import list

[7stud]

Dear Monks,

Can you point me to anything that documents a minus sign in an import list, e.g.:

use English qw( -no_match_vars );

Thanks.

[moritz]

The minus sign has no special syntactic meaning inside a quote-words qw(...) construct. It's just like ('-no_match_vars');.

Only the import routine in English.pm gives it a special meaning, there it excludes a group of variables from exporting. See also perlvar.

[Perlbotics]

I think it's mainly defined by pure convention as intended by the module author's idea of the modules interface. Seems, the module author's intention here was to let the argument resemble a command-line option switch.

Basically use Module qw(a b c d); is translated to BEGIN{ require Module; Module->import(qw(a b c d)); } (see use).

In Module::import(), the author has the freedom to do anything with the given list (after removing the module's name from the parameter list) like using it as-is ... @list = qw(a b c d) ... or interpreting it as a flattened hash ... %pairs = (a => 'b', c => 'd') ... or as a parameter list ... my($a,$b,$c,$d) = qw(a b c d) ... and so on. Then s/he can treat the arguments as needed, like filtering, grouping, normalizing, etc.

In this case: less `perldoc -l English`

...
# Grandfather $NAME import
sub import {
    my $this = shift;                              # 'English'
    my @list = grep { ! /^-no_match_vars$/ } @_ ;  # anything that is 
+not '-no_match_vars'
    local $Exporter::ExportLevel = 1; 
    if ( @_ == @list ) {
...

[tobyink]

It doesn't have any special meaning per se, but by convention the arguments passed to a module are normally a list of subroutines that you wish to import into your namespace. Thus if a module wants to allow you to specify other options, it makes things clearer if those arguments don't "look like" subroutine names. So you often see such options starting with characters that are not allowed in subroutine names, such as '-', '!', ':' or '+'.

The minus sign has an added advantage over other characters that due to fairly convoluted Perl parsing rules, the following:

use Foo "-bar";

... can be written as:

use Foo -bar;

... without violating use strict. In fact, you can even write it as:

use Foo-bar;

[chromatic]

perldoc perlop's Symbolic Unary Operators section explains this unary minus:

Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned.

In effect, it makes a bareword an unambiguous string constant. There's nothing special about this syntax when used in import lists (except that you'd have to write something like use English '-no_match_vars'; as an equivalent.)

[GrandFather]

Actually the - is just a character inside a qw() so it has no special meaning and no syntax beyond the fact that it isn't a white space character and isn't the closing character for the qw() applies.

[chromatic]

Yes, true.

[kcott]

Exporter - Specialised Import Lists shows non-alphanumeric characters which do have a special meaning in this position. The minus sign is not among them: it uses "!" for negation.