Re^2: Doing "it" only once

The keyword 'once' is very misleading. We're talking about optimizing away checking of a conditional - but it's not true that this conditional is checked only once. It continues to be checked (and needs to be checked) until it happens to be true.

Perl6 will have a FIRST block, meaning the block will be executed only once, and to be specific, the first time the surrounding block was entered. If you write this:

while (...) {
    if (/foo/) {
        FIRST {...; next}
    }
    ...
}
[download]

and perl can determine there are no side-effects in the conditional, perl could be made to optimize away the conditional once it was true once.

Having said that, the original problem isn't clearly defined. Suppose there would be this 'once-only if', what should the following print:

my @list = qw /foo bar foo/;
for my $x (1, 2) {
    for (@list) {
        if (/foo/) {  # Assume a 'once-only if'
            print "foo";
        }
     }
}
[download]

Should it print 'foo' once, or twice? That is, is the "once-only" a property of the enclosing loop, to be reset when that loop is reentered, or a property of the condition itself, meaning that after it became true once, it will never be true in the life-time of the program?

Comment on Re^2: Doing "it" only once Select or Download Code

Replies are listed 'Best First'.
Re^3: Doing "it" only once by QM (Parson) on Sep 22, 2005 at 17:51 UTC
That is, is the "once-only" a property of the enclosing loop, to be reset when that loop is reentered, or a property of the condition itself, meaning that after it became true once, it will never be true in the life-time of the program? Good point. I wonder if we could make a parallel with the `our/my` distinction, such that a `my once` would reset each time the enclosing block is entered, but `our once` would not? Or perhaps some sort of `reset` operator, along with labels, to reset a `once` conditional. Seems to be a lot of possible paths there. We should probably agree on the fundamental property first. I see a few choices: 1) a conditional, once true, never tested again 2) a block, once entered, never entered again (including testing for any attached condition). 3) a statement modifier, indicating the attached block will only be executed once. And then further choices about whether it's resettable, and the reset mechanism. [I can imagine taking the language duality of "once" as eleven in spanish, and having it context or parameter sensitive. If it's true 1 time, it's pronounced "wuns", if it's reset, it's pronounced "on-say".] Lots of potential. Still, is it useful? Is it practical? Or is it just cool? Update: Clarified (2) to be more general than (1). -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]
Re^4: Doing "it" only once by Anonymous Monk on Sep 23, 2005 at 09:06 UTC
2) a block, once entered, never entered again (including testing for the attached condition). Point 2 raises some questions. You talk about an attached condition. If you only talk about blocks with an attached condition, what's the difference between 1) and 2)? Also, suppose you had a 'once-only' construct, what should the following do: `for (1 .. 4) { if ($_ % 2) {print "odd"} # Assume 'once only blocks' else {print "even"} }` [download] Should it print "odd even even even"? But then, the else block isn't "once only". Print "odd even"? But then the conditional is still checked after being true once. Should it just print "odd"? That would violate the principle of least surprise. Lots of potential. Still, is it useful? Is it practical? Or is it just cool? Well, you (sort of) can do this in Perl6. Perl6 has `FIRST` blocks, which will be executed the first time the surrounding block is entered. So, effectively, you can make a "executed only once" block this way: `{ FIRST { ... } }` [download] that is, have nothing in the block, except a `FIRST` sub-block. However, that will not prevent a conditional from being executed. But in my opinion, the frequency of such construct isn't high enough for the extra syntactical sugar (and the subsequent pain when reset mechanisms need to be bolted on). It's really easy to simulate: `my $flag = 1; ... if ($flag && CONDITION) { $flag = 0; ... }` [download] The then block will be executed only once, but it the programmer has an very easy mechanism to reset the 'once only' property. And depending on the scope of the flag, it's a 'once-only in the life time of the program' or 'once-only for each time the surrounding block was entered'. Or anything in between. Sure, there's the slight overhead of an extra 'if (0 && ...)' - but if your program can't deal with that, and there's nothing else to be optimized, you probably shouldn't have used Perl in the first place.	[reply] [d/l] [select]
Re^5: Doing "it" only once by QM (Parson) on Sep 23, 2005 at 12:07 UTC
My intent was 2) a block, once entered, never entered again (including testing for any* attached condition).* Meaning that a block may or may not have a condition attached. This is more generic than (1). -QM -- Quantum Mechanics: The dreams stuff is made of	[reply]