Re: first class ':'

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

On Mon, Sep 14, 2009 at 5:00 PM,  wrote:
> A proposal for Lua 5.2: Make ':' an operator which creates a function
> with a bound first parameter.

We should decide how stack level behaves under this proposal.  Consider:

  local o = {}
  function o:test()
    return debug.getinfo(1, 'f').func, debug.getinfo(2, 'f').func,
           debug.getinfo(3, 'f').func
  end

  -- [1]
  local a,b = o.test(o)
  assert(a == o.test)
  assert(b == debug.getinfo(1,'f').func)

  -- [2]
  local a,b = o:test()
  assert(a == o.test)
  assert(b == debug.getinfo(1,'f').func)

  -- [3]
  local a,b,c = (function(...) return o.test(o, ...) end)()
  assert(a == o.test)
  assert(b == nil) -- tail call in anonymous function
  assert(c == debug.getinfo(1,'f').func)

  print 'done'  -- passes under Lua 5.1

In Lua 5.1, [1] and [2] execute at the same stack level, but [3]
executes at a deeper stack level.

To preserve the equivalence of [1] and [2] under the proposal, we
would need to make [2] like [3] but not introduce a new stack level.
Currently, Lua lacks a documented way to express that type of currying
operation.