当我使用“given”时为什么会出现语法错误？

Question

I'm trying to run the following code:

foreach my $k (keys %rec) {
    #switch for watchlist figures
    given ($k) { #line 93

        # Code omitted

        when ("p") { #positive breakout
            if ($row{cls} > $rec{$k}) {                       
                $email .= "positive";   
            } # line 104            
        }                       
        when ("n") { #negative breakout
            if ($row{cls} < $rec{$k}) { #line 107

But I get a syntax error:

syntax error at check_watch.pl line 93, near ") {"
syntax error at check_watch.pl line 104, near "}"
Unterminated <> operator at check_watch.pl line 107.

Why?

Answer 1

given and when are only available when either use feature "switch"; or use v5.10; (or some later value) is in effect. Place one of those lines at the top of your source file.

Answer 2

It is because it is not on by default. Add the line:

use 5.10.1;

to the code

Answer 3

Because pre-5.10 versions of Perl 5 don't have the given or when keywords, you're allowed to define custom functions with those names, which of course would have different syntax. To avoid breaking backward-compatibility with programs that do that, given and when are only enabled if you specifically ask for them, by putting either

use 5.010;

or

use feature 'switch';

at the top of a lexical scope you want to use the keywords in. In addition, the semantics keep changing. For example, given was originally designed to use lexical $_ by default, but lexical $_ turns out to be a really poor fit for Perl 5, an issue they're still revising. At some point, given stopped lexicalizing $_ , but of course that's a backward-incompatible change. when is (mainly) designed to use ~~ , but that operator had very complicated semantics in 5.10; they've been revised once and there are plans to revise them again. (This is why the Perl 5 developers have decided to just mark all new features as 'experimental' when they're first included in a development release).

Because they are experimental, to use them without warnings you also need to include:

no if $] >= 5.017011, warnings => 'experimental::smartmatch';

or add a 5.18 dependency and use

no warnings 'experimental::smartmatch';

, or add a dependency on the experimental CPAN module and use

use experimental 'smartmatch';

.

Now, on the other hand, the Perl 5 developers (of course) need people to use given , when , and ~~ in non-production-critical (or thoroughly unit-tested!) code and give them feedback. So definitely do use them if you can.