block-syntaxes-have-different-precedence.md

Block Syntaxes Have Different Precedence

There are two syntaxes for defining a block in Ruby. The semantically shorthand syntax uses the curly braces ({}). The semantically multi-line syntax uses do and end. For nearly all intents and purposes they are interchangable.

It is, however, worth noting that the do/end version has a lower precedence than the already low precedence of {}. That said, you have to write some weird code for this to become an issue.

Let's say we have two methods, method_one and method_two. They are both called on the same line like below and then followed by a block argument. Which method receives the block argument?

method_one method_two { |n|
  puts "Executing a block: #{n}"
}

method_one method_two do |n|
  puts "Executing a block: #{n}"
end

In the first case, with the curly braces, method_two receives the block as an argument. In the second case, with the do/end, method_one receives the block as an argument.

source