From: murphy <murphy@...>
Date: 2007-10-28T23:21:21+09:00
Subject: General hash keys for colon notation

Dear language designer(s) and parser wizards,

can we allow hash literals like {1:2, 3:4, 5:6} (for numbers, or even
better, for general keys)? I think that would be the best replacement
for the old list notation. my guess is that people are using the
following types of keys most (sorted by importance):
- Symbols
- Strings
- Special objects (not containers)
- Fixnums
- constants (eg. classes)
- (who uses containers for keys? yugh!)

I'd love Ruby 1.9 to allow the following:

Symbols:  { foo: 23, bar: 42 }  (already supported in 1.9)

Strings:  { 'foo': 23, 'bar': 42 }  (used a lot)

Numbers:  { 1:1, 3:9, 2:4 }  (shortcut, looks like a multiset)

Objects: typically given as variables. Tricky, because if foo is a local
variable, { foo: 3 } would be ambiguous. So we need parentheses:

  foo = Foo.new 313
  { default: 0, (foo): 'Hash me!', (Math::PI): 3.14 }

That would of course allow basically everything:

  { (/default/): 0, (()): (()), ({a:0}):0, ([1,2,3])::a,
    (ENV.keys.map(&:upcase)): :":", (foo): 7, (Foo): 7, foo: 7 }

but the classic => notation also allows this:

  { /default/ => 0, () => (()), {:a=>0} => 0, [1,2,3] => :a,
    ENV.keys.map(&:upcase) => :":", foo => 7, Foo: 7, :foo => 7 }


Another solution would be just to handle keys that look like identifiers
(local variables, methods and constants) special and interpret them as
Symbols, and make the colon an alternative to => for everything else.
you only have to use parentheses around local variables and constants then:

  { /default/: 0, (): (()), {a:0}:0, [1,2,3]::a,
    ENV.keys.map(&:upcase): :":", (foo): 7, (Foo): 7, foo: 7 }

(code golfers, cheer!)

Since the current implementation even allows => and : to be mixed in a
literal, it seems already to be smart enough to parse everything up to
the colon:

  SyntaxError: (irb):10: parse error, expecting `tASSOC'
  { {a: 3}: 3}
           ^

So, are there design or technical reasons not to allow general hask keys
in the new colon notation?

(or am I asking an old question that can be answered with a link?)
[murphy]