Re: Injecting names & method chaining (was Re: [ANN] Penlight Libraries, First release)

[David Manura <dm.lua@...>]

On Tue, May 5, 2009 at 2:20 PM, Petite Abeille wrote:
> Well, Objective-C had the concept of "category" for the longest time,
> and it hasn't collapse on its own weight yet.

That [1] looks like fun ;)  The run-time check
"OBJC_PRINT_REPLACED_METHODS" apparently offers some help though,
perhaps somewhat similar to the "name conflict for module" error in
Lua's module function (loadlib.c).

I think it would be ideal if we could inject only lexically (the
injection limited to the lexical scope).  With discipline, the wrapper
allows something like that:

On Mon, May 4, 2009 at 3:32 AM, steve donovan wrote:
> function module_fun (s1, s2)
>   s1,s2 = S(s1),S(s2)
>   ...
>   ... s:trim() etc
> end

but we must not carelessly pass the wrapped object to functions
outside of the lexical scope expecting a string.  We cannot guarantee
that our wrapper behaves exactly like a string because Lua strings are
not fully virtualizable [2].

An alternate idea is to modify the string metatable to be scope aware:

  -- framework
  local mt = debug.getmetatable('')
  local scope = {}
  function mt.__index(s, k)
    local f = debug.getinfo(2, 'f').func
    return scope[f] and scope[f][k] or string[k]
  end
  local function scoped_string_methods(t)
    local f = debug.getinfo(2, 'f').func
    scope[f] = t
  end

  -- test example libraries
  local stringx = {}
  function stringx.trim(self)  return self:match('^%s*(%S*)%s*$') end
  local stringxx = {}
  function stringxx.trim(self) return self:match('^%s?(.-)%s?$') end

  -- test example
  function test2(s)
    assert(s.trim == nil)
    scoped_string_methods(stringxx)
    assert(s:trim() == ' 123 ')
  end
  function test(s)
    scoped_string_methods(stringx)
    assert(s:trim() == '123')
    test2(s)
    assert(s:trim() == '123')
  end
  local s = '  123  '
  assert(s.trim == nil)
  test(s)
  assert(s.trim == nil)
  print 'DONE'


[1] http://kevincathey.com/code/detecting-conflicting-objective-c-category-methods/
[2] http://lua-users.org/wiki/LuaVirtualization