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.
For instance, in Javascript I can do something like:
class Foo {
x = 'baz';
bar() {
const someVar = 'x';
console.log(this[someVar]);
// Output: 'baz';
}
}
Hopefully that's relatively clear - it boils down to accessing a member variable by another variable's contents. How is this achieved in Dart?
This is not trivial in Dart. Dart doesn't have a syntax to access class properties with [].
There are a couple of approaches though:
Mirrors:
https://api.dartlang.org/stable/2.6.1/dart-mirrors/dart-mirrors-library.html
Basically you have access to everything and offers the biggest freedom. You can check what properties a class has, access them via names and so on. Big disadvantage is that the generated JS (if targeting web) will be huge. Flutter doesn't support it at all.
Reflectable
To deal with the large generated JS, you can use package:reflectable. Never tried it with Flutter. It's a bit more to set up and start using bit it works.
Dart only solution 1
You can overload [] operator on a class:
class Foo {
final _backing = <String, String>{
'foo': 'bar'
};
operator [](String val) {
return _backing[val];
}
}
void main() {
final inst = Foo();
print(inst['foo']);
}
Dart only solution 2
Just use a map :) Well sort of... If you are dealing with complex types and you want to add some extra functionality to your map, you can do something like this:
import 'dart:collection';
class StringMap extends Object with MapMixin<String, String> {
final _backing = <String, String>{};
#override
String operator [](Object key) {
return _backing[key];
}
#override
void operator []=(String key, String value) {
_backing[key] = value;
}
#override
void clear() {
_backing.clear();
}
#override
Iterable<String> get keys => _backing.keys;
#override
String remove(Object key) {
return _backing.remove(key);
}
}
In Java when you are defining an enum, you can do something similar to the following, i.e. add members to an enum. Is this possible in Dart?
enum Foo {
one(1), two(2);
final num value;
Foo(this.value);
}
Starting with Dart 2.6 you can define extensions on classes (Enums included).
enum Cat {
black,
white
}
extension CatExtension on Cat {
String get name {
switch (this) {
case Cat.black:
return 'Mr Black Cat';
case Cat.white:
return 'Ms White Cat';
default:
return null;
}
}
void talk() {
print('meow');
}
}
Example:
Cat cat = Cat.black;
String catName = cat.name;
cat.talk();
Here's one more live example (uses a constant map instead of a switch):
https://dartpad.dartlang.org/c4001d907d6a420cafb2bc2c2507f72c
Dart Enhanced Enum Classes
Starting with Dart 2.17, the Enhanced Enum Classes feature has been introduced. With that, the example from the question would look like this:
enum Foo {
one(1),
two(2);
const Foo(this.value);
final num value;
}
Now, you can just use the enum class like this:
void main() {
const foo = Foo.one;
print(foo.value); // 1
}
Note that you need to update your SDK constraint as the feature requires Dart 2.17:
environment:
sdk: '>=2.17.0-0 <3.0.0'
Adding members
With enhanced enums, you can add any member to your enum as long as the constructor is const.
This also means that you can add getters or methods to existing enums, for example:
enum Cake {
cherry,
apple,
strawberry;
String get description => '$name cake';
}
Generics
Enhanced enum classes also enable you to use generics for you enums. If you combine this with members, you can do the following:
enum Bar<T extends Object> {
number<int>(42),
name<String>('creativecreatorormaybenot'),
baz(true); // Note that type inference also works.
const Bar(this.value);
final T value;
}
Mixins and interfaces
In addition to declaring members, you can also mixin mixins and implement interfaces with enhanced enums and override any missing implementations.
mixin Foo {
int get n;
}
abstract class Bar {
void printNumber();
}
enum Baz with Foo implements Bar {
one(1),
two(2);
const Baz(this.n);
#override
final int n;
#override
void printNumber() => print(n);
}
Multiple arguments
Finally note that even if I did not make use of it in any of the examples above, it is possible to have an arbitrary number of arguments (and an initializer list):
enum Foo {
bar(42, description: 'The answer to life, the universe, and everything.'),
baz(0, enabled: false, description: 'noop');
const Foo(
int number, {
this.enabled = true,
required this.description,
}) : n = number;
final int n;
final bool enabled;
final String description;
}
Dart enums are used only for the simplest cases. If you need more powerful or more flexible enums, use classes with static const fields like shown in https://stackoverflow.com/a/15854550/217408
This way you can add whatever you need.
