I am learning Dart, and I can't understand the logic behind this code if anyone can help:
Function applyMultiplier(num multiplier) {
return (num value) {
return value * multiplier;
};
}
final triple = applyMultiplier(3);
print(triple(6)); //output 18
There is an anonymous function inside a named function.
We assigned a function to a variable.
What I don't understand is how did we pass from triple to value. I can't understand the logic behind.
Well, the function applyMultiplier takes a num as argument and returns a function that itself returns the value it is given multiplied by another multiplier. final tripple = applyMultiplier(3) stores this function that is returned from applyMultiplier in the variable triple. Because the variable triple then stores a function it can also be used like a function.
EDIT: Turns out this is not something possible with Lua, having the __index method AND methods like class instance methods. It's either or.
Trying to get my Lua interface to work where both fields and instance methods are supported. It seems that by manipulating the initialization, I can only get the functions (_f), or the methods (_m) to work, not both.
I feel like it's something really simple I'm just missing.
How I initialize the library:
void PushUserdata(const void *data, const char *metatable)
{
const void **wrapped_ptr = (const void**)lua_newuserdata(l, sizeof(const void*));
*wrapped_ptr = data;
luaL_getmetatable(l, metatable);
lua_setmetatable(l, -2);
}
static int player_head(lua_State *L)
{
if (!Player::playerHead)
lua_pushnil(L);
else
PushUserdata(Player::playerHead, "player");
return 1;
}
static int playerget(lua_State *L)
{
Player *player = *CHECKPLAYER(L, 1); // Get double pointer and dereference to get real pointer
const char *field = luaL_checkstring(L, 2);
if (!strcmp(field, "next"))
{
if (!player->next)
lua_pushnil(L);
else
PushUserdata(player->next, "player");
}
else if (!strcmp(field, "prev"))
{
if (!player->prev)
lua_pushnil(L);
else
PushUserdata(player->prev, "player");
}
else if (!strcmp(field, "obj"))
{
if (!player->obj)
lua_pushnil(L);
else
PushUserdata(player->obj, "wobj");
}
else if (!strcmp(field, "AddCollisionObjHook")) // This ends up here if __index is in the table below...
{
}
else
return 0;
return 1;
}
static const struct luaL_Reg playerlib_f[] = {
{"head", player_head},
{"AddPreThinker", AddPreThinker},
{"AddPostThinker", AddPostThinker},
{NULL, NULL}
};
static const struct luaL_Reg playerlib_m[] = {
{"__tostring", player2string},
{"__index", playerget},
{"__newindex", playerset},
{"AddCollisionObjHook", AddCollisionObjHook},
{NULL, NULL}
};
int Lua_PlayerLib(lua_State *L)
{
luaL_newmetatable(L, "player");
lua_pushvalue(L, -1); // duplicates the metatable...but why?
luaL_setfuncs(L, playerlib_m, 0);
luaL_newlib(L, playerlib_f, 0);
lua_setglobal(L, "player");
return 1;
}
Lua script:
me = playerlib.head()
me:AddCollisionObjHook(playerHitObj)
Error message:
Warning: [string "postload.lua"]: attempt to call method 'AddCollisionObjHook' (a nil value)
'me' is absolutely a valid non-nil value.
What you're trying to do is possible, but not in the way you're trying to do it.
I think it's worth reviewing how method calls and metatables/metamethods work, and what your code, as written, is actually doing. The tl;dr is:
method calls are just normal field lookups
metatables, and the metamethods they contain, are operator overloads, not method definitions
if you're implementing this for userdata you need an __index metamethod that can handle both field and method lookups
First of all, Lua has no inbuilt distinction between "methods" and "fields". You may find it convenient to differentiate the two when organizing your code, but as far as the Lua language is concerned methods and fields are the same thing. A method is just a field where the key is a valid lua identifier and the value is a function.
So, when you write something like me:AddCollisionObjHook(playerHitObj), what actually happens is something like:
local self = me
local method = self["AddCollisionObjHook"]
method(self, playerHitObj)
(Two notes on this:
no actual new locals are created; this all happens in the internals of the Lua interpreter.
self["AddCollisionObjHook"] and self.AddCollisionObjHook are two ways of writing the same thing; the latter is just a convenient shortcut for the former.)
