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I'm trying to run the following in Lua 5.3
function new_t()
local self = {}
setmetatable(self, {
__add = function(lhs,rhs)
print('ok so',lhs,'+',rhs)
end
})
return self
end
local t1 = new_t()
local t2 = new_t()
t1 + t2
I get an error saying syntax error near '+'. However if I change the last line to x = t1 + t2, it runs and prints without error.
Is it possible to use a binary operator without using the resulting value? Why doesn't Lua let me do t1 + t2 or even 1 + 2 by itself?
Lua doesn't allow this, because all the operators (except function calls) are intended to always calculate a result. There's no good reason to throw away the result of an expression, and it usually indicates a coding mistake.
If you just want to test your code, I suggest using assert:
assert(not (t1 + t2))
I use not here, because your __add function doesn't return anything.
EDIT: Normally, when we add two numbers, we expect to get a new number, without changing the original numbers. Lua's metamethods are designed to work the same way. To do side-effects like printing or modifying an operand, it's easier and clearer to use a regular named method.
I'm new to Lua and trying to figure out how the return statement in the squares function below is being used in the following code snippet:
function squares(iteratorMaxCount)
return square,iteratorMaxCount,0
end
The square parameter in the return statement refers to a function with the following signature:
function square(iteratorMaxCount,currentNumber)
What's confusing me is that the return statement looks like it's returning three values. What I think it's actually doing, however, is passing iteratorMaxCount and 0 as the arguments to a square function call.
Can anyone explain to me what's happening with this syntax? How is this serving as a function call as opposed to returning three values? In my mind, it feels as though the return statement should be written return square(iteratorMaxCount, 0) as opposed to return square, iteratorMaxCount, 0. I know that this is obviously wrong, but I can't figure out why.
I've tried searching through the Lua Manual, Lua Reference Guide, and searching Google, but I can't seem to find anything that explains this particular syntax. Can anyone point me in the right direction, please?
Thanks in advance.
Full code below via
Tutorialspoint
function square(iteratorMaxCount,currentNumber)
if currentNumber<iteratorMaxCount
then
currentNumber = currentNumber+1
return currentNumber, currentNumber*currentNumber
end
end
function squares(iteratorMaxCount)
return square,iteratorMaxCount,0
end
for i,n in squares(3)
do
print(i,n)
end
squares really does return three values, the first of which is a function. squares does not call square at all.
The trick here is how the for ... in syntax works. In the Lua 5.3 Reference Manual, section 3.3.5 says:
A for statement like:
for var_1, ···, var_n in explist do block end
is equivalent to the code:
do
local f, s, var = explist
while true do
local var_1, ···, var_n = f(s, var)
if var_1 == nil then break end
var = var_1
block
end
end
So the keyword "in" needs to be followed by three values:
an "iterator function" for getting the variables in each iteration
a "state" value to pass to the function each time
an initial value to pass to the function the first time
After the first time the function is called, the first value from the previous call is passed back into the next function call. When the first value returned from the function is nil, the for loop ends.
So in this example, squares(max) is designed to be used after "in", using square as the iterator function, max as the "state", 0 as the initial value, and a number and its square as the loop data values.
I learned how to get the function name inside a function by using debug.getinfo(1, "n").name.
Using this feature, I found out the strange behavior in Lua.
Here's my code:
function myFunc()
local name = debug.getinfo(1, "n").name
return name
end
function foo()
return myFunc()
end
function boo()
local name = myFunc()
return name
end
print(foo())
print(boo())
Result:
nil
myFunc
As you can see, the function foo() and boo() calls the same function myFunc() but they return different results.
If I replace debug.getinfo(1, "n").name with other string, they return the same results as expected but I don't understand the unexpected behavior caused by using the debug.getinfo().
Is it possible to correct myFunc() function so calling both foo() and boo() functions return the same result?
Expected result:
myFunc
myFunc
In Lua, any return statement of the form return <expression_yielding_a_function>(...) is a "tail call". Tail calls essentially don't exist in the call stack, so they take up no additional space or resources. The function you call effectively gets erased from the debug information.
Is it possible to correct myFunc() function so calling both foo() and boo() functions return the same result?
