til/ruby/refer-to-implicit-block-argument-with-it.md

Refer To Implicit Block Argument With It

One of the key features of the Ruby 3.4 release is the it implicit block argument.

The vast majority of inline blocks defined in Ruby code receive a single block argument. Typically we name and reference a block argument explictly like so:

items.map { |item| item * item }

Ruby likes to cut away excess syntax when possible. To that end, the implicit it block argument has been added. This is an identifier we can reference in the context of a block and its value is the current

items = [1,2,3,4,5]

squares = items.map { it * it }

pp squares
#=> [1, 4, 9, 16, 25]

Note: we cannot mix numbered parameters (_1, _2) with the it parameter. If we do, we'll get the following error:

def method_using_block(a, b)
  yield(a, b) if block_given?
end

puts method_using_block(4,5) { _2 ** _1 } #=> 625
puts method_using_block(4,5) { _2 ** it }
# it_block.rb:12: syntax error found (SyntaxError)
#   10 |
#   11 | puts method_using_block(4,5) { _2 ** _1 }
# > 12 | ... it }
#      |     ^~ `it` is not allowed when a numbered parameter is already used

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