block syntax difference causes “LocalJumpError: no block given (yield)” [duplicate]

问题

This question already has answers here:

Code block passed to each works with brackets but not with 'do'-'end' (ruby) (3 answers)

Closed 5 years ago.

Saw a strange case come up, trying to figure out what is happening here:

> def test
>   p yield
> end
=> nil
> test { 1 }
1
=> 1
> p test { 1 }
1
1
=> 1
> p test do
>   1
> end
LocalJumpError: no block given (yield)

回答1:

The parser recognizes this

p test do
  1
end

as this

(p test) do
  1
end

The block is passed to p, not test. Therefore, yield can't yield and raises that error.

回答2:

do and {} to denote blocks attached to methods are not completely interchangeable.

p test do
  1
end

Precedence is screwing with you. This is actually this:

p(test()) do
  1
end

So the block is getting passed to p, not test.

{} has higher precedence than do, and so binds more tightly to the syntactically closer method. This is also true for other ruby keywords that have symbolic equivalents, such as and/&& and or/||, which is why the symbols are usually recommended over the words.

来源：https://stackoverflow.com/questions/18623447/block-syntax-difference-causes-localjumperror-no-block-given-yield