Lambdas

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# lambda using the arrow syntax
hello_world = -> { 'Hello World!' }
hello_world[]
# 'Hello World!'

# lambda using the arrow syntax accepting 1 argument
hello_world = ->(name) { "Hello #{name}!" }
hello_world['Sven']
# "Hello Sven!"

the_thing = lambda do |magic, ohai, dere|
  puts "magic! #{magic}"
  puts "ohai #{dere}"
  puts "#{ohai} means hello"
end

the_thing.call(1, 2, 3)
# magic! 1
# ohai 3
# 2 means hello

the_thing.call(1, 2)
# ArgumentError: wrong number of arguments (2 for 3)

the_thing[1, 2, 3, 4]
# ArgumentError: wrong number of arguments (4 for 3)

You can also use -> to create and .() to call lambda

the_thing = ->(magic, ohai, dere) {
  puts "magic! #{magic}"
  puts "ohai #{dere}"
  puts "#{ohai} means hello"
}

the_thing.(1, 2, 3)
# => magic! 1
# => ohai 3
# => 2 means hello

Here you can see that a lambda is almost the same as a proc. However, there are several caveats:

The arity of a lambda’s arguments are enforced; passing the wrong number of arguments to a lambda, will raise an ArgumentError. They can still have default parameters, splat parameters, etc.
returning from within a lambda returns from the lambda, while returning from a proc returns out of the enclosing scope:

def try_proc
  x = Proc.new {
    return # Return from try_proc
  }
  x.call
  puts "After x.call" # this line is never reached
end

def try_lambda
  y = -> {
    return # return from y
  }
  y.call
  puts "After y.call" # this line is not skipped
end

try_proc # No output
try_lambda # Outputs "After y.call"

Found a mistake? Have a question or improvement idea? Let me know.

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