Ruby's argument binding semantics are already pretty 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]]
Can you tell at first glance what the result of the following invocations will be?
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
Now, imagine adding optional parameters with default arguments after the splat parameter to that list. It's not impossible to find sane semantics for that, but it may lead to some non-obvious results.
Can you come up with simple, sane, backwards-compatible, and non-surprising semantics?
BTW: here's the cheatsheet for the method at the top:
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: # }
