block syntax difference causes "LocalJumpError: no block given (yield)"

Question

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)

Here is the answer... http://stackoverflow.com/questions/533008/what-is-the-difference-or-value-of-these-block-coding-styles-in-ruby/533040#533040 — Arup Rakshit, Sep 04 '13 at 20:47
here is the answer - http://stackoverflow.com/questions/2122380/using-do-block-vs-brackets?lq=1 — Arup Rakshit, Sep 04 '13 at 20:51
This is why you use parentheses, even though Ruby lets you choose. — Linuxios, Sep 04 '13 at 21:12

score 22 · Accepted Answer · edited Feb 24 '20 at 18:12

22

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.

edited Feb 24 '20 at 18:12

Alex Wayne

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answered Sep 04 '13 at 20:48

Sergio Tulentsev

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score 17 · Answer 2 · edited Sep 26 '17 at 07:03

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.

block syntax difference causes "LocalJumpError: no block given (yield)"

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