Nope. In Dart, enums can only contain the enumerated items:
enum Color {
red,
green,
blue
}
However, each item in the enum automatically has an index number associated with it:
print(Color.red.index); // 0
print(Color.green.index); // 1
You can get the values by their index numbers:
print(Color.values[0] == Color.red); // True
See: https://www.dartlang.org/guides/language/language-tour#enums
It may not be "Effective Dart" , I add a static method inside a Helper class ( there is no companion object in Dart) .
In your color.dart file
enum Color {
red,
green,
blue
}
class ColorHelper{
static String getValue(Color color){
switch(color){
case Color.red:
return "Red";
case Color.green:
return "Green";
case Color.blue:
return "Blue";
default:
return "";
}
}
}
Since the method is in the same file as the enum, one import is enough
import 'package:.../color.dart';
...
String colorValue = ColorHelper.getValue(Color.red);
extension is good, but it cannot add static methods. If you want to do something like MyType.parse(string), consider using a class with static const fields instead (as Günter Zöchbauer suggested before).
Here is an example
class PaymentMethod {
final String string;
const PaymentMethod._(this.string);
static const online = PaymentMethod._('online');
static const transfer = PaymentMethod._('transfer');
static const cash = PaymentMethod._('cash');
static const values = [online, transfer, cash];
static PaymentMethod parse(String value) {
switch (value) {
case 'online':
return PaymentMethod.online;
break;
case 'transfer':
return PaymentMethod.transfer;
break;
case 'cash':
return PaymentMethod.cash;
default:
print('got error, invalid payment type $value');
return null;
}
}
#override
String toString() {
return 'PaymentMethod.$string';
}
}
I found this much handier than using a helper function.
final method = PaymentMethod.parse('online');
assert(method == PaymentMethod.online);
I did this (inspired form the accepted answer by #vovahost)
enum CodeVerifyFlow {
SignUp, Recovery, Settings
}
extension CatExtension on CodeVerifyFlow {
String get name {
return ["sign_up", "recovery", "settings"][index];
}
}
// use it like
CodeVerifyFlow.SignUp.name
thank me later!
There's an upcoming feature in Dart known as enhanced enums, and it allows for enum declarations with many of the features known from classes. For example:
enum Blah {
one(1), two(2);
final num value;
const Blah(this.value);
}
The feature is not yet released (and note that several things are not yet working), but experiments with it can be performed with a suitably fresh version of the tools by passing --enable-experiment=enhanced-enums.
The outcome is that Blah is an enum declaration with two values Blah.one and Blah.two, and we have Blah.one.value == 1 and Blah.two.value == 2. The current bleeding edge handles this example in the common front end (so dart and dart2js will handle it), but it is not yet handled by the analyzer.
As an improvement on the other suggestions of using Extensions, you can define your assigned values in a list or map, and the extension will be concise.
enum Numbers {
one,
two,
three,
}
// Numbers.one.value == 1
// Numbers.two.value == 2
// Numbers.three.value == 3
example with list
extension NumbersExtensionList on Numbers {
static const values = [1, 2, 3];
int get value => values[this.index];
}
example with map
extension NumbersExtensionMap on Numbers {
static const valueMap = const {
Numbers.one: 1,
Numbers.two: 2,
Numbers.three: 3,
};
int get value => valueMap[this];
}
Note: This approach has the limitation that you can not define a static factory method on the Enum, e.g. Numbers.create(1) (as of Dart 2.9). You can define this method on the NumbersExtension, but it would need to be called like NumbersExtension.create(1)
For String returns :
enum Routes{
SPLASH_SCREEN,
HOME,
// TODO Add according to your context
}
String namedRoute(Routes route){
final runtimeType = '${route.runtimeTypes.toString()}.';
final output = route.toString();
return output.replaceAll(runtimeType, "");
}
You can add extra fields and methods with my package enum_extendable.
