I'm trying to pass a Kotlin Object to a C-Library which will pass that object over to a callback function. This function is written in Kotlin, thus I can use this Object and work with it. Howerver i could not find a way to convert a Kotlin Object to a CPointer. The only thing I found which may be what I need, is fun createKotlinObjectHolder(any: Any?): NativePtr and fun <reified T : Any> unwrapKotlinObjectHolder(holder: Any?): T. But even if I use the just created NativePtr to pass it over to the unwrapper function, it will fail with the error message unrecognized selector sent to instance 0xXYZ. What am I doing wrong or aren't those the functions I should use?
An example Code with libcURL:
fun writeCallback(ptr: CPointer<ByteVar>, ignored: ULong, size: ULong, userData: COpaquePointer?): ULong {
if (userData != null) {
unwrapKotlinObjectHolder<StringBuilder>(userData.rawValue).append("Hello")
}
return size
}
fun main() {
...
val curl = curl_easy_init()
...
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, staticCFunction(::writeCallback))
val stringBuilder = StringBuilder()
val nativePtr = createKotlinObjectHolder(stringBuilder)
val cPtr = interpretCPointer<CPointed>(nativePtr)
curl_easy_setopt(curl, CURLOPT_WRITEDATA, cPtr)
...
}
The code is compiled on macOS.
There is a StableRef class, that should be used to wrap Kotlin instances in such cases. This code should work correctly:
fun writeCallback(ptr: CPointer<ByteVar>, ignored: ULong, size: ULong, userData: COpaquePointer?): ULong {
if (userData != null) {
userData.asStableRef<StringBuilder>().get().append("Hello")
}
return size
}
fun main() {
/* get a curl handle */
val curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, staticCFunction(::writeCallback))
val stringBuilder = StringBuilder()
val stableRef = StableRef.create(stringBuilder)
val cPtr = stableRef.asCPointer()
curl_easy_setopt(curl, CURLOPT_WRITEDATA, cPtr)
curl_easy_perform(curl)
curl_easy_cleanup(curl)
stableRef.dispose()
}
See this page of the documentation for additional information.
Related
I'm trying to call SomeClass().call, but am running into compiler errors.
Specifically, running tests for
const std = #import("std");
test "doing a thing" {
{
const calc_result = SomeClass().call(.{});
try std.testing.expectEqual(calc_result, 42);
}
}
fn SomeClass() type {
return struct {
fn call(context: .{}) u32 {
_ = context;
return 42;
}
};
}
results in the error message
src/test.zig:12:17: error: expected type 'type', found '#TypeOf(.{})'
fn call(context: .{}) u32 {
^~~~~~~
referenced by:
test.doing a thing: src/test.zig:5:40
remaining reference traces hidden; use '-freference-trace' to see all reference traces
How do I call a generic type method that takes an empty context?
What you're doing is equivalent to fn foo(value: 0) void {}. Which is obviously wrong. A function definition cannot have values.
You need to define the type of the context:
const std = #import("std");
const Context = struct {
};
fn SomeClass() type {
return struct {
fn call(context: Context) u32 {
_ = context;
return 42;
}
};
}
test "doing a thing" {
{
const calc_result = SomeClass().call(.{});
try std.testing.expectEqual(calc_result, 42);
}
}
Or, use anytype:
fn call(context: anytype) u32 { ... }
I am trying to write some externs to some Lua libraries that require to pass dictionary tables and I want to make them type safe.
So far, I have been declaring abstract classes with public inline constructors, but this gets tedious really fast:
abstract JobOpts(Table<String, Dynamic>) {
public inline function new(command:String, args:Array<String>) {
this = Table.create(null, {
command: command,
arguments: Table.create(args)
});
}
}
Is there a better way that allows me to keep things properly typed but that does not require that much boilerplate?
