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Why am I getting a syntax error when I use "given"?

I am 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 am getting 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?

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3 answers


given

and when

are available only when either use feature "switch";

or use v5.10;

(or some later meaning) is in effect. Place one of these lines at the top of your source file.



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This is because it is not enabled by default. Add the line:

use 5.10.1;


to the code

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Since Perl 5 version 5.10 does not have keywords given

or when

, you are allowed to define UDFs with those names, which will of course have a different syntax. To avoid breaking backward compatibility with programs that do this given

and when

are only activated if you specifically request them by adding either

use 5.010;

or

use feature 'switch';

at the top of the lexical area in which you want to use keywords. Also, the semantics keep changing. For example, given

it was originally intended to use the $_

default lexical , but the lexical $_

turned out to be very bad for Perl 5, an issue they are still revising. At some point, given

stopped lexicalization $_

, but, of course, this is an incompatible change with the reverse change. when

(mostly) intended to be used ~~

, but this operator had very complex semantics in 5.10; they were revised once and there are plans to revise them again. (This is why the Perl 5 developers decided to simply mark all new features as "experimental" when they were first included in a development release).

Since they are experimental, to use them without warning, you also need to enable:

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

or add dependency 5.18 and use

no warnings 'experimental::smartmatch';

or add a dependency on the CPAN module experimental

and use

use experimental 'smartmatch';

...

Now, on the other hand, Perl 5 developers (naturally) need people to use given

, when

and ~~

in non-production critical (or fully unit testing!) Code and give them feedback. So be sure to use them if you can.

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