perlopref needs you

The perlopref document is nearing completion. I need to add the file test, quote-like, regex quote-like, and I/O operators, but now is a good time for other people to go over the document and make sure I haven't made any mistakes or omissions.

I am also thinking about changing the name to perlopquick since it isn't really a reference (it doesn't contain everything about the operators).

adventures in ignorance: what I have learned from perlopref so far

While working on perlopref I have learned a few things about Perl I had never known. This is a short list of the things I learned (besides the bit about modulo that I have already blog'ed about).

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Only the plain assignment operator (=) can do list assignment. I found this out when I expected @a x= 5 to do something (besides throw an error that is). This makes sense for most of the assignment operators. For instance, @x += 2 makes very little sense, but I think @a x= 3 makes some sense, but alas it is an error.

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The bitwise and operator (&) has different behavior with respect to strings than the bitwise or and xor operators (| and ^ respectively). Bitwise and truncates to the length of the shorter string, but bitwise or and xor extend the shorter string to the length of the longer one with nul characters.

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The arrow operator has surprising behavior with respect to coderefs. This code throws an error as I would expect:

#!/usr/bin/perl

use strict;
use warnings;

sub func {
    return [qw/a b c/];
}

print func()[0], "\n";

But this code prints "a\n":

#!/usr/bin/perl

use strict;
use warnings;

sub func {
    return [qw/a b c/];
}

my $ref = \&func;

print $ref->()[0], "\n";

This seems to be an extension of the rule that lets you pretend that an AoA is really a multi-dimensional array. I knew this rule allowed you to say $aoa->[0][1]("arg1", "arg2"), but I had never tried it with the function call first.

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Some people use ~~X (i.e. ~(~X)) to force scalar context on X, boy are they going to be annoyed by the smartmatch operator.

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I go through bouts of knowing and forgetting this, but the flip-flop operators (.. and ...) actually return useful information, not just true or false:

#!/usr/bin/perl

use strict;
use warnings;

for my $i (1 .. 10) {
 my $range = ($i == 2 or $i == 6) .. ($i == 4 or $i == 8);
 print "$range: $i\n" if $range;
}

The code above prints

1: 2
2: 3
3E0: 4
1: 6
2: 7
3E0: 8

---

The comparison operators (<=> and cmp) return -1, 0, and 1 not negative, zero, and positive. I had always though it wasn't guaranteed what the return value was, but apparently it is.

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Perl 5.10.1 does not have the err operator (i.e. the low precedence version of //. I could have sworn it did, but it is not there. It looks like I am not the only one confused by its non-existence though, perlop has a section titled "Logical or, Defined or, and Exclusive Or" that only describes or and xor.

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The left bit shift operator (<<) is not defined if you you shift past the boundary of your native integer. I had always assumed that it dropped those bits, but apparently that is just the behavior of the versions of C I have used in the past (ANSI/ISO C does not define the behavior of overflow due to shifting, so it is a crap shoot).

---

The and and or operators have different precedence levels. The and operator binds more tightly than or, so

$x == 5 and $y == 6 or $x == 6 and $y == 5

is the same as

(($x == 5) and ($y == 6)) or (($x == 6) and ($y == 5))

What to call the ... operator?

What should we call Perl 5's ... operator? In list context it is identical to the range operator (..) and in scalar context it is almost identical to the flip-flop operator (also ..). The Camel and perlop are strangely silent on this. I am currently calling it the alternate range/flip-flop operator for lack of a better term. What do you call it?

project: Anouncing perlopref

In my last post I discussed the need for a perlopref document and even posted a portion of the version I am working on. I got some positive feedback and no negative feedback, so I have created a GitHub repository and uploaded what I currently have to it. There has already been one fork that fixed some of my POD stupidity.

If you want to help, just fork my repository, file an issue against my repository claiming an operator, make your change, and then submit a pull request. Note: not all of the operators are currently listed in the file (e.g. the filetest operators and the quote-like operators are not yet listed).

There is no guarantee that this document will make it into a release of Perl, but at the very least I will be creating a CPAN distribution for it.