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

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

Noncompliant Code Example

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.

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. Instead, this code behaves as follows:

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