What is the behavior when using both positional and keyword arguments in Ruby?

Question

Ruby 2.0 supports keyword arguments. I was wondering, what are the 'rules' for mixing regular with keyword arguments? Something like this would not work:

def some_method(a: 'first', b: 'second', c)
  [a, b, c]
end

but this will:

def some_method(c, a: 'first', b: 'second')
  [a, b, c]
end

So why does putting a positional argument before the keyword arguments (and not after) work?

Is there some reference on the web on this (mixing keyword and positional arguments)? I haven't found any.

Answer 1

1. Arguments with defaults and splat argument must be grouped together;
2. Splat argument must appear after positional arguments with default values but before keyword arguments;
3. Keyword arguments must appear after positional arguments and before double splat argument;

4. Double splat argument must appear last but before block argument.

   def foo(a, b=1, c=2, *d, e, f: 1, g: 2, **kwargs, &block)

Answer 2

The order is as follows:

• required arguments
• arguments with default values (arg=default_value notation)
• optional arguments (*args notation, sometimes called "splat parameter")
• required arguments, again
• keyword arguments
  • optional (arg:default_value notation, since 2.0.0)
  • intermixed with required (arg: notation, since 2.1.0)
• arbitrary keyword arguments (**args notation, since 2.0.0)
• block argument (&blk notation)

For example:

def test(a, b=0, *c, d, e:1, f:, **g, &blk)
  puts "a = #{a}"
  puts "b = #{b}"
  puts "c = #{c}"
  puts "d = #{d}"
  puts "e = #{e}"
  puts "f = #{f}"
  puts "g = #{g}"
  puts "blk = #{blk}"
end

test(1, 2, 3, 4, 5, e:6, f:7, foo:'bar') { puts 'foo' }
# a = 1
# b = 2
# c = [3, 4]
# d = 5
# e = 6
# f = 7
# g = {:foo=>"bar"}
# blk = #<Proc:0x007fb818ba3808@(irb):24>

More detailed information is available from the official Ruby Syntax Documentation.

Answer 3

A pseudo-regex for parameter lists in Ruby (this applies equally to methods, blocks and lambda literals) is something like this:

mand* opt* splat? mand* (mand_kw | opt_kw)* ksplat? block?

Here's an example:

def foo(m1, m2, o1=:o1, o2=:o2, *splat, m3, m4, 
          ok1: :ok1, mk1:, mk2:, ok2: :ok2, **ksplat, &blk)
  Hash[local_variables.map {|var| [var, eval(var.to_s)] }]
end

method(:foo).arity
# => -5

method(:foo).parameters
# => [[:req, :m1], [:req, :m2], [:opt, :o1], [:opt, :o2], [:rest, :splat], 
#     [:req, :m3], [:req, :m4], [:keyreq, :mk1], [:keyreq, :mk2], 
#     [:key, :ok1], [:key, :ok2], [:keyrest, :ksplat], [:block, :blk]]

foo(1, 2, 3, 4)
# ArgumentError: missing keywords: mk1, mk2

foo(1, 2, 3, mk1: 4, mk2: 5)
# ArgumentError: wrong number of arguments (3 for 4+)

foo(1, 2, 3, 4, mk1: 5, mk2: 6)
# => { m1: 1, m2: 2, o1: :o1, o2: :o2, splat: [], m3: 3, m4: 4, 
#      ok1: :ok1, mk1: 5, mk2: 6, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, mk1: 6, mk2: 7)
# => { m1: 1, m2: 2, o1: 3, o2: :o2, splat: [], m3: 4, m4: 5, 
#      ok1: :ok1, mk1: 6, mk2: 7, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, mk1: 7, mk2: 8)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [], m3: 5, m4: 6, 
#      ok1: :ok1, mk1: 7, mk2: 8, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, mk1: 8, mk2: 9)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5], m3: 6, m4: 7, 
#      ok1: :ok1, mk1: 8, mk2: 9, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, mk1: 9, mk2: 10)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: :ok1, mk1: 9, mk2: 10, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: 9, mk1: 10, mk2: 11, ok2: :ok2, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11, ok2: 12)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: 9, mk1: 10, mk2: 11, ok2: 12, ksplat: {}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11, ok2: 12, k3: 13)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: 9, mk1: 10, mk2: 11, ok2: 12, ksplat: {k3: 13}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, 
      ok1: 9, mk1: 10, mk2: 11, ok2: 12, k3: 13, k4: 14)
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: 9, mk1: 10, mk2: 11, ok2: 12, ksplat: {k3: 13, k4: 14}, 
#      blk: nil }

foo(1, 2, 3, 4, 5, 6, 7, 8, 
      ok1: 9, ok2: 10, mk1: 11, mk2: 12, k3: 13, k4: 14) do 15 end
# => { m1: 1, m2: 2, o1: 3, o2: 4, splat: [5, 6], m3: 7, m4: 8, 
#      ok1: 9, mk1: 10, mk2: 11, ok2: 12, ksplat: {k3: 13, k4: 14}, 
#      blk: #<Proc:0xdeadbeefc00l42@(irb):15> }

[Note: mandatory keyword arguments will be introduced in Ruby 2.1, all the rest already works.]