Um... yes, but before I tell you how, allow me to try to convince you not to do this.
As previously mentioned, tail calls are part of the Lua language. The removal of tail calls from the stack is not an "optimization" any more than it is an "optimization" for a for loop to exit when you use break. It is a part of Lua's grammar, and Lua programmers have just as much a right to expect a tail call to be a tail call as they have the right to expect break to exit loops.
Lua, as a language, specifically states that this:
local function recursive(...)
--some terminating condition
return recursive(modified_args)
end
will never, ever, run out of stack space. It will be just as stack space efficient as performing a loop. This is a part of the Lua language, just as much a part of it as the behavior of for and while.
If a user wants to call your function via a tail call, that is their right as the user of a language that makes tail calls a thing. Denying users of a language the right to use the features of that language is rude.
So don't do it.
Furthermore, your code suggests that you are attempting to rely on functions having names. That you're doing something significant and meaningful with those names.
Well, Lua is not Python; Lua functions do not have to have names, period. As such, you should not write code that meaningfully relies upon the name of a function. For debugging or logging purposes, fine. But you should not break user expectations just for debugging and logging. So if the user made a tail call, just accept that's what the user wanted and that your debugging/logging will suffer slightly.
OK, so, do we agree that you shouldn't do this? That Lua users have the right to tail calls, and you don't have the right to deny them? That Lua functions are not named and you shouldn't write code that requires them to maintain a name? OK?
What follows is terrible code that you should never use! (in Lua 5.3):
function bypass_tail_call(Func)
local function tail_call_bypass(...)
local rets = table.pack(Func(...))
return table.unpack(rets, rets.n)
end
return tail_call_bypass
end
Then, simply replace your real function with the return of the bypass:
function myFunc()
local name = debug.getinfo(1, "n").name
return name
end
myFunc = bypass_tail_call(myFunc)
Note that the bypass function has to build an array to hold the return values, then unpack them into the final return statement. This obviously requires additional memory allocations that don't have to happen in regular code.
So there's another reason not to do this.
You can run your code through luac -l -p
...
function <stdin:6,8> (4 instructions at 0x555f561592a0)
0 params, 2 slots, 1 upvalue, 0 locals, 1 constant, 0 functions
1 [7] GETTABUP 0 0 -1 ; _ENV "myFunc"
2 [7] TAILCALL 0 1 0
3 [7] RETURN 0 0
4 [8] RETURN 0 1
function <stdin:10,13> (4 instructions at 0x555f561593b0)
0 params, 2 slots, 1 upvalue, 1 local, 1 constant, 0 functions
1 [11] GETTABUP 0 0 -1 ; _ENV "myFunc"
2 [11] CALL 0 1 2
3 [12] RETURN 0 2
4 [13] RETURN 0 1
Those are the two function that are of interest to us: foo and boo
As you can see, when boo calls myFunc, it's just a normal CALL, so nothing interesting there.
foo, however, does something called a tail call. That is, the return value of foo is the return value of myFunc.
What makes this kind of call special is that there is no need for the program to jump back into foo; once foo calls myFunc it can just hand over the keys and say "You know what to do"; myFunc then returns its results directly to where foo was called. This has two advantages:
The stack frame of foo can be cleaned up before myFunc is called
once myFunc returns, it doesn't need two jumps to return to the main thread; only one
Both of those are insignificant in examples like yours, but once you have a chain of lots and lots of tail calls, it becomes significant.
The downside of this is that, once the stack of foo gets cleaned up, Lua also forgets all the debugging information associated with it; it only remembers that myFunc was called as a tail call, but not from where.
An interesting side note, is that boo is almost also a tail call. If Lua didn't have multiple return values, it'd be exactly identical to foo, and a smarter compiler like LuaJIT might compile it to a tail call. PUC Lua won't though, since it needs a literal return some_function() to recognize the tail call.
The difference is that boo only returns the first value returned by myFunc, and while in your example, there will only ever be one, the interpreter can't make that assumption (LuaJIT might make that assumption during JIT compilation, but that's beyond my understanding)
Also note that, technically, the word tail call just describes a function A directly returning the return value of another function B.
It often gets used interchangeably with tail call optimization, which is what the compiler does when it re-uses the stack frame and turns the function call into a jump.
