In Lua, calling a function with excess arguments will simply discard those. Is there a possibility with the debug library to access these discarded arguments? (I'm not looking for variadic functions)
function test()
local info = debug.getinfo(1, "u")
print(info.nparams) -- just 0 :(
-- how to access the discarded arguments?
end
test(42, "Hello World!", 3.14)
Edit: As Luiz-Henrique informed me in the comments, this is not possible in standard Lua. He also asked for more context, so I'm going to repeat the problem that I was trying to solve to answer another question. In this other question the OP wanted to access the name that module has inside package.preload from within the loader function but without taking it as an argument as it is passed by require. I don't know why but this led me to the question whether it is possible to access discarded arguments.
package.preload["test"] = function()
local info = debug.getinfo(1, "u")
print(info.nparams) -- just 0 :(
-- how to access the name without taking it as an argument
return {}
end
require("test")
Related
How to output the contents of a function?
function a()
print("Hello,World")
end
print(func_toString(a))
I hope the result of func_toString(a) could be
function a()
print("Hello,World")
end
or just
print("Hello,World")
It is assumed that the source code is executed directly without precompiling or embedding.
How to do this?
well, it's not completely impossible, the code in some cases can be read from a lua file, for example:
function a()
print("Hello,World")
end
local function get_source_code(f)
local t = debug.getinfo (f)
if t.linedefined < 0 then print("source",t.source); return end
local name = t.source:gsub("^#","")
local i = 0
local text = {}
for line in io.lines(name) do
i=i+1
if i >= t.linedefined then text[#text+1] = line end
if i >= t.lastlinedefined then break end
end
return table.concat(text,"\n")
end
print( get_source_code(a) )
maybe that will be enough.
This is not possible from within the running Lua program.
Lua provides the debug library to inspect functions and variable. This allows you to obtain the source (the path or string where the function is defined) and the line range on which the function is defined. In the vast majority of cases this might actually be enough: Just find the first occurrence of function(...) on the first line and the first occurrence of end on the last line, then string.sub the relevant portion. This is however rather error prone; consider e.g.
function a() function b() end end
a()
func_toString(a)
since both functions are on the same line, you can't distinguish them - the debug library only provides you line info, nothing more. You could try to distinguish them by their signature since hacks exist to obtain the method signature, but that would fail here as well since both functions have the same signature. You could try gsubing them out based on their names, but remember that functions can be anonymous in Lua.
Lua also provides the string.dump function to obtain the bytecode of a function. I highly doubt that this is of any use to you; theoretically you could decompile it to get back a "Lua" representation of what the function does, but it would hardly be recognizable or readable.
I'm still ramping up in lua, and I am not quite familiar with this syntax. What is happening when you pass in a function as a parameter like below?
Comm.setRouting(function(url)
for i = 1,4 do
local portIndex = "Path"..i
if url:match(portConfig[portIndex]) ~= nil then
return Comm.slots()[1], Comm.groups()[i]
end
end
end)
The other answers are correct, but it might help you if you wrote your own function that calls another function:
function CallAFunction(func_to_call)
func_to_call()
end
You could pass a named function (an anonymous function assigned to a variable) or an anonymous function written on the fly.
function SayHello()
print("Hello!")
end
--[[
^This is equivalent to:
SayHello = function()
print("Hello!")
end
]]--
CallAFunction(SayHello)
CallAFunction(function()
print("Goodbye!")
end)
output:
Hello!
Goodbye!
and this can be done with parameters
function CallAFunction(func)
func("Bonjour")
end
CallAFunction(function(parameter)
print(parameter)
end)
Here, func is the anonymous function, which accepts 1 parameter parameter.
When you call func("Bonjour") you are passing Bonjour as parameter, like a normal function call.
Here you're passing an argument to setRouting that is an "anonymous function"
Functions are first-class values in Lua and can be stored in local variables, global variables and table fields. Here the function is being passed anonymously on the call stack to the setRouting function.
setRouting would be called a "higher-order function" because it accepts a function as its input.
The following page has some more information about functions in Lua:
https://www.lua.org/pil/6.html
A couple of things are happening in this example:
You're passing a function as a parameter. The callee (e.g. setRouting()) can invoke that function. This is often referred to as a callback function.
You're defining the function itself, on-the-fly. This is an example of an "anonymous function", or a lambda function.
