Inconsistent behaviour of empty "while" EXPR

[Clyde Ingram]

Odd one this:an empty EXPRession in a "while" loop seems to take a defined
value in this call from either ksh on solaris 2.6 to Perl 5.004, or Cygwin
on Win2k or Win98 to Perl 5.8.0 (ActiveState binary 806):

perl -e 'while () { print "Aha "; }'

because it loops endlessly, printing "Aha Aha Aha Aha . . .".
Same behaviour if I beef this up into:

perl -wTe 'use strict; while () { print "Aha "; }'

or even it I put this into a file (fred), make it executable (chmod 755
fred), and run it (perl fred).

Contrast this with:

perl -e 'do { print "Aha "; } while ();'

which only prints "Aha " once, and then stops.
Contrast still further with:

perl -e 'if () { print "Aha "; }'

which fails with:

syntax error at -e line 1, near "() "
Execution of -e aborted due to compilation errors.

Note also, these print nothing, as expected:

perl -e 'while (undef) { print "Aha "; }'
perl -e 'while ("") { print "Aha "; }'
perl -e 'while (0) { print "Aha "; }'
perl -e 'while (defined) { print "Aha "; }'

Now the Perl manual says:

if (EXPR) BLOCK
. . .
LABEL while (EXPR) BLOCK
. . .
The while statement executes the block as long as the expression is
true (does not evaluate to the null string "" or 0 or "0").

Can anyone explain why my very first "while ()" looped endlessly, but the
"do . . . while ()" stopped?
And should I be surprised that the "if ()" gave a syntax error instead?

Thank-you,
Clyde

[Abigail]

Clyde Ingram
(clyde@nohamorspamgetofftheline.freeservenohamorsp am.co.uk.nohamorspam) wrote on MMMDCLXVII September MCMXCIII in <URL:news:3Jq9b.2166$966.1671@newsfep3-gui.server.ntli.net>:
?? Odd one this:an empty EXPRession in a "while" loop seems to take a defined
?? value in this call from either ksh on solaris 2.6 to Perl 5.004, or Cygwin
?? on Win2k or Win98 to Perl 5.8.0 (ActiveState binary 806):
??
?? perl -e 'while () { print "Aha "; }'
??
?? because it loops endlessly, printing "Aha Aha Aha Aha . . .".

Yes. That's a feature (but poorly documented):

$ perl -MO=Deparse -e 'while () {}'
while (1) {
;
}
-e syntax OK

Here's another, similar trick:

$ perl -MO=Deparse -e 'for (;;) {}'
while (1) {
;
}
-e syntax OK


The latter trick comes from C.


?? Contrast this with:
??
?? perl -e 'do { print "Aha "; } while ();'
??
?? which only prints "Aha " once, and then stops.
?? Contrast still further with:
??
?? perl -e 'if () { print "Aha "; }'
??
?? which fails with:
??
?? syntax error at -e line 1, near "() "
?? Execution of -e aborted due to compilation errors.


Only while () BLOCK is special. Not if () BLOCK, not do BLOCK while (),
and neither EXPR while ().



Abigail
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