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Perl also provides three alternative logical operators: 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]. 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 often leads to confusing, counterintuitive behavior. Therefore, every Perl expression should use either the early-precedence operators or the late-precedence ones, never both.
[Damian Conway 2005] recommends avoiding the use of not and and entirely and using or only in control-flow operations, as a failure mode [Conway 2005]:
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print $filehandle $data or croak("Can't write to file: $!");
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This noncompliant code example checks a file for suitability as an output file. It does this by checking to see that the file does not exist.
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if (not -f $file) {
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This code is perfectly fine. However, it is later amended to also work if the file does exist but can be overwritten.
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if (not -f $file || -w $file) {
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This code will not behave as expected because the binding rules are lower for the not operator than for the ! operator. Instead, this code behaves as follows:
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if (not (-f $file || -w $file)) {
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when the maintainer really wanted:
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if ((not -f $file) || -w $file) {
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if (! -f $file || -w $file) {
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if (not -f $file or -w $file) {
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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
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