Perl provides three logical operators: &&, ||, and !, and they have the same meaning as in C.
Wiki Markup and}}, {{or}}, and {{not}}. They have the same meanings as {{&&}}, {{||}}, and {{\!}}. They have much lower binding precedence, which makes them useful for control flow \ [[Wall 2011|AA. Bibliography#Manpages]\]. They are called the late-precedence logical operators, whereas {{&&}}, {{||}}, and {{!}} are called the early-precedence logical operators.
It is possible to mix the early-precedence logical operators with the late-precedence logical operators, but this mixture of precedence will often lead leads to confusing, counterintuitive behavior. Therefore, every Perl expression should use either the early-precedence operators or the late-precedence ones, never both.
Wiki Markup not and and entirely and using or only in control-flow operations, as a failure mode [Conway 2005]:
| Code Block | ||
|---|---|---|
| ||
print $filehandle $data or croak("Can't write to file: $!");
|
...
This noncompliant code example checks a file to see if it is suitable for suitability as an output file. It does this by checking to see that the file does not exist.
| Code Block | ||
|---|---|---|
| ||
if (not -f $file) {
|
This code is perfectly fine. However, it is later amended to also work if the file does exist , but can be overwritten.
| Code Block | ||||
|---|---|---|---|---|
| ||||
if (not -f $file || -w $file) {
|
This code will not behave as expected , because the binding rules are lower for the not operator than for the ! operator. This code will instead behave like the followingInstead, this code behaves as follows:
| Code Block | ||
|---|---|---|
| ||
if (not (-f $file || -w $file)) {
|
when the maintainer really wanted:
| Code Block | ||
|---|---|---|
| ||
if ((not -f $file) || -w $file) {
|
...
This compliant solution uses the ! operator in conjunction with the || operator. This code has the desired behavior of determining if a file either does not exist , or does exist but is overwritable.
| Code Block | ||||
|---|---|---|---|---|
| ||||
if (! -f $file || -w $file) {
|
...
This compliant solution uses the early-precedence operators consistently. Again, the code works as expected.
| Code Block | ||||
|---|---|---|---|---|
| ||||
if (not -f $file or -w $file) {
|
...
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
EXP04-PL | low Low | unlikely Unlikely | low Low | P1 P3 | L3 |
Automated Detection
Tool | Diagnostic |
|---|---|
Perl::Critic | ValuesAndExpressions::ProhibitMixedBooleanOperators |
Bibliography
...
...
| [CPAN |
|---|
...
...
| Perl-Critic-1.116 ProhibitMixedBooleanOperators | |
|---|---|
| [Conway 2005] | "Low-Precedence Operators," p. 70 |
| [Wall 2011] | perlop |
...
|http://search.cpan.org/~elliotjs/Perl-Critic-1.116/]. [ProhibitMixedBooleanOperators|http://search.cpan.org/~elliotjs/Perl-Critic-1.112_001/lib/Perl/Critic/Policy/ValuesAndExpressions/ProhibitMixedBooleanOperators.pm].
\[[Conway 2005|AA. Bibliography#Conway 2005]\], pg 70
\[[Wall 2011|AA. Bibliography#Manpages]\] [perlop|http://perldoc.perl.org/perlop.html]
02. Expressions