In Dart, we can declare a local variable without initializing it (and yes, it works with null safety too):
void conditionalInit(bool something) {
int x;
if (something) {
x = 1;
print(x); // OK
}
}
If so, what is the added benefit of late modifier? The only one I can think of is that it silences the error of messy conditions like this:
void conditionalInit(bool something) {
int x;
if (something) {
x = 1;
print(x); // OK
}
if (something) {
print(x); // This only compiles with late declaration
}
}
But that should be avoided anyway.
So is there a legitimate use for local late variables?
Here is one possible use for a local late variable:
import 'dart:async';
Stream<int> countToFive() {
late StreamController<int> controller;
controller = StreamController(
onListen: () {
for (int i = 0; i < 5; i++) {
controller.add(i + 1);
}
controller.close();
},
);
return controller.stream;
}
Future<void> main() async {
await for (final number in countToFive()) {
print(number);
}
}
Because controller is late it is possible to reference controller within the onListen callback passed into the constructor of the StreamController.
I think the main use case is lazy initialization. A late variable will only be evaluated when it is accessed, which can be useful for costly computations which may not be required at runtime.
late is also useful to assign immutable final variables after initialization.
void conditionalInit(bool something) {
late final int x;
if (something) {
x = 1;
print(x); // OK
}
}
}
Related
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;
I can conveniently change opsCount variable directly from inside the function,
because there is only one of that type of variable.
int opsCount = 0;
int jobXCount = 0;
int jobYCount = 0;
int jobZCount = 0;
void doStats(var jobCount) {
opsCount++;
jobCount++;
}
main() {
doStats(jobXCount);
}
But there are many jobCount variables, so how can I change effectively that variable, which is used in parameter, when function is called?
I think I know what you are asking. Unfortunately, the answer is "you can't do this unless you are willing to wrap your integers". Numbers are immutable objects, you can't change their value. Even though Dart's numbers are objects, and they are passed by reference, their intrinsic value can't be changed.
See also Is there a way to pass a primitive parameter by reference in Dart?
You can wrap the variables, then you can pass them as reference:
class IntRef {
IntRef(this.val);
int val;
#override
String toString() => val.toString();
}
IntRef opsCount = new IntRef(0);
IntRef jobXCount = new IntRef(0);
IntRef jobYCount = new IntRef(0);
IntRef jobZCount = new IntRef(0);
void doStats(var jobCount) {
opsCount.val++;
jobCount.val++;
}
main() {
doStats(jobXCount);
print('opsCount: $opsCount; jobXCount: $jobXCount; jobYCount: $jobYCount; jobZCount: $jobZCount');
}
EDIT
According to Roberts comment ..
With a custom operator this would look like:
class IntRef {
IntRef(this.val);
int val;
#override
String toString() => val.toString();
operator +(int other) {
val += other;
return this;
}
}
void doStats(var jobCount) {
opsCount++;
jobCount++;
}
How I can return Future value from Future object?
This code does not work.
import 'dart:async';
void main() {
var temp = foo();
temp.then((Future<int> future) {
future.then((int result) {
print(result);
});
});
}
Future<Future<int>> foo() {
return new Future<Future<int>>(() {
return new Future<int>(() => 5);
});
}
How to prevent unnecessary unwrapping?
In this case in async library 'Future' declared as generic class.
abstract class Future<T> {
}
If I create expression as the following
new Future<Future<int>>();
Then with type T specified as Future<int> which result expected from generic class Future?
I thing that result must be as specified in type argument T.
I.e. Future<int>.
But result is not as expected.
There is no information found about this abnormal behavior on Dart API site.
If this is a "feature" (but I think that abnormal behavior wrongly to call "feature') then why it not documented in Dart API?
How can be explained this discrepancy?
Why this code not generated errors and warnings?
Another IDENTICAL example but w/o using Future.
void main() {
var temp = foo();
temp.baz((Foo<int> foo) {
foo.baz((int result) {
print(result);
});
});
}
Foo<Foo<int>> foo() {
return new Foo<Foo<int>>(() {
return new Foo<int>(() => 5);
});
}
If in this case result will be as when using Future (i.e. unexpected) then how we can call this code?
Normal or abnormal?
Or maybe the Future in Dart some special (magic)?
Look at the api documentation
http://api.dartlang.org/docs/releases/latest/dart_async/Future.html
It says there:
If the returned value is itself a Future, completion of the created future will wait until
the returned future completes, and will then complete with the same result.
I guess that means you can't return a Future from a Future.
But you could return a list of futures.
void main() {
var temp = foo();
temp.then((List<Future<int>> list) {
list[0].then((int result) {
print(result);
});
});
}
Future<List<Future<int>>> foo() {
return new Future<List<Future<int>>>(() {
return [new Future<int>(() => 5)];
});
}
There is no need for any of that extra wrapping. According to the Future documentation:
If the returned value is itself a [Future], completion of the created
future will wait until the returned future completes, and will then
complete with the same result.
This means you can rewrite your code as:
import 'dart:async';
void main() {
var temp = foo();
temp.then((int result) {
print(result);
});
}
Future<int> foo() {
return new Future<int>(() {
return new Future<int>(() => 5);
});
}
This is a lot cleaner to work with and provides the expected result.
I still havn't understood how to deal with Iterable/Iterator in Dart.
I think I have to give up and simply return Lists but that's not what I want since it will
lead bad performance in my case.
What I want is to understand how to implement my own Iterable/Iterator.
Why do both of these attempts fail?
library foo;
import 'dart:collection';
// Both attemps below raises the following error:
// ==============================================
//
// Closure call with mismatched arguments: function 'moveNext'
//
// NoSuchMethodError: incorrect number of arguments passed to method named 'moveNext'
// Receiver: Closure: (dynamic) => Iterator<int> from Function 'iterator':.
// Tried calling: moveNext()
main() {
Iterable<int> iterable1 = new OddsIterableDartStyle([1,2,4,6,7,8,9]);
for (int i in iterable1)
print("ODD: $i");
Iterable<int> iterable2 = new OddsIterableJavaStyle([1,2,4,6,7,8,9]);
for (int i in iterable2)
print("ODD: $i");
}
// ------------------------------------------
class OddsIterableDartStyle extends Object with IterableMixin<int> {
List<int> _ints;
OddsIterableDartStyle(this._ints);
Iterator<int> iterator() {
return new OddsIterator(this);
}
}
// ------------------------------------------
class OddsIterableJavaStyle implements Iterable<int> {
List<int> _ints;
OddsIterableJavaStyle(this._ints);
Iterator<int> iterator() {
return new OddsIterator(this);
}
}
// ------------------------------------------
class OddsIterator implements Iterator<int> { // Iterate over odd numbers
List<int> _ints;
int _index;
OddsIterator(this._ints) {
_index = -1;
}
bool moveNext() {
while (++_index < _ints.length) {
if (_ints[_index].isOdd)
return true;
}
return false;
}
int get current => (_index < 0) ? null : _ints[_index];
}
I see two immediate problems:
iterator is a getter. The code shouldn't read Iterator<int> iterator() { ... }, it should be Iterator<int> get iterator { ... } instead.
Your iterators are expecting the underlying integer lists, but you are passing in the wrapper. You probably want to construct your iterator like new OddsIterator(_ints), not like new OddsIterator(this).
Btw, Iterator is supposed to return null if you call current and you have already moved beyond the end.
class Count extends Iterable with Iterator {
Count([this.limit = 10]);
int limit;
int i = 0;
#override
int get current => i;
#override
bool moveNext() {
i++;
return i <= limit;
}
#override
Iterator get iterator => this;
}
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;
}