Let's say that I call some functions indirectly, via a variable. For example:
obj = {
on_init = function()
print "hello."
end,
on_destroy = function()
print "bye."
end,
on_do_something = function()
print "doing something."
error("Hi de hi, hi de ho!")
end,
}
local event = "do_something"
local func = obj["on_" .. event]
func()
All works fine.
However, the problem is that when the called function raises an exception (as in the code above) the error message isn't quite clear. It is thus:
lua: test.lua:13: Hi de hi, hi de ho!
stack traceback:
[C]: in function 'error'
test.lua:13: in function 'func'
test.lua:20: in main chunk
It says "in function 'func'". I'd prefer it to say "in function 'on_do_something'" instead.
I'd imagine this scenario to be very common. Is there a solution for this?
I tried calling the function thus:
obj["on_" .. event]()
But then the error message says "in function '?'", which isn't helpful either.
(I tried this code on Lua 5.1, 5.2 and LuaJIT without notable differences.)
This is a limitation of the heuristics used by Lua to provide names for functions.
In Lua, all functions are anonymous. A given function can be the value of several variables: global, local, and table fields. The Lua debug system, which is used in error handling, tries to find a reasonable name for a value based on where it came from by looking into the bytecode being executed.
See
Why is 'name' nil for debug.getinfo(1).
You have a few options. The debug module will try to produce something useful. For instance, you might be able to get the file name and line number where it was defined, if this is your own code. See
http://www.lua.org/pil/23.1.html for the list of what is available via debug module. Or, you might be able to define the functions in the module, then add them to the table:
-- module something
function a()
...
end
tt = {
fn1 = a,
...
}
Depending on where you trap the error (error handler installed via debug hook?), you could check if filename is the module where your table of functions is defined, and if it is, then use the debug module to print appropriate info based on your table structure etc; if it is not, just print the default traceback etc.
What is the syntax to create the function, but then add it's implementation further down in code?
So roughly like this:
Define function doX
Call doX (further down in the code)
doX implemention (i.e. all functions down at the bottom of the file)
You only need to have a variable to reference. local funcName is sufficient for your purposes with one caveat. This will work:
local funcName
function callIt()
print(funcName())
end
function defineIt()
funcName = function() return "My Function" end
end
defineIt()
callIt()
As long as you define it (defineIt) before you call it (callIt), it should work as expected. You can't do something like this though (and this is the caveat):
local funcName
print(funcName())
funcName = function() return "My Function" end
You will get an error: attempt to call local 'funcName' (a nil value).
oh...so there's really no way to call funcName prior to having actually defined the function then? i.e. you still need to make sure defineIt is called before your first call to funcName itself?
I wanted to clarify this point, and I felt that an answer would be the better way than a comment.
Lua is a much simpler language than C or C++. It is built on some simple foundations, with some syntactic sugar to make parts of it easier to swallow.
There is no such thing as a "function definition" in Lua. Functions are first-class objects. They are values in Lua, just like the number 28 or the string literal "foo" are values. A "function definition" simply sets a value (namely, the function) into a variable. Variables can contain any kind of value, including a function value.
All a "function call" is is taking the value from a variable and attempting to call it. If that value is a function, then the function gets called with the given parameters. If that value is not a function (or a table/userdata with a __call metamethod), then you get a runtime error.
You can no more call a function that hasn't been set in a variable yet than you can do this:
local number = nil
local addition = number + 5
number = 20
And expect addition to have 25 in it. That's not going to happen. And thus, for the same reason, you can't do this:
local func = nil
func(50)
func = function() ... end
As Paul pointed out, you can call a function from within another function you define. But you cannot execute the function that calls it until you've filled in that variable with what it needs to contain.
As others have written, you cannot call a function at runtime that has not been assigned prior to the call. You have to understand that:
function myFunc() print('Something') end
Is just a syntax sugar for this:
myFunc = function() print('Something') end
Now, it makes sense that this kind of code would not work the way you want it to:
print(greeter(io.read())) -- attempt to call global 'greeter' (a nil value)
function greeter(name) return 'Hello '..name end
When you use the greeter variable, its value is nil, because its value is set only on the next line.
