为什么Perl功能“映射”给出错误“地图的参数不足” [英] Why does Perl function "map" give the error "Not enough arguments for map"

问题描述

这是我不明白的东西。

此脚本正常工作（请注意地图中的拼接功能）：

This script works correctly (notice the concatenation in the map functin):

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %aa = map { 'a' . '' => 1 } (1..3);

print Dumper \%aa;

__END__
output:

$VAR1 = {
          'a' => 1
        };

但是没有并置，地图不起作用。这是我期望工作的脚本，但它不是：

But without concatenation the map does not work. Here is the script I expect to work, but it does not:

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my %aa = map { 'a' => 1 } (1..3);

print Dumper \%aa;
__END__
output:

Not enough arguments for map at e.pl line 7, near "} ("
syntax error at e.pl line 7, near "} ("
Global symbol "%aa" requires explicit package name at e.pl line 9.
Execution of e.pl aborted due to compilation errors.

你能解释一下这种行为吗？

Can you please explain such behaviour?

推荐答案

Perl使用启发式方法来决定是否使用：

Perl uses heuristics to decide whether you're using:

map { STATEMENTS } LIST;   # or
map EXPR, LIST;

因为虽然{通常是块的开始，但也可能是开始一个hashref。

Because although "{" is often the start of a block, it might also be the start of a hashref.

这些启发式在令牌流（IIRC两个令牌）中看起来并不遥远。

These heuristics don't look ahead very far in the token stream (IIRC two tokens).

您可以使用以下方式强制{被解释为块：

You can force "{" to be interpreted as a block using:

map {; STATEMENTS } LIST;    # the semicolon acts as a disambigator

你可以强制{被解释为啊使用：

You can force "{" to be interpreted as a hash using:

map +{ LIST }, LIST;    # the plus sign acts as a disambigator

grep 同样受苦。 （技术上也是这样， do ，因为一个hashref可以作为一个参数给出，然后将其作为一个文件名进行字符串处理，这只是奇怪的。

grep suffers similarly. (Technically so does do, in that a hashref can be given as an argument, which will then be stringified and treated as if it were a filename. That's just weird though.)

这篇关于为什么Perl功能“映射”给出错误“地图的参数不足”的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！