...
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
...
...
].
...
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]:
| Code Block | ||
|---|---|---|
| ||
\[[Conway 2005|AA. Bibliography#Conway 2005]\] recommends avoiding the use of {{not}} and {{and}} entirely, and only using {{or}} in control-flow operations, as a failure mode: {code|lang=perl} print $filehandle $data or croak("Can't write to file: $!"); {code} h2. 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. {code| |
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.
| Code Block | ||
|---|---|---|
| ||
lang=perl} if (not -f $file) { {code} |
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) { {code} |
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:
| Code Block | ||
|---|---|---|
| ||
This code will instead behave like the following: {code|lang=perl} if (not (-f $file || -w $file)) { {code} |
when
...
the
...
maintainer
...
really
...
wanted:
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} if ((not -f $file) || -w $file) { {code} h2 |
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.
| Code Block | ||||
|---|---|---|---|---|
| ||||
. 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. {code:bgColor=#ccccff|lang=perl} if (! -f $file || -w $file) { {code} h2. Compliant Solution This compliant solution uses the |
Compliant Solution
This compliant solution uses the early-precedence
...
operators
...
consistently.
...
Again
...
, the
...
code
...
works
...
as
...
expected.
| Code Block | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| =
| |
| =
| |||||||
} if (not -f $file or -w $file) { {code} h2. Risk Assessment Mixing |
Risk Assessment
Mixing early-precedence
...
operators
...
with
...
late-precedence
...
operators
...
can
...
produce
...
code
...
with
...
unexpected
...
behavior.
...
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
EXP04-PL | Low | Unlikely | Low | P3 | L3 |
Automated Detection
Tool | Diagnostic |
|---|---|
Perl::Critic | ValuesAndExpressions::ProhibitMixedBooleanOperators |
Bibliography
...
| ProhibitMixedBooleanOperators | |
|---|---|
| [Conway 2005] | "Low-Precedence Operators," p. 70 |
| [Wall 2011] | perlop |
...