Please note that typedefs and anonymous structures are not valid options, because they introduce nasty fields in the created table and also do a function execution to assign a metatable to them:
--typedef X = {cmd: String}
_hx_o({__fields__={cmd=true},cmd="Yo"})
My abstract code example compiles to a clean lua table, but it is a lot of boilerplate
Some targets support #:nativeGen to strip Haxe-specific metadata from objects, but this does not seem to be the case for typedefs on Lua target. Fortunately, Haxe has a robust macro system so you can make the code write itself. Say,
Test.hx:
import lua.Table;
class Test {
public static function main() {
var q = new JobOpts("cmd", ["a", "b"]);
Sys.println(q);
}
}
#:build(TableBuilder.build())
abstract JobOpts(Table<String, Dynamic>) {
extern public inline function new(command:String, args:Array<String>) this = throw "no macro!";
}
TableBuilder.hx:
import haxe.macro.Context;
import haxe.macro.Expr;
class TableBuilder {
public static macro function build():Array<Field> {
var fields = Context.getBuildFields();
for (field in fields) {
if (field.name != "_new") continue; // look for new()
var f = switch (field.kind) { // ... that's a function
case FFun(_f): _f;
default: continue;
}
// abstract "constructors" transform `this = val;`
// into `{ var this; this = val; return this; }`
var val = switch (f.expr.expr) {
case EBlock([_decl, macro this = $x, _ret]): x;
default: continue;
}
//
var objFields:Array<ObjectField> = [];
for (arg in f.args) {
var expr = macro $i{arg.name};
if (arg.type.match(TPath({ name: "Array", pack: [] } ))) {
// if the argument's an array, make an unwrapper for it
expr = macro lua.Table.create($expr, null);
}
objFields.push({ field: arg.name, expr: expr });
}
var objExpr:Expr = { expr: EObjectDecl(objFields), pos: Context.currentPos() };
val.expr = (macro lua.Table.create(null, $objExpr)).expr;
}
return fields;
}
}
And thus...
Test.main = function()
local this1 = ({command = "cmd", args = ({"a","b"})});
local q = this1;
_G.print(Std.string(q));
end
Do note, however, that Table.create is a bit of a risky function - you will only be able to pass in array literals, not variables containing arrays. This can be remedied by making a separate "constructor" function with the same logic but without array➜Table.create unwrapping.
I can't figure out how to wrap a GLib.Array in a GLib.Value.
I tried this code.
public int main (string[] args) {
var value = Value(typeof (Array));
var a = new Array<string>();
a.append_val("test");
value.set_object((Object) a);
return 0;
}
But it resulted in these errors.
(process:1797): GLib-GObject-WARNING **: invalid uninstantiatable type '(null)' in cast to 'GObject'
(process:1797): GLib-GObject-CRITICAL **: g_value_set_object: assertion 'G_VALUE_HOLDS_OBJECT (value)' failed
I think of GValue being used as a container for a single type of value, whereas GArray is a collection. GValue does have a type_compatible () method. Using that method as a check before your code shows GLib doesn't allow a GArray to be stored as a GObject:
public int main (string[] args) {
if (!Value.type_compatible (typeof (Array), typeof (Object))) {
message ("Incompatible types");
return 1;
}
var value = Value(typeof (Array));
var a = new Array<string>();
a.append_val("test");
value.set_object((Object) a);
return 0;
}
This code stops where the check for compatible types is made.
I don't know what you are trying to achieve, but I suggest looking at GVariant:
void main () {
string[] a = {"test"};
Variant variant = a;
print (variant.print(true) + "\n");
}
If you need a GValue then you can store a GVariant in a GValue using GValue's take_variant () method.
GArray is not a GObject. However, it is a boxed type (a more primitive version of type tagging used in GValue), so you should be able to store it in the GValue using value.set_boxed (a).
Is there a native (language supported) lazy evaluation syntax? Something like lazy val in Scala.
I've gone through the docs, and could not find anything. There is only a chapter about "lazily loading a library", but it's not what I am asking.
Based on this research I incline to believe (please correct me if I'm wrong) that currently there is no such thing. But maybe you know of any plans or feature requests which will provide the functionality? Or maybe it was considered and rejected by the Dart team?
If indeed there is no native support for this, then what is the best practice (best syntax) for implementing lazy evaluation? An example would be appreciated.
Edit:
The benefits of the feature that I am looking for are mostly the same as in implementation in other languages: Scala's lazy val or C#'s Lazy<T> or Hack's __Memorize attribute:
concise syntax
delayed computation until the value is needed
cache the result (the by-need laziness)
don't break pure functional paradigm (explanation below)
A simple example:
class Fibonacci {
final int n;
int _res = null;
int get result {
if (null == _res) {
_res = _compute(this.n);
}
return _res;
}
Fibonacci(this.n);
int _compute(n) {
// ...
}
}
main(List<String> args) async {
print(new Fibonacci(5).result);
print(new Fibonacci(9).result);
}
The getter is very verbose and has a repetitive code. Moreover I can't make the constructor const because the caching variable _res has to be computed on demand. I imagine that if I had a Scala-like lazy feature then I would also have language support for having a constant constructor. That's thanks to the fact, that the lazy evaluated _res is referentially transparent, and would not be in the way.
class Fibonacci {
final int n;
int lazy result => _compute(this.n);
const Fibonacci(this.n); // notice the `const`
int _compute(n) {
// ...
