Re: Use of uninitialized value in substr

The warning is easily avoided by using '' instead of undef, but that seems a little unperlish to me?

I totally agree with you. Why should perl complain if I explicitly state undef ? -

Use of uninitialized value in substr at (eval 7) line 1, <STDIN> line 
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makes me think "Yes, silly. I know. I put it there on purpose. I wrote that. It's nothing you calculated" which is pretty close to an insult.

An explicit undef is seen by the compiler, and since the compiler does flip warning bits, it should do that for explicit undef.

Sometimes it does:

qwurx [shmem] ~ > perl -le 'print "foo bar ", "baz", ^@^ , " niente"'
Useless use of bitwise xor (^) in void context at -e line 1.
Useless use of a constant in void context at -e line 1.
foo bar baz
qwurx [shmem] ~ > perl -le 'print "foo bar ", "baz", ^@^ , undef'
Useless use of bitwise xor (^) in void context at -e line 1.
foo bar baz
qwurx [shmem] ~ > perl -le 'print "foo bar ", "baz", ^@^ , ""'
Useless use of bitwise xor (^) in void context at -e line 1.
Useless use of a constant in void context at -e line 1.
foo bar baz
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The 'useless use of a constant' warning goes away using undef and reappears using an empty string - "which I did write, silly." Ah well... ;-)

And I also agree that undef for substr as per your example expresses the intended purpose far better than using an empty string. But that's debatable, and maybe the right place to do that is over there at p5porters. Monasteries gather dogmas...

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Re^2: Use of uninitialized value in substr by ikegami (Patriarch) on May 03, 2010 at 23:49 UTC
since the compiler does flip warning bits I know it's off topic, but could you give an example of where the compiler flips warning bits or suppresses a warnings? I can't think of any. Sometimes it does `undef()` never gives a warning in void context. It's not an example of the compiler messing with the warning bits. It probably should warn if it's in void context and has no args, but it has nothing to do with flipping warning bits. The current implementation probably can't handle the "and has no args" part trivially. An explicit undef is seen by the compiler, and since the compiler does flip warning bits, it should do that for explicit undef. Your suggested implementation wouldn't work for a number of reasons (`substr` is already compiled. `substr` has more than one argument and the warning bits apply to all of them), but it doesn't mean it's not possible. In fact, I think it's quite easy to implement by having the parser convert `undef` to `''` (like it converts `while (<>)` to `while (defined($_ = <>))`).	[reply] [d/l] [select]

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Re^2: Use of uninitialized value in substr
by ikegami (Patriarch) on May 03, 2010 at 23:49 UTC

since the compiler does flip warning bits

I know it's off topic, but could you give an example of where the compiler flips warning bits or suppresses a warnings? I can't think of any.

Sometimes it does

undef() never gives a warning in void context. It's not an example of the compiler messing with the warning bits. It probably should warn if it's in void context and has no args, but it has nothing to do with flipping warning bits. The current implementation probably can't handle the "and has no args" part trivially.

An explicit undef is seen by the compiler, and since the compiler does flip warning bits, it should do that for explicit undef.

Your suggested implementation wouldn't work for a number of reasons (substr is already compiled. substr has more than one argument and the warning bits apply to all of them), but it doesn't mean it's not possible. In fact, I think it's quite easy to implement by having the parser convert undef to '' (like it converts while (<>) to while (defined($_ = <>))).

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