser will – unfortunately, in my opinion – also accept a trailing ; after an expression, but this is not necessary or generally encouraged. In your case, the semicolon inside end; else begin is problematic because it does not come after an expression: it’s in the middle of an if ... then ... else expression where the parser does not expect to find one.

Hope that clears things up!

3 Likes
3

Thank you! :grinning: