问题
This is possible in Ruby:
class SomeClass
def initialize(a, *b, c)
end
end
but this is not:
class SomeClass
def initialize(a, *b, c='anything here')
end
end
Why?
Edit: This question does NOT have an answer. In the answer linked, the first answer is:
The splat means "use up all of the remaining arguments" but then you provide an optional argument, so how could the interpreter know if the last argument is part of the "numbers" splat or the optional "options"?
Which is NOT a valid answer because according to this logic, Ruby would also not allow mandatory parameters after a splat.
回答1:
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: #<Proc:0xdeadbeefc00l42@(irb):15> }
来源：https://stackoverflow.com/questions/27051567/why-can-i-have-required-parameters-after-a-splat-in-ruby-but-not-optional-ones