Till Varoquaux wrote:

Actually it doesn't:

   log (lazy (Printf.printf "%s" (awfully_long_computation ())))

when log_val is false (or sylvain's solution, which I prefer), will
not behave like

   Printf.ifprinf "%s" (awfully_long_computation ())

(it won't evaluate its arguments).

Till

IMHO: evaluating the arguments of your log statement will avoid some really ugly heisenbugs- what if awfully_long_computation performs I/O or otherwise has side effects?. Note that having side-effects in your arguments to the log statements is a really bad idea, but people will do it, and finding where they do it is non-trivial.

Also, creating a lazy thunk in Ocaml is expensive (like 140+ clock cycles), while passing an argument into a function is cheap- and the common case will be that the argument won't need to be evaluated, just passed in.

Translation: don't try to be too clever to avoid evaluating arguments.

Brian