Why do I have the required parameters after splat in Ruby, but not as optional?

Question

This is possible in Ruby:

class SomeClass
  def initialize(a, *b, c)
  end
end

but this is not so:

class SomeClass
  def initialize(a, *b, c='anything here')
  end
end

Why?

Edit: This question has no answer. In the response, the first answer:

The splat symbol means “use all other arguments”, but then you specify an optional argument, and how could the interpreter know if the last argument is part of the “number” symbol or the optional “options”?

This is NOT the correct answer because, according to this logic, Ruby would also not allow the required parameters after splat.

Answer 1

Ruby's argument binding semantics are already quite complex. Consider this method:

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

foo(1, 2, 3, mk1: 4, mk2: 5)

foo(1, 2, 3, 4, mk1: 5, mk2: 6)

foo(1, 2, 3, 4, 5, mk1: 6, mk2: 7)

foo(1, 2, 3, 4, 5, 6, mk1: 7, mk2: 8)

foo(1, 2, 3, 4, 5, 6, 7, mk1: 8, mk2: 9)

foo(1, 2, 3, 4, 5, 6, 7, 8, mk1: 9, mk2: 10)

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11)

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11, ok2: 12)

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11, ok2: 12, k3: 13)

foo(1, 2, 3, 4, 5, 6, 7, 8, ok1: 9, mk1: 10, mk2: 11, ok2: 12, k3: 13, k4: 14)

foo(1, 2, 3, 4, 5, 6, 7, 8, 
      ok1: 9, ok2: 10, mk1: 11, mk2: 12, k3: 13, k4: 14) do 15 end

splat . , .

, , ?

: - :

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> }