Strictly speaking, C (for example) has tail calls, but it has no tail call optimization, meaning something like
int recursive(n) { return recursive(n+1); }
is valid C code, but will eventually cause a stack overflow, while in Lua
local function recursive(n) return recursive(n+1) end
will just run forever. Both are tail calls, but only the second gets optimized.
EDIT: As always with C, some compilers may, on their own, implement tail call optimization, so don't go around telling everyone that "C never ever does it"; it's just not a requried part of the language, while in Lua it's actually defined in the language specification, so it's not Lua until it has TCO.
This is a result of tail call optimisation, which Lua does.
In this case, Lua translates the function call into a "goto" statement, and does not use any extra stack frame to perform the tail call.
You can add traceback statement to check it:
function myFunc()
local name = debug.getinfo(1, "n").name
print(debug.traceback("Stack trace"))
return name
end
Tail call optimisation happens in Lua when you return with a function call:
-- Optimized
function good1()
return test()
end
-- Optimized
function good2()
return test(foo(), bar(5 + baz()))
end
-- Not optimised
function bad1()
return test() + 1
end
-- Not optimised
function bad2()
return test()[2] + foo()
end
You can refer to the following links for more information:
- Programming in Lua - 6.3: Proper Tail Calls
- What is tail call optimisation? - Stack Overflow
This is my first time working with Lua, but not with programming. I have experience in Java, Action Script, and HTML. I am trying to create an addon for Elder Scroll Online. I managed to find the ESO API at the following link:
http://wiki.esoui.com/API#Player_Escorting
I am trying to make a function that returns a count of how many items each guild member has deposited in the bank. The code I have so far is as follows
function members()
for i=0, GetNumGuildEvents(3, GUILD_EVENT_BANKITEM_ADDED)
do
GetGuildEventInfo(3, GUILD_EVENT_BANKITEM_ADDED, i)
end
I am having trouble referencing the character making the specific deposit. Once I am able to do that I foresee making a linked list storing character names and an integer/double counter for the number of items deposited. If anyone has an idea of how to reference the character for a given deposit it would be much appreciated.
I don't have the game to test and the API documentation is sparse, so what follows are educated guesses/tips/hints (I know Lua well and programmed WoW for years).
Lua supports multiple assignment and functions can return multiple values:
function foo()
return 1, "two", print
end
local a, b, c = foo()
c(a,b) -- output: 1, "two"
GetGuildEventInfo says it returns the following:
eventType, secsSinceEvent, param1, param2, param3, param4, param5
Given that this function applies to multiple guild event types, I would expect param1 through param5 are specific to the particular event you're querying. Just print them out and see what you get. If you have a print function available that works something like Lua's standard print function (i.e. accepts multiple arguments and prints them all), you can simple write:
print(GetGuildEventInfo(3,GUILD_EVENT_BANKITEM_ADDED,i))
To print all its return values.
If you don't have a print, you should write one. I see the function LogChatText which looks suspiciously like something that would write text to your chat window. If so, you can write a Lua-esque print function like this:
function print(...)
LogChatText(table.concat({...}, ' '))
end
If you find from your experimentation that, say, param1 is the name of the player making the deposit, you can write:
local eventType, secsSinceEvent, playerName = GetGuildEventInfo(3,GUILD_EVENT_BANKITEM_ADDED, i)
I foresee making a linked list storing character names and an integer/double counter for the number of items deposited.
You wouldn't want to do that with a linked list (not in Lua, Java nor ActionScript). Lua is practically built on hashtables (aka 'tables'), which in Lua are very powerful and generalized, capable of using any type as either key or value.
local playerEvents = {} -- this creates a table
playerEvents["The Dude"] = 0 -- this associates the string "The Dude" with the value 0
print(playerEvents["The Dude"]) -- retrieve the value associated with the string "The Dude"
playerEvents["The Dude"] = playerEvents["The Dude"] + 1 -- this adds 1 to whatever was previous associated with The Dude
If you index a table with a key which hasn't been written to, you'll get back nil. You can use this to determine if you've created an entry for a player yet.