But if you want to have your "main" program on the top and the functions at the bottom, there is simple way to achieve this: create a "main" function and call it as the last thing on the bottom. By the time the function is called, all the functions will be set to the corresponding global variables:
-- start of program, your main code at the top of source code
function main()
local name = readName()
local message = greeter(name)
print(message)
end
-- define the functions below main, but main is not called yet,
-- so there will be no errors
function readName() io.write('Your name? '); return io.read() end
function greeter(name) return 'Hello, ' .. name end
-- call main here, all the functions have been assigned,
-- so this will run without any problems
main()
How do I call a function that needs to be called from above its creation? I read something about forward declarations, but Google isn't being helpful in this case. What is the correct syntax for this?
Lua is a dynamic language and functions are just a kind of value that can be called with the () operator. So you don't really need to forward declare the function so much as make sure that the variable in scope when you call it is the variable you think it is.
This is not an issue at all for global variables containing functions, since the global environment is the default place to look to resolve a variable name. For local functions, however, you need to make sure the local variable is already in scope at the lexical point where you need to call the value it stores, and also make sure that at run time it is really holding a value that can be called.
For example, here is a pair of mutually recursive local functions:
local a,b
a = function() return b() end
b = function() return a() end
Of course, that is also an example of using tail calls to allow infinite recursion that does nothing, but the point here is the declarations. By declaring the variables with local before either has a function stored in it, those names are known to be local variables in lexical scope of the rest of the example. Then the two functions are stored, each referring to the other variable.
You can forward declare a function by declaring its name before declaring the actual function body:
local func1
local func2 = function()
func1()
end
func1 = function()
--do something
end
However forward declarations are only necessary when declaring functions with local scope. That is generally what you want to do, but Lua also supports a syntax more like C, in which case forward declaration is not necessary:
function func2()
func1()
end
function func1()
--do something
end
Testing under the embedded lua in Freeswitch, forward declaration does not work:
fmsg("CRIT", "It worked.")
function fmsg(infotype, msg)
freeswitch.consoleLog(infotype, msg .. "\n")
end
result:
[ERR] mod_lua.cpp:203 /usr/local/freeswitch/scripts/foo.lua:1: attempt to call global 'fmsg' (a nil value)
Reversing the order does (duh) work.
To comprehend how forward referencing in Lua works compared to C, you must understand the a fundamental difference between C compilation and the Lua execution.
In C, forward referencing is a compile time mechanism. Hence if you include a forward declaration template in a C module then any of your code following will employ this template in compiling the call. You may or may not include the function implementation in the same module, in which case both declarations must be semantically identical or the compiler will error. Since this is a compile time construct, the compiled code can be executed in any order.
In Lua, forward referencing is runtime mechanism, in that the compiled function generates a function prototype internally within the code, but this is only accessible as a runtime Lua variable or value after the execution has
passed over the declaration creating a Lua closure. Here the declaration order within the source is immaterial. It is the execution order that is important: if the closure hasn't been bound to the variable yet, then the execution will throw a "nil value" exception.If you are using a local variable to hold the function value, then normal local scoping rules still apply: the local declaration must precede its use in the source and must be within scope, otherwise the compiler will compile in the wrong global or outer local reference. So forward referencing using locals as discussed in other answer will work, but only if the Protos are bound to closures before the first call is executed.
Doesn't work for me if I try to call the function before definition. I am using this Lua script in nginx conf.
lua entry thread aborted: runtime error: lua_redirect.lua:109: attempt to call global 'throwErrorIfAny' (a nil value)
Code snippet -
...
throwErrorIfAny()
...
function throwErrorIfAny()
ngx.say("request not allowed")
ngx.exit(ngx.HTTP_OK)
end
Given some other answers have also pointed out that it didn't work for them either, it is possible that forward declaration of Lua doesn't work with other tools.
PS : It works fine if I put the function definition before and then call it after wards.
If you use OOP you can call any function member prior its "definition".
local myClass = {}
local myClass_mt = { __index = myClass }
local function f1 (self)
print("f1")
self:later() --not yet "delared" local function
end
local function f2 (self)
print("f2")
self:later() --not yet "declared" local function
end
--...
--later in your source declare the "later" function:
local function later (self)
print("later")
end
function myClass.new() -- constructor
local this = {}
this = {
f1 = f1,
f2 = f2,
later = later, --you can access the "later" function through "self"
}
setmetatable(this, myClass_mt)
return this
end
local instance = myClass.new()
instance:f1()
instance:f2()
Program output:
f1
later
f2
later