So, how does that self["AddCollisionObjHook"] lookup work? The same way any other field lookup works. The Lua Manual goes into detail on this, including pseudocode, but the part relevant to your code is:
-- We're looking up self[key] but self is userdata, not table
local mt = getmetatable(self)
if mt and type(mt.__index) == 'function' then
-- user provided an __index function
return mt.__index(self, key)
elseif mt and mt.__index ~= nil then
-- user provided an __index table (or table-like object)
-- retry the lookup using it
return mt.__index[key]
else
-- no metatable, or metatable lacks __index metamethod
error(...) -- don't know how to do field lookup on this type!
end
Note that at no point in this process are fields other than __index looked up in the metatable. The metatable exists only to tell Lua how to implement operators for types that don't normally have them; in this case the field lookup ("index") operator ([], and its aliases . and :) for a specific kind of userdata. It's entirely down to __index itself to handle the actual process of turning field names into values, either by being a table that the lookup can be retried with or by being a function that can return the associated value.
So, this brings us to the answer of how to support both (settable) fields and (callable) methods:
__newindex needs to understand how to set fields
__index needs to understand how to return both field values and method implementations
Since, from Lua's perspective, both field lookups and method lookups are the same operation, and thus __index gets used for both.
In light of that, how should we organize the code to support both, and how can we restructure your code to work? There's lots of ways to do this, although for the purposes of this answer I'm going to make a few assumptions:
fields are stored entirely C-side, with no corresponding data to manage in Lua
methods cannot be overwritten by Lua code
metamethods are stored separately from instance methods
The last one is not strictly necessary; in fact it's very common to store both metamethods and instance methods in the same table (I usually do this myself). However, I think this also tends to engender confusion among Lua newbies about the distinction between them, so the interest of making the code as clear as possible, I'm separating them out in this answer.
With that in mind, let's rework the setup code. I've looked at your edits to try to reconstruct what you originally had in mind.
static int playerget(lua_State *L)
{
Player *player = *CHECKPLAYER(L, 1);
const char *field = luaL_checkstring(L, 2);
// Check if it's a method, by getting the method table
// and then seeing if the key exists in it.
// This code can be re-used (or factored out into its own function)
// at the start of playerset() to raise an error if the lua code tries
// to overwrite a method.
lua_getfield(L, LUA_REGISTRYINDEX, "player-methods");
lua_getfield(L, -1, field);
if (!lua_isnil(L, -1)) {
// Lookup in methods table successful, so return the method impl, which
// is now on top of the stack
return 1;
} else {
// No method, so clean up the stack of both the nil value and the
// table of methods we got it from.
lua_pop(L, 2);
}
if (!strcmp(field, "next"))
// ... code for reading fields rather than methods goes here ... //
}
// Functions that are part of the player library rather than tied to any
// one player instance.
static const struct luaL_Reg playerlib_api[] = {
// player.head() -> returns the first player
{"head", player_head},
{NULL, NULL}
};
// Metamethods defining legal operators on player-type objects.
static const struct luaL_Reg playerlib_metamethods[] = {
// Overrides the tostring() library function
{"__tostring", player2string},
// Adds support for the table read operators:
// t[k], t.k, and t:k(...)
{"__index", playerget},
// Adds support for the table write operators:
// t[k]=v and t.k=v
{"__newindex", playerset},
{NULL, NULL}
};
// Instance methods for player-type objects.
static const struct luaL_Reg playerlib_methods[] = {
// player_obj:AddCollisionObjHook(hook)
{"AddCollisionObjHook", AddCollisionObjHook},
// player_obj:AddPreThinker(thinker)
{"AddPreThinker", AddPreThinker},
// player_obj:AddPostThinker(thinker)
{"AddPostThinker", AddPostThinker},
{NULL, NULL}
};
int Lua_PlayerLib(lua_State *L)
{
// Create the metatable and fill it with the stuff from playerlib_metamethods.
// Every time a player object is pushed into Lua (via player_head() or similar)
// this metatable will get attached to it, allowing lua to see the __index,
// __newindex, and __tostring metamethods for it.
luaL_newmetatable(L, "player");
luaL_setfuncs(L, playerlib_metamethods, 0);
lua_pop(L, 1);
// Create the method table and fill it.
// We push the key we're going to be storing it in the registry under,
// then the table itself, then store it into the registry.
lua_pushliteral(L, "player-methods");
luaL_newlib(L, playerlib_methods, 0);
lua_settable(L, LUA_REGISTRYINDEX);
// Initialize the `player` library with the API functions.
luaL_newlib(L, playerlib_api, 0);
// Set that table as the value of the global "player".
