[LUA] Local function cannot access itself

[Ranguna259]

Code: Select all

local a = function(i)
  i=i+1
  if i == 5 then
    return i
  else
    return a(i)
  end
end

print(a(1))

This code produces the following error:

Code: Select all

input:6: attempt to call a nil value (global 'a')

"a" should access itself because it was declaired before the compiler read it so the scope it's in should be reachable by itself but by the looks of it, this is not how it works so how am I supposed to create function "a" while keeping it local ?

[Rucikir]

Taking inspiration from kikito's middleclass

Finally, a subtle point regarding recursive private methods. If you need to create a private method that calls himself, you will need to declare the variable first, and then (on the next line) initialize it with the function value. Otherwise the variable will not be available when the function is created

Code: Select all

MyClass3 = class('MyClass3')

local _secretRecursiveMethod -- variable declared here
_secretRecursiveMethod= function(self, n) -- and initialized here
  if(n<=0) then
    print( 'Last recursion')
  else
    print ( 'Recursion level ' .. n )
    _secretRecursiveMethod(self, n-1)
  end
end

MyClass3:recurseOver(n)
  _secretRecursiveMethod(self, n)
end

So you have to declare a before setting the function:

Code: Select all

local a
a = function(i)
  i=i+1
  if i == 5 then
    return i
  else
    return a(i)
  end
end

print(a(1))

[marco.lizza]

You need to declare the function as

Code: Select all

local function a(i)
  -- code
end

in order for it to work, due to Lua scope mechanism (see http://www.lua.org/pil/6.2.html).

[Ranguna259]

Yep, both ways work and here I was experimenting with the debug lib. It's in my blood, I got a tendency to make stuff harder than they seem
Anyway, thank you both

Code: Select all

local a = function(i)
  local a = {debug.getlocal(2,1)}
  a = a[2]
  i = i+1
  if i == 5 then
    return i
  end
  return a(i)
end

print(a(1))

[zorg]

So, to sum it up:

Code: Select all

-- The syntax sugar
local function name(arg)
  -- something
end
-- is actually equivalent to:
local name; name = function(arg)
  -- something
end
-- and NOT to:
local name = function(arg)
  -- something
end
-- for the exact reason that the last one can not call itself.

As a sidenote, i also believe that something similar happens when one would want to define fields in a table via the "filling the curly brackets" way, and they can't refer to any "previously defined" key, since technically they don't exist yet; the whole thing will be created at once.

Code: Select all

-- Just to illustrate what i mean:
-- this works:
local t = {a = 1, b = 2}
t.b = t.a
-- these do not:
local t = {a = 1, b = a} -- b is set to nil.
local t = {a = 1, b = t[a]} -- errors because i basically did nil[something], where something in this case was nil as well.