From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-19T23:13:52+09:00
Subject: Re: [RCR] nil for unassigned keys

On Tue, 19 Jul 2005, Simon Strandgaard wrote:

> On 7/19/05, Yukihiro Matsumoto <matz@ruby-lang.org> wrote:
>>     on Tue, 19 Jul 2005 20:40:47 +0900, Simon Strandgaard <neoneye@gmail.com> writes:
>>
>> |Sometimes I find myself writing  :key=>true,
>> |it would be nice if one didn't had to do assignment,
>> |so its value instead defaulted to nil.
>> |
>> |def test(hash={})
>> |       p hash
>> |end
>> |
>> |test('x', 'y'=>2, 'z')         #  {"x"=>nil, "y"=>2, "z"=>nil}
>>
>> In that case, how can we distinguish a hash key without value and
>> ordinal mandatory argument?
>
> Maybe I don't understand the mandatory argument thing?
> But can't you just count them somehow?

not when there are zero or more allowed, for instance

   def test(*args, &block)
     x = args.shift || 42
     hash = Hash === x ? x : (args.shift || {})
     y, z = hash.values_at 'x', 'z'
     p [x,y,z]
   end

which allows

   test()
   test('y' => 42)
   test('y' => 42, 'z' => 42.0)
   test('x as forty-two')
   test('x as forty-two', 'y' => 42)
   test('x as forty-two', 'y' => 42, 'z' => 42.0)

maybe it's possible - but it seems like kleeny star makes it pretty tough.

why not a little helper function to transform argument lists in the way you
desire?

   def h(*list)
     list.inject({}) do |hash,elem|
       hash.update(Hash === elem ? {elem => nil} : elem)
     end
   end

which at least allows

   test(h('x', 'z', 'y' => 42))

though not

   test(h('x', 'y' => 42, 'z'))

cheers.

-a
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