返回字符串的子例程警告“无效上下文中常量的无用使用”。

Question

​

在一个返回文本的模块中，我有一个简单的函数blurb

package Il::NetApp::Dox::FlashCache;

use strict;
use warnings;
no if $] >= 5.018, warnings => "experimental::smartmatch";

use Carp;
use Carp::Heavy;
use Data::Dumper;

use FindBin qw($Bin);
use Il::SysMon::Tools 3.2.1 qw( :debug :special_chars :text_format :help_snippets);
use Il::NetApp::Tools qw( :help_snippets ); 

# ===========================================================================
# = Texte  - ab hier wird übersetzt =
# ===========================================================================

# ===========================================================================
# Markdown Syntax in blurb, extra und examples=>txt!
#
# Verwendbare Variablen in den Texten:
#
# $VERBOSE_HINT = Hinweis -v einzusetzen
#
# ===========================================================================

sub blurb {
q{Checks several metrics of NetApps FlashCache (PAM II).}; # Line 27
}

sub extra {
<<END_EXTRA,
This plugin checks various performance counters of the NetApp-system.

A list of supported counters is printed via the `--counter` switch.

$HELP_DISCOVER_COUNTERS
END_EXTRA
} 

#
# Examples: Hier ist jeweils nur txt => zu übersetzen
# 

sub simple_examples {
    my $examples = 
        [
            {
                cmd => q{--explore=counters},
                txt => q{List all available and supported counters on the target system.}
            },
            {
                cmd => q{-z hit_percent -w 0 -c 0},
                txt => q{Monitor the hitrate for trendanalyses but do not alarm.}
            },
        ]
    ; # Ende von my $examples =
    return $examples;
}

# sub advanced_examples {
#     my $examples = 
#         [
#             {
#                 cmd => q{},
#                 txt => q{}
#             },
#         ]
#     ; # Ende von my $examples =
#     return $examples;
# }


# ===========================================================================
# = ENDE der Texte - ab hier ist nichts mehr zu übersetzen =
# ===========================================================================

1;  # return true

在一台服务器上，我们偶尔会收到警告：

常量的无用使用(“检查若干度量标准”.)在空上下文中，在./lib/Il/NetApp/Dox/FlashCache.pm第27行。

如果Perl子例程是表达式，则返回执行的最后一条语句的值，而且这种技术以前是有效的。我无法重现Perl5.10.1或5.18.2版本的问题。

具有这些警告的站点正在运行Perl v5.16.3

# perl --version

This is perl 5, version 16, subversion 3 (v5.16.3) built for x86_64-linux-thread-multi
(with 33 registered patches, see perl -V for more detail)

这可能是特定Perl版本中的错误吗？

Answer 1

Void是这样一个上下文，在这个上下文中，没有任何东西可以使用返回的内容。

当使用一个空的原型定义子时，就会出现警告：

use warnings;
sub blurb () { q(some string) }
blurb();

您能显示NetApp/Dox/FlashCache.pm第27行吗？

Answer 2

哦，不，我们终于在一些客户服务器上找到了错误信息“常量的无用使用”的原因。这个问题是由配送链中的一个错误引入到客户的路上的。

我们发送的代码(例如，减少)：

#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";

say blurb();

say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();

sub blurb {
q{I am the blurb - print me wherever you want me to be shown.};
}


sub extra {
<<END;
Some extra documentation - spanning several lines including indents.

EXAMPLES:

    # so something
    do_something.pl -H <hostname>

Does something with hostname.

HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
}

实际上在客户服务器上运行并导致该错误的代码略有不同：

#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";

say blurb();

say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();

sub blurb {
q{I am the blurb - print me wherever you want me to be shown.};
my $status;
}


sub extra {
<<END;
Some extra documentation - spanning several lines including indents.

EXAMPLES:

    # so something
    do_something.pl -H <hostname>

Does something with hostname.

HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
}

产生错误消息的原因是blurb-sub中的附加行。

结论

除了修复分发链之外，通过将文本存储在变量中并在稍后返回该变量，或者省略最后一个分号，这些代码将变得更加健壮，后者将在编译时抛出一个明显的错误，并将节省调试时间。

#!/usr/local/bin/perl -w
use warnings;
use strict;
use feature ":5.10";

say blurb();

say "\n\n\nDESCRIPTION and EXAMPLES\n\n" . extra();

sub blurb {
q{I am the blurb - print me wherever you want me to be shown.}
}


sub extra {
    my $extra = 
<<END;
Some extra documentation - spanning several lines including indents.

EXAMPLES:

    # so something
    do_something.pl -H <hostname>

Does something with hostname.

HINT: Da also lag der Hund begraben (German saying, "that's where the dog is buried" translates to "there's the fly in the ointment")
END
return $extra;
}

对于第一个错误代码示例，很抱歉，谢谢您的评论！