OCaml function calls happening in wrong order

Question

Right, so this is an odd problem that really took me by surprise. Basically, I'm working on a build system that gives you the option of running shell commands before and after the main build. To execute these commands I'm just using Sys.command. The problem is that whenever I use that function it changes the order in which functions are called. For example:

Sys.command "echo 'Hi!'";
Printf.printf "second\n";
Sys.command "echo 'Bye!'"

outputs

Hi!
Bye!
second

from both the REPL and compiled executables. However, if I use any other function it seems to work fine. Interestingly, if I define a function to call Sys.command it still executes in the wrong order. I've tested this on both 4.02.1 running on GNU/Linux and 4.01.0 in Cygwin and get the same behavior on both. As far as I am aware the ; does not affect call order. Am I missing something here?

Answer 1

You have a buffering problem.

Try this:

Sys.command "echo 'Hi!'";
Printf.printf "second\n%!";
Sys.command "echo 'Bye!'"

The %! specifier says to flush the buffer at that point.

Because it's a mixed paradigm language (with side effects), OCaml expressions are executed in a predictable order. You can depend on this. If things seem to be executed out of order, there is something else going on.

(As a side comment, note that order of evaluation of parameters to a function is not constrained.)