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2017-07-11
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Blocks and Procs and Lambdas

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Syntax

  • Proc.new(block)
  • lambda { |args| code }
  • ->(arg1, arg2) { code }
  • object.to_proc
  • { |single_arg| code }
  • do |arg, (key, value)| code end

Remarks

Be careful about operator precedence when you have a line with multiple methods chained, like:

str = "abcdefg"
puts str.gsub(/./) do |match|
  rand(2).zero? ? match.upcase : match.downcase
end

Instead of printing something like abCDeFg, like you'd expect, it prints something like #<Enumerator:0x00000000af42b28> -- this is because do ... end has lower precedence than methods, which means that gsub only sees the /./ argument, and not the block argument. It returns an enumerator. The block ends up passed to puts, which ignores it and just displays the result of gsub(/./).

To fix this, either wrap the gsub call in parentheses or use { ... } instead.

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