Blocks and Procs and Lambdas

Versions

[{“Name”:“2.0”,“GroupName”:null},{“Name”:“2.1”,“GroupName”:null},{“Name”:“2.2”,“GroupName”:null},{“Name”:“2.3”,“GroupName”:null}]

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.