Do not invoke a function in a context for which it is not defined

Perl functions can be invoked in two contexts: list and scalar. These contexts indicate what is to be done with the return value. Functions can return different values in list context than in scalar context. For instance, the grep() function takes a list and a block or expression, and filters out elements of the list for whom the block or expression evaluates to false. The grep() function returns the filtered list when called in list context, but when called in scalar context, it merely returns the size of this list. That is, it returns the number of elements for whom the block or expression evalues to true.

Some functions do not define what they return in list or scalar context. For instance, according to the {[perlfunc}} manpage, the sort() function "sorts the LIST and returns the sorted list value. In scalar context, the behaviour of "sort()" is undefined.".

Do not invoke a function in a context for which it is undefined.

Noncompliant Code Example (sort())

This noncompliant code example inadvertantly assigns a scalar to the result of the sort() function.

sub ret {
  my $list = shift;
  my @list = @{$list};
  # ...
  return sort @list;
}

my @list = ( "foo", "bar", "baz");
my $result = ret @list;

The contents of $result are undefined, because the sort() function's return value is not defined in a scalar context.

Compliant Solution (sort())

This compliant solution guarantees that the ret() function is only called in list context.

sub ret {
  my $list = shift;
  my @list = @{$list};
  # ...
  return sort @list;
}

my @list = ( "foo", "bar", "baz");
my @result = ret @list;

In this case, the @result array will contain the list {{{ "bar", "baz", "foo" }}}.

Risk Assessment

Using an unspecified value can lead to erratic program behavior.

Automated Detection

Bibliography

[Conway 2005]
[Manpages] perlfunc

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