But it's more important whether a language can do something nicely, which I don't necessarily claim Lisp can. After all, Turing machines can do all that, too. :)

would it collide if other people did something else with their braces?

My guess is: yes, they'd probably clash to no end. I'm not familiar with Common Lisp "in the wild", so I don't really know. http://www.faqs.org/faqs/lisp-faq/part2/section-18.html seems to back me up a little.

You could hypothetically package the reader changes into their own modules so that you'd need to explicitly say which braces you're using, modules keep their changes local, etc. Anyone know of work done on this front?

Yes it's like python's op-overload but a bit better because of __index and __newindex.

So are __index and __newindex roughly like in the following (silly) Python code?

  $ python
  Python 2.5.4 (r254:67916, Jan 24 2010, 12:18:00)
  [GCC 4.4.3] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> class foo:
  ...     def __init__(self, x):
  ...         self.x = x
  ...     def __getitem__(self, index):
  ...         return self.x + index
  ...     def __setitem__(self, index, value):
  ...         self.y = index + value
  ...
  >>> bar = foo(5)
  >>> bar[10]
  15
  >>> bar[20] = 5
  >>> bar.y
  25

Hmm, giving the job of custom readers to modules strikes me as making implementing both modules and custom readers complicated and difficult; and makes unnecessary impositions on the programmer: I might well want to implement one part of my module using one syntax and another part of my module using a different syntax.

On the other hand I think being able to choose your custom reader by source code file would simple to use, simple to understand, easy to implement, would make it easy to customize your editor or IDE to understand your different syntaxes by file, and means that modules can also be easily implemented as they could do their work entirely after the source code has been read in.

I guess I use the words "module" and "namespace" interchangeably. I'm not sure what else "module" ("package", "namespace", sometimes "library" (loosely), etc.) would mean.

What I meant was that, say, you have two files:

a.arc

  (use "braces-for-table.arc")

  (def table-I-really-want ()
    {foo 1 bar 2 baz 3})

  (use "braces-for-set-builder-notation.arc")
  (load "a.arc")

  (def set-I-really-want ()
    { (* v v) for v in (vals (table-I-really-want)) })

With a fancier module system, you might even have

b.arc

  (qualify "braces-for-set-builder-notation.arc" s)
  (load "a.arc") ; doesn't qualify its braces for tables

  (def set-I-really-want ()
    s{ (* v v) for v in (vals (table-I-really-want)) })

  (def another-table-I-want ()
    {foo 5 bar 10 baz 15})

c.arc

  (use "braces-for-table.arc")

  (def some-table ()
    {x 5 y 10})

  (use "braces-for-set-builder-notation.arc")
  ; braces hereafter are for set-builder notation

  (def some-set ()
    { x for x in (vals sig) })

  arc> (load "sscontract.arc") ; only really needs to provide sscontract
  nil
  arc> (priority #\!) ; but the user gets all of the implementation details!
  1

Even some simple public/private control would be nice. I've played around with this in Arc before. E.g.,

  (let localize nil

    (assign localize
            (fn (expr fs)
              (when (caris expr 'def)
                (if (~mem (cadr expr) fs)
                    `(assign ,(cadr expr) (fn ,@(cddr expr)))
                    expr))))

    (mac provide (fs . body)
      (let private (trues [and (caris _ 'def)
                               (~mem (cadr _) fs)
                               (cadr _)]
                          body)
        `(let ,private nil
           ,@(map [localize _ fs] body))))
    )

  arc> (macex1 '(provide (f g)
                  (def f (x) (h (+ x 1)))
                  (def g (x) (+ (h x) 1))
                  (def h (x) (* x x))))
  (let (h) nil
    (def f (x) (h (+ x 1)))
    (def g (x) (+ (h x) 1))
    (assign h (fn (x) (* x x))))

  arc> (def foo () (prn "outer foo") nil)
  #<procedure: foo>
  arc> (provide (bar)
         (def foo () (prn "inner foo") nil)
         (def bar () (prn "bar") (foo)))
  #<procedure: bar> ; note: no redef warning, since def changed to assign
  arc> (bar)
  bar
  inner foo
  nil
  arc> (foo)
  outer foo
  nil

By "namespace", I was thinking specifically of MzScheme namespaces, which is where global variables live.

    Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
    > t_mt =  { __index = function(self, k)
    >>         return self.x + k
    >>     end,
    >>     __newindex = function(self, k, v)
    >>         self.y = k + v
    >>     end }
    > 
    > function foo(x)
    >>     return setmetatable({x = x, y = 0}, t_mt)
    >> end
    > 
    > bar = foo(5)
    > print(bar[10]) --> 15
    15
    > 
    > bar[20] = 5
    > print(bar.y) --> 25
    25

  return setmetatable({x = x, y = 0}, t_mt)