We're going to pretend that param1 contains the player name. Fix this when you find out where it's actually located:
local itemsAdded = {}
function members()
for i=0, GetNumGuildEvents(3, GUILD_EVENT_BANKITEM_ADDED ) do
local eventType, secsSinceEvent, playerName = GetGuildEventInfo(3, GUILD_EVENT_BANKITEM_ADDED, i)
itemsAdded[playerName] = (itemsAdded[playerName] or 0) + 1
end
end
itemsAdded now contains the number of items added by each player. To print them out:
for name, count in pairs(itemsAdded) do
print(string.format("Player %s has added %d items to the bank.", name, count))
end
I'm currently working on a game using Roblox (which uses Lua). It is a basically made up of several minigames. At the beginning of each round, all the players in game are put in a table and teleported to an area. That is where the coroutine comes into play. As the round is in progress, I want a coroutine to start. Every second that coroutine checks if the player's health is below zero, and removes them from the currentPlayer table if it is.
Sorry if I am not describing the problem correctly, but the coroutine will not yield. I haven't used coroutines before, so I am probably trying to yield it the wrong way. I know most of you will not be familiar with Roblox, but the Lua syntax is the same.
Can someone please give me an example of how I would end a looping coroutine?
currentPlayers = {}
roundTime = 60
local lookForWinners = coroutine.create(function()
while coroutine.running do
wait(1)
for i, v in pairs(currentPlayers) do
if v.Character.Humanoid.Health <= 0 then
table.remove(currentPlayers, v)
end
end
end
end)
while wait() do
repeat display("Two or more players need to be in the game.", 1) until #_G.plrs > 1 --Ignore, just checks if two+ players are in game.
display("Picking a map...", 3) pickMap()
teleport(0, 500, 0)
coroutine.resume(lookForWinners)
wait(roundTime)
print("Round over")
coroutine.yield(lookForWinners)
end
Lua is a single-threaded language. Coroutines do not cause functions to execute in parallel.
Coroutines are effectively just a way to make a function that can pause its own execution (using coroutine.yield), that can be resumed from outside (using coroutine.resume). There is no "coroutine.running": there's only one line "running" at any given time.
If Roblox were meant for you to use wait() to jump out of the Lua thread, you would write this as a series of loops that check their condition and then call wait():
local currentPlayers={}
local roundTime = 60
while #_G.plrs > 1 do
display("Two or more players need to be in the game.", 1)
wait()
end
display("Picking a map...", 3) pickMap()
teleport(0, 500, 0)
for i=0, roundTime do
for i, v in pairs(currentPlayers) do
if v.Character.Humanoid.Health <= 0 then
table.remove(currentPlayers, v)
end
end
wait(1)
end
print("Round over")
However, this is bad code. (Whenever you write code, let loops with a "wait" function in them serve to indicate that something is being done incorrectly.) You should be using Roblox's Events to handle your game's logic.
Check to see if the game should start only when the number of players changes.
"Look For Winners" only when a Humanoid's health changes (the HealthChanged event).
Run the timer on some kind of timer or interval (don't forget that you'll probably want to end your game early once somebody has won).
Events have many, many advantages over a busy loop; the most visible one will be that your checks occur when the thing they're checking for happens, and not later.
I suggest you follow Stuart's advice to use events; this is mostly to provide additional information on what coroutines are to help you use them correctly.
Think of coroutines as functions that may return values, but with a twist: while a "normal" function completes when it executes return, when you yield from a coroutine, it saves its state, so that resume can then continue from the point where you yielded as if nothing happened. Note that you only yield from a coroutine and only to the point where the resume of that coroutine was done (this is no different from calling a function and returning from it; the control returns to the point where you called the function).
In addition to that, resume and yield calls allow you to pass values to the coroutine and return (intermediate) values from the coroutine. See this SO answer for an example of how this can be used.
One can still return from a coroutine and it's no different from returning from a function, which completes its execution. If you check the status of the coroutine at that time (coroutine.status), it should be "dead".
So, to answer your question how you can end a looping coroutine: you can return from it, you can yield() from it (and never resume it again), or you can call error(), which you can then catch and check for in the result of the resume call. Having said that, I agree with Stuart that it may be the wrong way to solve your problem.