Mixing keyword with regular arguments in Ruby?

问题:

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 regular argument before the keyword arguments (and not after) works?

Is there some reference on the web on this (mixing keyword and regular arguments)? I can't seem to find any.

回答1:

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.]

回答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.

回答3:

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)

回答4:

Keyword arguments is just an optional hash argument, as I know.

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

is same as

def some_method(c, { a: 'first', b: 'second' }) // will not compile actually

Ruby just interprets last arguments with hash syntax as hash. So the first example doesn't work because ruby can't understand such syntax (has syntax at first place).