Why is the null while-loop condition expression true?

AnomalousMonk has asked for the wisdom of the Perl Monks concerning the following question:

In a recent post, I saw a question about why

while ()
     {
     <>;
     last unless /\S/;
     print FILE;
     }
[download]

produced a "Use of uninitialized value $_ in pattern match (m//) ..." warning. This is easily fixed by assigning the value returned by the <> (diamond) operator (aka the readline function) to the appropriate variable:

c:\@Work\Perl>perl -wMstrict -le
"while ()
      {
      $_ = <>;
      last unless /\S/;
      print qq{>$_<};
      }
"
qeert
>qeert
<
ghjk
>ghjk
<
[download]

(This example works the same under ActiveState 5.8.9 and Strawberry 5.14.4.)

What caught my eye was the null loop condition expression in the
while () { ...; last unless ...; ...; }
infinite loop. I would have thought of something along the lines of
while (1) { ...; last unless ...; ...; }

I'm surprised that a null loop condition expression is accepted by the compiler or, being accepted, that it evaluates as true. Because other boolean "nullities" (undef, the empty string, 0) are false, I would have expected a null condition expression, if it compiled at all, to be false. (A B::Deparse of while () { ... } shows it to compile to while (1) { ... } in both Perl versions I tried.)

So, my questions:

Is there any documentation or discussion of this behavior? (Offhand, I can't find any in a quick scan of perlsyn.)
What is the rationale for this behavior? (Well, it does make a neat shorthand.)

Give a man a fish: <%-{-{-{-<

Comment on Why is the null while-loop condition expression true? Select or Download Code

Replies are listed 'Best First'.
Re: Why is the null while-loop condition expression true? by Paladin (Vicar) on Mar 28, 2016 at 18:31 UTC
From perlsyn (5.22.1 at least): As a special case, if the test in the "for" loop (or the corresponding "while" loop) is empty, it is treated as true. That is, both for (;;) { ... } and while () { ... } are treated as infinite loops.	[reply]
Re: Why is the null while-loop condition expression true? by kennethk (Abbot) on Mar 28, 2016 at 18:33 UTC
Documented in For Loops in perlsyn: As a special case, if the test in the for loop (or the corresponding while loop) is empty, it is treated as true. That is, both `for (;;) { ... }` [download] and `while () { ... }` [download] are treated as infinite loops. Not very well placed documentation, and counter-intuitive from my perspective, but documented. Update:: A little Googling pulled up while () vs. while (1), particularly Re: while () vs. while (1). #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re: Why is the null while-loop condition expression true? by hexcoder (Curate) on Mar 28, 2016 at 19:02 UTC
The rationale might have been that in former art (in this case C) `for (;;) { ... }` was a common idiom for an infinite loop. While C did not allow an empty condition in a `while` loop, the canonical treatment of the loop condition in `for` and `while` in Perl might have led to this result. I think it is more consistent than in C.	[reply] [d/l] [select]
Re: Why is the null while-loop condition expression true? by AnomalousMonk (Archbishop) on Mar 28, 2016 at 20:58 UTC
Paladin, kennethk and hexcoder: I sought enlightenment only in my local version 5.14 documentation and did not find it. (Neither is the `for (;;) { ... }` variation mentioned.) I should have thought to consult the scrolls on-line. I have to agree the infinitude of `while () { ... }` does seem a bit counterintuitive (I was aware of and comfortable with the `for`-loop version in both C and Perl), but I'm a programmer: I can believe three counterintuitive things before my first cup of coffee. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^2: Why is the null while-loop condition expression true? by BrowserUk (Patriarch) on Mar 28, 2016 at 22:12 UTC
I have to agree the infinitude of while () { ... } does seem a bit counterintuitive I agree with your reaction, but not your conclusion. With `while(1){` doing the intuitively obvious, I can see absolutely no justification for equivalence between that and `while(){` Seems most likely to be a bug that slipped through rather than a deliberate act; and remains because of some misguided backward compatibility justifiction; despite that no one in their right mind would code an infinite loop that way in order to save 1 character. A bug masquerading as a feature is my assessment. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :) In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^3: Why is the null while-loop condition expression true? by Anonymous Monk on Mar 28, 2016 at 22:45 UTC
The source code from v1 seems to make it pretty clear it was deliberate, never a bug. I am with AnomalousMonk. While `while(){}` is an odd synonym at first blush for `while(1){}` the idea of ever writing `while(){}` to mean… all the enclosed code is just for laughs because I never want it to run is even sillier.	[reply] [d/l] [select]
Re^4: Why is the null while-loop condition expression true? by BrowserUk (Patriarch) on Mar 28, 2016 at 22:51 UTC
Re^5: Why is the null while-loop condition expression true? by soonix (Chancellor) on Mar 29, 2016 at 08:28 UTC
Re: Why is the null while-loop condition expression true? by SimonPratt (Friar) on Mar 29, 2016 at 13:26 UTC
I agree, that seems very counter-intuitive. It would make more sense to me for `while(){...}` to function more like `while($_){...}` instead of `while(1){...}`	[reply] [d/l] [select]