}
}
main(List<String> args) async {
// now these makes more sense:
print(const Fibonacci(5).result);
print(const Fibonacci(9).result);
}
Update 2021
Lazy initialization is now part of dart from the release 2.12.
Simply add late modifier to the variable declaration
late MyClass obj = MyClass();
And this object will be initialized only when it is first used.
From the docs:
Dart 2.12 added the late modifier, which has two use cases:
Declaring a non-nullable variable that’s initialized after its
declaration.
Lazily initializing a variable.
Checkout the example here:
https://dartpad.dev/?id=50f143391193a2d0b8dc74a5b85e79e3&null_safety=true
class A {
String text = "Hello";
A() {
print("Lazily initialized");
}
sayHello() {
print(text);
}
}
class Runner {
late A a = A();
run() async {
await Future.delayed(Duration(seconds: 3));
print("First message");
a.sayHello();
}
}
Here class A will be initialized only after "First message" has been displayed.
update2
From #lrn s comment - using an Expando for caching makes it work with const:
class Lazy<T> {
static final _cache = new Expando();
final Function _func;
const Lazy(this._func);
T call() {
var result = _cache[this];
if (identical(this, result)) return null;
if (result != null) return result;
result = _func();
_cache[this] = (result == null) ? this : result;
return result;
}
}
defaultFunc() {
print("Default Function Called");
return 42;
}
main([args, function = const Lazy(defaultFunc)]) {
print(function());
print(function());
}
Try it in DartPad
update
A reusable Lazy<T> could look like below in Dart but that also doesn't work with const and can't be used in field initializers if the calculation needs to refer instance members (this.xxx).
void main() {
var sc = new SomeClass();
print('new');
print(sc.v);
}
class SomeClass {
var _v = new Lazy<int>(() {
print('x');
return 10;
});
int get v => _v();
}
class Lazy<T> {
final Function _func;
bool _isEvaluated = false;
Lazy(this._func);
T _value;
T call() {
if(!_isEvaluated) {
if(_func != null) {
_value = _func();
}
_isEvaluated = true;
}
return _value;
}
}
Try it in DartPad
original
Dart version of http://matt.might.net/articles/implementing-laziness/ using a closure to lazy evaluate:
void main() {
var x = () {
print ("foo");
return 10;
}();
print("bar");
print(x);
// will print foo, then bar then 10.
print('===');
// But, the following Scala program:
x = () {
print("foo");
return 10;
};
print ("bar");
print (x());
// will print bar, then foo, then 10, since it delays the computation of x until it’s actually needed.
}
Try it in DartPad
Update
int _val;
int get val => _val ??= 9;
Thanks #Nightscape
Old
I think this little snippet might help you...
int _val;
int get val => _val ?? _val = 9;
Does Dart support the concept of variable functions/methods? So to call a method by its name stored in a variable.
For example in PHP this can be done not only for methods:
// With functions...
function foo()
{
echo 'Running foo...';
}
$function = 'foo';
$function();
// With classes...
public static function factory($view)
{
$class = 'View_' . ucfirst($view);
return new $class();
}
I did not found it in the language tour or API. Are others ways to do something like this?
To store the name of a function in variable and call it later you will have to wait until reflection arrives in Dart (or get creative with noSuchMethod). You can however store functions directly in variables like in JavaScript
main() {
var f = (String s) => print(s);
f("hello world");
}
and even inline them, which come in handy if you are doing recusion:
main() {
g(int i) {
if(i > 0) {
print("$i is larger than zero");
g(i-1);
} else {
print("zero or negative");
}
}
g(10);
}
The functions stored can then be passed around to other functions
main() {
var function;
function = (String s) => print(s);
doWork(function);
}
doWork(f(String s)) {
f("hello world");
}
I may not be the best explainer but you may consider this example to have a wider scope of the assigning functions to a variable and also using a closure function as a parameter of a function.
void main() {
// a closure function assigned to a variable.
var fun = (int) => (int * 2);
// a variable which is assigned with the function which is written below
var newFuncResult = newFunc(9, fun);
print(x); // Output: 27
}
//Below is a function with two parameter (1st one as int) (2nd as a closure function)
int newFunc(int a, fun) {
int x = a;
int y = fun(x);
return x + y;
}