Non-zero return value of a callback function is used as a return value of lbfgs()

[msakai]

I noticed that when a callback function returns non-zero value, the value is used as a return value of lbfgs().

liblbfgs/lib/lbfgs.c

Lines 495 to 500 in 5ad02fb

	/* Report the progress. */
	if (cd.proc_progress) {
	if ((ret = cd.proc_progress(cd.instance, x, g, fx, xnorm, gnorm, step, cd.n, k, ls))) {
	goto lbfgs_exit;
	}
	}

But this behavior is not documented, e.g. at

liblbfgs/include/lbfgs.h

Lines 403 to 404 in 5ad02fb

	* @retval int Zero to continue the optimization process. Returning a
	* non-zero value will cancel the optimization process.

or

liblbfgs/include/lbfgs.h

Lines 461 to 465 in 5ad02fb

	* @param proc_progress The callback function to receive the progress
	* (the number of iterations, the current value of
	* the objective function) of the minimization
	* process. This argument can be set to \c NULL if
	* a progress report is unnecessary.

The current situation is not desirable as a user might write a callback function that returns a value that is not intended to be used as the return value of lbfgs().

I think it's better to either:

1. document the current behavior (#37), or
2. make lbfgs() always return LBFGSERR_CANCELED on cancel, ignoring the actual return value of the callback (#38)