块定义 - 大括号和 do-end 之间的区别？

发布于 2024-11-10 16:00:28 字数 735 浏览 1 评论 0原文

任何人都可以解释为什么以下代码会因错误而中止，

irb(main):186:0> print ((1..10).collect do |x| x**2 end)
SyntaxError: (irb):186: syntax error, unexpected keyword_do_block,
expecting ')'
print ((1..10).collect do |x| x**2 end)
                         ^
(irb):186: syntax error, unexpected keyword_end, expecting $end
print ((1..10).collect do |x| x**2 end)
                                      ^
        from /usr/bin/irb:12:in `<main>'

而以下具有相同功能的代码会按预期工作？

irb(main):187:0> print ((1..10).collect { |x| x**2 })
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]=> nil

我确实相信大括号“{}”可以在块中任意替换“do end” 定义。

我知道我可以通过省略 print 之间的空格来“修复”第一个调用方法和第一个括号，但我对解释感兴趣 为什么它的行为不同。

原文

can anybody explain why the following code aborts with an error

irb(main):186:0> print ((1..10).collect do |x| x**2 end)
SyntaxError: (irb):186: syntax error, unexpected keyword_do_block,
expecting ')'
print ((1..10).collect do |x| x**2 end)
                         ^
(irb):186: syntax error, unexpected keyword_end, expecting $end
print ((1..10).collect do |x| x**2 end)
                                      ^
        from /usr/bin/irb:12:in `<main>'

whereas following code with the same functionality works as expected ?

irb(main):187:0> print ((1..10).collect { |x| x**2 })
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]=> nil

I did believed curly-braces "{ }" can substitute "do end" arbitrarily at block
definition.

I know I can "fix" the first call by omitting a space between print
method and the first parenthesis, but I'm interested in an explanation
why it behaves different.

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顾挽 2024-11-17 16:00:28

区别在于优先级：

# Equivalent to puts( (1..10).map { |i| i*2 } )
> puts (1..10).map { |i| i*2 }
2
4
6
8
10
12
14
16
18
20
 => nil 

# Equivalent to puts( (1..10).map ) { |i| i*2 }
> puts (1..10).map do |i| i*2 end
#<Enumerator:0x928f24>
 => nil

在第一种情况下，块被传递给 map，并且一切正常。
在第二种情况下，该块被传递给 puts，它不会对其执行任何操作。 map 不接收块，只返回一个枚举器。

至于语法错误，如果删除 print 和 ( 一切正常;) 之间的空格，

区别在于 ruby 是否将括号视为方法参数分隔符，或者是否将其视为方法参数分隔符。通用语句分组。我不确定其中的确切区别，但它很微妙且烦人

The difference is precedence:

# Equivalent to puts( (1..10).map { |i| i*2 } )
> puts (1..10).map { |i| i*2 }
2
4
6
8
10
12
14
16
18
20
 => nil 

# Equivalent to puts( (1..10).map ) { |i| i*2 }
> puts (1..10).map do |i| i*2 end
#<Enumerator:0x928f24>
 => nil

In the first case, the block is passed to map, and everything works properly.
In the second case, the block is passed to puts, which doesn't do anything with it. map doesn't receive a block and just returns an enumerator.

As for the syntax error, if you remove the space between print and ( everything works ;)

The difference is whether ruby is treating your parentheses as method argument delimiters, or whether it's a generic statement grouping. I'm not sure of the exact difference there but it's subtle and annoying

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