EXP34-PL. Do not modify $_ in list or sorting functions

Perl provides several functions for list manipulation. For instance, the map() function takes an expression or block, applies it to each element in a list, and returns the list of mapped elements. If it is given a block, the block is executed with $_ assigned to each element of the list in turn. The perlfunc manpage adds:

Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regular "foreach" loop for this purpose would be clearer in most cases.

While supported, using map() to modify a list in-place can lead to surprises in maintainability, and is thus forbidden.

Many other list functions provide similar functionality, using a block on various list elements. The grep() function is one such example, as are the first() and reduce() functions in List::Util and all of the functions in List::MoreUtils.

Finally, the sort() function also provides aliases to its comparison blocks, so a comparison block for sort() must also not modify its variables.

Noncompliant Code Example (grep())

This noncompliant code example reads the /etc/passwd file, and lists each user that uses /bin/sh as their login shell.

open( PASSWD, "<", "/etc/passwd");
@users = <PASSWD>;
@shell_users = grep +(s|/bin/sh||), @users;
foreach $user (@shell_users) {
  print "Shell User: $user";
}

However, since the grep() block removes /bin/sh from any input line that contains it, it modifies the @users list so that no user has /bin/sh!

Compliant Solution grep()

This compliant colution does the same thing, but does not modify the @users array.

open( PASSWD, "<", "/etc/passwd");
@users = <PASSWD>;
@shell_users = grep +(m|/bin/sh|), @users;
foreach $user (@shell_users) {
  $user ~= s|/bin/sh||;
  print "Shell User: $user";
}

If any error occurs, the program calls the croak() function, passing it a string that includes both the source file being opened, and the $! variable, which contains a system error string based on the value of errno, which is set to a useful value when the open(2) or close(2) functions fail.

Risk Assessment

Failure to handle error codes or other values returned by functions can lead to incorrect program flow and violations of data integrity.

Automated Detection

Bibliography

[Conway 05] pg. 114 "List Processing Side Effects"
[Wall 2011] perlfunc
[CPAN] Bar, Graham. List::Utils
[CPAN] Kennedy, Adam
List::MoreUtils

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