Re^3: Wait and retry loop without goto

[einhverfr]

I think it seems magical because it isn't really a goto. I see why Perl 6 is changing it to &sub.nextwith(). It would be nicer IMO if it had been given its own keyword like tailcall().

[Laurent_R]

IMHO, it is a form of goto in the sense that the code never returns from that call. The Camel book says the following on this form of goto:

The goto &NAME form is highly magical and sufficiently removed from the ordinary goto to exempt its users from the opprobrium to which goto users are customarily subjected. It substitutes a call to the named subroutine for the currently running subroutine.

Having said that, in his book Modern Perl, chromatic also suggest to use a goto to eliminate tail-recursion:

Perl does not eliminate tail calls automatically, but you can get the same effect by using a special form of the goto builtin. Unlike the form which often produces spaghetti code Named because control flow is as simple and straightforward as a plate of spaghetti., the goto function form replaces the current function call with a call to another function. You may use a function by name or by reference. You can even modify the arguments passed to the replacement function by modifying @_:

    # split the array down and recurse
    if ($item < $miditem)
    {
        @_ = ($item, @array[0 .. $midpoint]);
        goto &elem_exists;
    }

    # split the array up and recurse
    else
    {
        @_ = ($item, @array[$midpoint + 1 .. $#array] );
        goto &elem_exists;
    }

Sometimes optimizations are ugly, but if the alternative is highly recursive code which runs out of memory, embrace the ugly and rejoice in the practical.

So, you are right, may be it should have been called something like tailcall(), except that the goto &NAME form can also be used for other things than tail reduction elimination.