强制成为标量

发布于 2024-12-11 13:49:32 字数 211 浏览 0 评论 0原文

#!/usr/bin/perl
use Modern::Perl;
while (<>)
{ chomp;
say reverse;  
}

上面的代码不起作用，但是当我将倒数第二行更改为 say scalar reverse; 时，它就可以正常工作。为什么我需要显式地强制它成为标量？ Perl 不能 DWIM 吗？

原文

#!/usr/bin/perl
use Modern::Perl;
while (<>)
{ chomp;
say reverse;  
}

The above code doesn't work but when I change 2nd last line to say scalar reverse; then it works fine. Why do I need to force it to be a scalar explicitly? Can't Perl DWIM?

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佼人 2024-12-18 13:49:32

如果我对文档的理解正确的话，reverse 通常在列表上运行。在不带参数的列表上下文中，它返回一个空列表，并且默认情况下不会将其分配到任何地方。在您的示例中，假设输出未更改的 $_;

强制 reverse 进入标量上下文会更改其行为并使其反转字符串，并默认使用 $_。由于 say 可用于打印列表和标量，因此它不会强制将其参数放入标量上下文中。

Perl 可能会执行 DWIM，只是针对给定的“I”值。

详细了解反向操作在以下情况下的作用：

#!/usr/bin/env perl
use strict;
use v5.12;

my $inscalar = "foo bar";

# `reverse` is in list context, $in is coerced to a single-element list
my @outlist = reverse $inscalar;

# `reverse` is in scalar context, so it reverses strings
my $outscalar = reverse $inscalar;

say join(" | ", @outlist); # prints "foo bar"
say $outscalar; # prints "rab oof"

# the "normal" behaviour of `reverse`
my @inlist = qw(foo bar);
@outlist = reverse @inlist;
say join(" | ", @outlist); # prints "bar | foo"

If I understand the documentation right, reverse normally operates on lists. In a list context used without arguments, it returns an empty list and by default doesn't assign it anywhere. In your example, say outputs the unchanged $_;

Forcing reverse into scalar context changes its behaviour and makes it reverse character strings, and use $_ by default. Because say can be used to print lists as well as scalars, it doesn't force its arguments into scalar context.

Perl probably does DWIM, just for given values of "I".

A breakdown of what reverse does when:

#!/usr/bin/env perl
use strict;
use v5.12;

my $inscalar = "foo bar";

# `reverse` is in list context, $in is coerced to a single-element list
my @outlist = reverse $inscalar;

# `reverse` is in scalar context, so it reverses strings
my $outscalar = reverse $inscalar;

say join(" | ", @outlist); # prints "foo bar"
say $outscalar; # prints "rab oof"

# the "normal" behaviour of `reverse`
my @inlist = qw(foo bar);
@outlist = reverse @inlist;
say join(" | ", @outlist); # prints "bar | foo"

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