// This also pops it, so we duplicate it first...
lua_pushvalue(L, -1);
lua_setglobal(L, "player");
// ...so that we can also return it, so that constructs like
// local player = require 'player'; work properly.
return 1;
}
Breaking it down, this gives us three tables:
player, which holds the actual library API like player.head()
REGISTRY["player"], which holds the metatable shared by all player objects
__tostring which is invoked for prettyprinting
__newindex which is invoked for field writes
__index which is invoked for field reads (including method lookup!)
REGISTRY["player-methods"], which holds all the instance methods
__index looks here for method implementations
As noted above, I've kept the table of metamethods and the table of methods separate in the hopes of minimizing conceptual confusion; idiomatic code would probably store all the methods and metamethods together, and use luaL_getmetafield() at the start of playerset() and playerget() to do method lookup.
Functions in Dart are first-class objects, allowing you to pass them to other objects or functions.
void main() {
var shout = (msg) => ' ${msg.toUpperCase()} ';
print(shout("yo"));
}
This made me wonder if there was a way to modify a function a run time, just like an object, prior to passing it to something else. For example:
Function add(int input) {
return add + 2;
}
If I wanted to make the function a generic addition function, then I would do:
Function add(int input, int increment) {
return add + increment;
}
But then the problem would be that the object I am passing the function to would need to specify the increment. I would like to pass the add function to another object, with the increment specified at run time, and declared within the function body so that the increment cannot be changed by the recipient of the function object.
The answer seems to be to use a lexical closure.
From here: https://dart.dev/guides/language/language-tour#built-in-types
A closure is a function object that has access to variables in its
lexical scope, even when the function is used outside of its original
scope.
Functions can close over variables defined in surrounding scopes. In
the following example, makeAdder() captures the variable addBy.
Wherever the returned function goes, it remembers addBy.
/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
return (int i) => addBy + i;
}
void main() {
// Create a function that adds 2.
var add2 = makeAdder(2);
// Create a function that adds 4.
var add4 = makeAdder(4);
assert(add2(3) == 5);
assert(add4(3) == 7);
}
In the above cases, we pass 2 or 4 into the makeAdder function. The makeAdder function uses the parameter to create and return a function object that can be passed to other objects.
You most likely don't need to modify a closure, just the ability to create customized closures.
The latter is simple:
int Function(int) makeAdder(int increment) => (int value) => value + increment;
...
foo(makeAdder(1)); // Adds 1.
foo(makeAdder(4)); // Adds 2.
You can't change which variables a closure is referencing, but you can change their values ... if you an access the variable. For local variables, that's actually hard.
Mutating state which makes an existing closure change behavior can sometimes be appropriate, but those functions should be very precise about how they change and where they are being used. For a function like add which is used for its behavior, changing the behavior is rarely a good idea. It's better to replace the closure in the specific places that need to change behavior, and not risk changing the behavior in other places which happen to depend on the same closure. Otherwise it becomes very important to control where the closure actually flows.
If you still want to change the behavior of an existing global, you need to change a variable that it depends on.
Globals are easy:
int increment = 1;
int globalAdder(int value) => value + increment;
...
foo(globalAdd); // Adds 1.
increment = 2;
foo(globalAdd); // Adds 2.
I really can't recommend mutating global variables. It scales rather badly. You have no control over anything.
Another option is to use an instance variable to hold the modifiable value.
class MakeAdder {
int increment = 1;
int instanceAdd(int value) => value + increment;
}
...
var makeAdder = MakeAdder();
var adder = makeAdder.instanceAdd;
...
foo(adder); // Adds 1.
makeAdder.increment = 2;
foo(adder); // Adds 2.
That gives you much more control over who can access the increment variable. You can create multiple independent mutaable adders without them stepping on each other's toes.
To modify a local variable, you need someone to give you access to it, from inside the function where the variable is visible.
int Function(int) makeAdder(void Function(void Function(int)) setIncrementCallback) {
var increment = 1;
setIncrementCallback((v) {
increment = v;
});
return (value) => value + increment;
}
...
void Function(int) setIncrement;
int Function(int) localAdd = makeAdder((inc) { setIncrement = inc; });
...
foo(localAdd); // Adds 1.
setIncrement(2);
foo(localAdd); // Adds 2.
This is one way of passing back a way to modify the local increment variable.
It's almost always far too complicated an approach for what it gives you, I'd go with the instance variable instead.
Often, the instance variable will actually represent something in your model, some state which can meaningfully change, and then it becomes predictable and understandable when and how the state of the entire model changes, including the functions referring to that model.
