In Lua, how can you print the name of the current function, like the C99 __func__ identifier?

Question

Something like this:

function foo()
    print( __func__ )
   ...
end

How can it be done?

Solution 1

#!/usr/bin/lua

local function myFunc()
 print(debug.getinfo(1, "n").name);
end
myFunc()

Solution 2

You can't. In lua, functions are first class variables. So they don't have names. You might as well ask "what the name of 2" is. Just because some variable was assigned the value '2' doesn't make that variable the name of 2. Likewise "someFunc" is a variable - potentially one of many - that holds a particular function.

Solution 3

function __FILE__() return debug.getinfo(2, 'S').source end
function __LINE__() return debug.getinfo(2, 'l').currentline end
function __FUNC__() return debug.getinfo(2, 'n').name end

function printlinefilefunc()
    print("Line at "..__LINE__()..", FILE at "..__FILE__()..", in func: "..__FUNC__())
end

Output:

Line at 8, FILE at @./andydebug.lua, in func: printlinefilefunc

Solution 4

While I agree with Ephraim's answer, that code will not always report the same name as pointed out by Chris Becke. When the function is assigned to another variable, the "name" would be changed.

Here is another alternative. It just uses a string to identify the function. This method solves the changing name problem, but introduces a maintenance issue. The string would need to be kept in sync with the function name with future refactorization.

function foo()
  local __func__ = "foo"
  print( __func__ )
  --...
end

Alternatively, if the location of the function is more important than the name, the following may be better. It will give a name to the function that is based on the source and line number.

function getfunctionlocation()
  local w = debug.getinfo(2, "S")
  return w.short_src..":"..w.linedefined
end

function foo()
  print(getfunctionlocation()) --> foo.lua:6
  --...
end

If the __func__ still seems better, and standard Lua is not important, then the Lua parser can be modified as it is in this example for __FILE__ and __LINE__.

Solution 5

Use the Debug Library. It provides a getinfo(func) function that returns a table with information about the function.

View more solutions

Comments

[x-x]

See this post stackoverflow.com/questions/4021816/… for what I'm trying to achieve :)

[Admin]

in stacktraces, these functions appear as </path/to/filename.lua:$LINE>, , where $LINE is the line where the function appears.

[Potassium Ion]

Minor note (yes, I know this is an old thread): using debug.getinfo(1) or "" will work the same as debug.getinfo(1) and debug.getinfo(1) or ""

[phil294]

how does your answer comply with the accepted answer?

[Chris Becke]

It doesn't. Both answers are technically correct. If - by name - you want the unique unchanging assigned name of a lua function - what FUNC provides for C programs - you can't get it. If by name you simply want the name of an variable that has the function as its value, use the debug methods to get that, remembering that variables can be reassigned, and multiple variables can contain the same value - so the "name" returned this way is neither unique nor unchanging.

[Деян Добромиров]

Why Did you vote down when you know it. I am just trying to help you ??!!

[Potassium Ion]

1) This was two and a half years ago. 2) I didn’t downvote, someone else did.

[TarmoPikaro]

Depends on lua interpretter / version. debug.getinfo(2, 'S').source gave me whole source code of .lua script.

[AF7]

This currently returns "nil" in Lua 5.3.

[Roland Illig]

In Lua, functions are not variables. They are values that can be passed around.

[tsh]

First class citizen is not a valid reason to prevent access its name. For example, in Python, you can access it by myFunc.__name__. In JavaScript, it would be myFunc.name. But in C, after linking, there is nothing about function name for you. And we all know Python and JavaScript support first class function, but not C.