Splats and tuples

A method can receive a variable number of arguments by using a splat parameter (*), which can appear only once and in any position:

def sum(*elements)
  total = 0
  elements.each do |value|
    total += value
  end
  total
end

sum 1, 2, 3      # => 6
sum 1, 2, 3, 4.5 # => 10.5

The passed arguments become a Tuple in the method's body:

# elements is Tuple(Int32, Int32, Int32)
sum 1, 2, 3

# elements is Tuple(Int32, Int32, Int32, Float64)
sum 1, 2, 3, 4.5

Arguments past the splat parameter can only be passed as named arguments:

def sum(*elements, initial = 0)
  total = initial
  elements.each do |value|
    total += value
  end
  total
end

sum 1, 2, 3              # => 6
sum 1, 2, 3, initial: 10 # => 16

Parameters past the splat parameter without a default value are required named parameters:

def sum(*elements, initial)
  total = initial
  elements.each do |value|
    total += value
  end
  total
end

sum 1, 2, 3              # Error, missing argument: initial
sum 1, 2, 3, initial: 10 # => 16

Two methods with different required named parameters overload between each other:

def foo(*elements, x)
  1
end

def foo(*elements, y)
  2
end

foo x: "something" # => 1
foo y: "something" # => 2

The splat parameter can also be left unnamed, with the meaning "after this, named parameters follow":

def foo(x, y, *, z)
end

foo 1, 2, 3    # Error, wrong number of arguments (given 3, expected 2)
foo 1, 2       # Error, missing argument: z
foo 1, 2, z: 3 # OK

Splatting a tuple

A Tuple can be splat into a method call by using *:

def foo(x, y)
  x + y
end

tuple = {1, 2}
foo *tuple # => 3

Double splats and named tuples

A double splat (**) captures named arguments that were not matched by other parameters. The type of the parameter is a NamedTuple:

def foo(x, **other)
  # Return the captured named arguments as a NamedTuple
  other
end

foo 1, y: 2, z: 3    # => {y: 2, z: 3}
foo y: 2, x: 1, z: 3 # => {y: 2, z: 3}

Double splatting a named tuple

A NamedTuple can be splat into a method call by using **:

def foo(x, y)
  x - y
end

tuple = {y: 3, x: 10}
foo **tuple # => 7