Re: Self-awareness of functions?

[Drake Wilson <drake@...>]

Quoth Dirk Laurie <dpl@sun.ac.za>, on 2010-12-29 09:22:36 +0200:
> In Lua you must take care if you want to define a global recursive 
> function.  Thus:
> 
> do  local fact
>     fact = function(x) if x<2 then return 1 else return x*fact(x-1) end end
>     fac = fact
>     end
> fact = "Napoleon escaped from Elba."
> print(fac(5)) --> 120
> 
> You can't omit any of that padding: create a local variable, then 
> assign it to a global variable, all in an enclosing block.

Note that those first two are compressible into a « local function »
definition, and the do/end is only necessary if you care about the
local name being invisible afterwards, so really it's more like:

  local function identity(...) return ... end
  foo.identity = identity

> Trouble is, I can't see a way to write a C function "self" that 
> can be imported into the global namespace and would achieve this.
> "self" would have to be an implicitly defined local variable 
> referring to the function under construction, created by the Lua 
> compiler: in effect, a reserved word.

I wouldn't do it that way.  If it were necessary, I'd add the Y
combinator, something like (untested):

  function y(f)
    local function g(...) return f(g, ...) end
    return g
  end

and then for your example:

  y(function(rec, x) if x < 2 then return 1 else return x * rec(x-1) end end)

(Of course you probably wouldn't call it 'y' in practice even though
the combinator is named that.)

I wouldn't normally bother doing that, though; I tend to give all
local recursive function definitions names anyway, for clarity.

   ---> Drake Wilson