EXP04-PL. Do not mix the early-precedence logical operators with late-precedence logical operators

Perl provides three logical operators: &&, ||, and !, and they have the same meaning as in C.

Perl also provides three alternate 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 will often lead to confusing, counterintuitive behavior. Therefore, every Perl expression should use either the early-precedence operators or the late-precedence ones, never both.

[Conway 2005] recommends avoiding the use of not and and entirely, and only using or in control-flow operations, as a failure mode:

    print $filehandle $data    or croak("Can't write to file: $!");

Noncompliant Code Example

This noncompliant code example checks a file to see if it is suitable as an output file. It does this by checking to see that the file does not exist.

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.

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 following:

if (not (-f $file || -w $file)) {

when the maintainer really wanted:

if ((not -f $file) || -w $file) {

Compliant Solution

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.

if (! -f $file || -w $file) {

Compliant Solution

This compliant solution uses the early-precedence operators consistently. Again the code works as expected.

if (not -f $file or -w $file) {

Risk Assessment

Mixing early-precedence operators with late-precedence operators can produce code with unexpected behavior.

Automated Detection

Bibliography

[CPAN]. Elliot Shank, Perl-Critic-1.116. ProhibitMixedBooleanOperators.
[Conway 2005], pg 70
[Wall 2011] perlop

EXP11-C. Do not apply operators expecting one type to data of an incompatible type      03. Expressions (EXP)      EXP13-C. Treat relational and equality operators as if they were nonassociative