kw27.rb

class Foo
  def to_hash
    1
  end
end
foo = Foo.new

r1kr1 = proc { |a, b:| [a, b] }

p r1kr1.call(1, b: 2)
begin
  r1kr1.call(1)
rescue # #<ArgumentError: missing keyword: b>
  p $!
end
begin
  r1kr1.call(1, foo)
rescue # #<TypeError: can't convert Foo to Hash (Foo#to_hash gives Fixnum)>
  p $!
end
p r1kr1.call(1, {b: 2}, 3, {b: 6})

r1ko1 = proc { |a, b: 2| [a, b] } 

p r1ko1.call(1, b: 3)
p r1ko1.call(1)

# If required or optional or rest kwargs we potentially can have one
# more parameter.