为什么要映射的参数之间不需要逗号?

问题描述

以下为何有效?

my @ys = map { $_ * $_ } @xs;

以下内容无效吗?

my @ys = map { $_ * $_ }, @xs;

map是语言构造而不是真正的函数，还是块的特殊规则?

Is map a language construct and not really a function, or are there special rules for blocks?

推荐答案

map是列表运算符和核心函数.这是简单的Perl语法，在子程序的块参数之后不期望逗号. map的特殊之处在于它也可以采用map EXPR, LIST的形式.如果将其与标准子例程一起使用，则将仅对EXPR进行评估并将其作为第一个参数传递.

map is list operator and a core function. It is simple Perl syntax that expects no comma after a block parameter to a subroutine. The special thing about map is that it can also take the form map EXPR, LIST. If this was used with a standard subroutine the EXPR would just be evaluated and passed as the first parameter.

块参数对所有子例程均有效，并且在将原型应用于子例程定义时可以使用.例如，您可以定义一个mymap，其行为与编写时的行为相同

Block parameters are valid for all subroutines, and can be used if you apply prototypes to your subroutine definition. For instance, you could define a mymap that behaved just the same way by writing

use strict;
use warnings;
use 5.010;

sub mymap(&@) {
  use Data::Dump;
  my $sub = shift;
  my @newlist;
  push @newlist, $sub->($_) for @_;
  @newlist;
}

say for mymap { $_ * $_ } 1, 2, 3;

输出

1
4
9

但是一般来说，除非您确切地知道您在做什么，否则应该避免使用原型.通常，有一种更好的方式来编写代码.

But in general you should avoid prototypes unless you know exactly what you are doing. There is generally a better way to write your code.

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