Using partial function application
You can use a partial function application to bind arguments to functions.
If you have something like:
int add(int input, int increment) => input + increment;
and want to pass it to another function that expects to supply fewer arguments:
int foo(int Function(int input) applyIncrement) => applyIncrement(10);
then you could do:
foo((input) => add(input, 2); // `increment` is fixed to 2
foo((input) => add(input, 4); // `increment` is fixed to 4
Using callable objects
Another approach would be to make a callable object:
class Adder {
int increment = 0;
int call(int input) => input + increment;
}
which could be used with the same foo function above:
var adder = Adder()..increment = 2;
print(foo(adder)); // Prints: 12
adder.increment = 4;
print(foo(adder)); // Prints: 14
In the Lua C API I can store a number or a string from the stack with lua_tostring().
How can a “reference” (if that is the correct term) to a Lua function be passed to C through the Lua API? So it can be called later from C, with lua_call(), without having to reference it by its name.
(It really needs to be like that, the C program will call the function somewhere in the future and the program doesn't know anything about the function because the functions to be passed are defined in the Lua program)
In C you can't refer to Lua functions directly but you can represent numbers and strings. So, for a function to "be called later", you can store this function in some table and refer to it by a numeric or string key of the table.
Here's a simpleminded mechanism to start with:
On the Lua side:
funcs = {}
local function register_hanlder(key, fn)
funcs[key] = fn
end
register_handler("on_mouse_click", function()
print "You clicked me!"
end)
On the C side:
/* code not tested */
lua_getglobal(L, "funcs");
lua_getfield(L, -1, "on_mouse_click");
if (!lua_isnil(L, -1)) {
lua_call(L, 0, 0);
else {
// nothing registered
}
Instead of registering the functions in a global table you can register them in the registry table (see luaL_ref). You'll get some integer (that's the key in the registry table where the function value is) that you can pass around in you C code.
Note that if you don't need to store a Lua function "for use later" you don't need any of this: if your C function has some Lua function passed to it via argument you can call it outright.
== Edit:
As I mentioned, instead of using a global variable (the funcs above) you can store the reference to the function in the "registry". Conceptually there's no difference between this method and the previous one.
Let's re-use the previous example: you want the Lua programmer to be able to register a function that would be fired whenever a mouse is clicked in your application.
The Lua side would look like this:
register_mouse_click_handler(function()
print "the mouse was clicked!"
end)
On the C side you define register_mouse_click_handler:
static int the_mouse_click_handler = 0;
static int register_mouse_click_handler(lua_State* L) {
the_mouse_click_handler = luaL_ref(L, LUA_REGISTRYINDEX);
return 0;
}
(...and expose it to Lua.)
Then, in your application, when the mouse is clicked and you want to call the Lua function, you do:
...
if (the_mouse_click_handler != 0) {
lua_rawgeti(L, LUA_REGISTRYINDEX, the_mouse_click_handler);
lua_call(L, 0, 0);
} else {
// No mouse handler was registered.
}
...
(I may have typos in the code.)
Example code that produces the problem:
import std.stdio, core.thread;
void main() {
ThreadGroup tg = new ThreadGroup();
foreach (int i; 1 .. 5)
tg.create( () => writeln(i) );
tg.joinAll();
}
Sample output:
3
5
5
5
5
(The expected output was the integers 1 through 5.)
I don't get why this is happening -- i is not a reference type, nor is there a reference to it being used in the delegate, so why does each thread use the value of i as whatever value it happens to have when the thread is scheduled, instead of the presumably pass-by-value value it's given?
I've made a handful of lame attempts like this, but nothing's been successful:
foreach (int i; 1 .. 5) {
scope j = i;
tg.create( () => writeln(j) );
}
I'm curious as to why this doesn't work, either. Is it not declaring a fresh j each time? Why does each thread refer to the same j (whose value is usually 5 by the time the threads get scheduled)?
so why does each thread use the value of i as whatever value it happens to have when the thread is scheduled, instead of the presumably pass-by-value value it's given?
It's pass-by-value as far as the loop body goes, however that does not apply to the threads created in it. The threads will still refer to i by its address.
To fix this problem, you need to create a closure inside the loop:
import std.stdio, core.thread;
void main() {
ThreadGroup tg = new ThreadGroup();
foreach (int i; 1 .. 5)
(i =>
tg.create( () => writeln(i) )
)(i);
tg.joinAll();
}
The lambda parameter will be stored in the closure, giving each thread its own copy.