[tomp]

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.

[mercurial]

What would be the corresponding SML? I see how the way SML binds ';' would help here, but would it change anything about the last two badly indented matching? And it seems to provide you with the equivalent footgun if you attempt to do multiple side-effect operations in a match branch, which in my experience you want to do more often than you need to separate matches with ';'.

[tel]

I figured the indentation was a lie, but didn't realize you would have to introduce something like begin...end to make it work the way it appears.