It generates extensions on enum, so you can use your enum values in the similar way to instances of a regular Dart class.
For example, if you have enum MathOperator { plus, minus } the symbol and calculate(...) can be added to it.
So, the enum can be used in such way:
final n1 = 1;
final n2 = 2.0;
MathOperator.values.forEach((operator) {
print('$n1 ${operator.symbol} $n2 = ${operator.calculate(n1, n2)}');
});
Usage:
Add dependencies to pubspec.yaml:
dependencies:
enum_extendable_annotation:
dev_dependencies:
build_runner:
enum_extendable_generator:
Install these dependencies:
# Dart
pub get
# Flutter
flutter packages get
Add imports to your enum file:
import 'package:enum_extendable_annotation/enum_extendable_annotation.dart';
part '<your enum file name>.enum_extendable.g.dart';
Create a PODO class with fields and methods you wanted.
Create a map with instances of this PODO class for each enum value.
Annotate elements:
the enum with #ExtendableEnum();
the PODO class - #ExtendableEnumPodo();
the map of PODO instances - #ExtendableEnumValues().
Run code generator:
if your package depends on Flutter:
flutter pub run build_runner build
if your package does not depend on Flutter:
dart pub run build_runner build
The file with extensions should be generated.
Example of the enum file:
import 'package:enum_extendable_annotation/enum_extendable_annotation.dart';
part 'math_operator.enum_extendable.g.dart';
#ExtendableEnum()
enum MathOperator { plus, minus }
#ExtendableEnumPodo()
class _MathOperatorPodo {
final String symbol;
final num Function(num, num) calculate;
_MathOperatorPodo(
this.symbol,
this.calculate,
);
#ExtendableEnumValues()
static final Map<MathOperator, _MathOperatorPodo> _values = {
MathOperator.plus: _MathOperatorPodo(
'+',
(n1, n2) => n1 + n2,
),
MathOperator.minus: _MathOperatorPodo(
'-',
(n1, n2) => n1 - n2,
),
};
}
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'm using the style class below to mimick enums (from Does Dart support enumerations?)
It is working fine in that this snippet produces expected results.
void main() {
InterpolationType it = InterpolationType.LINEAR;
print("it is $it and stringified ${stringify(it)}");
print(InterpolationType.fromJson(it.toJson()));
}
But the DartEditor is complaining about "Expected constant expression" in the case statements of fromJson method. Is there a const I can throw in somewhere to get rid of this complaint?
class InterpolationType {
static const LINEAR = const InterpolationType._(0);
static const STEP = const InterpolationType._(1);
static const CUBIC = const InterpolationType._(2);
static get values => [
LINEAR,
STEP,
CUBIC
];
final int value;
const InterpolationType._(this.value);
String toString() {
switch(this) {
case LINEAR: return "LINEAR";
case STEP: return "STEP";
case CUBIC: return "CUBIC";
}
}
int toJson() {
return this.value;
}
static InterpolationType fromJson(int v) {
switch(v) {
case LINEAR.value: return LINEAR;
case STEP.value: return STEP;
case CUBIC.value: return CUBIC;
}
}
static InterpolationType fromString(String s) {
switch(s) {
case "LINEAR": return LINEAR;
case "STEP": return STEP;
case "CUBIC": return CUBIC;
}
}
}
As you discovered: accessing fields from a const object is not a constant operation. So the editor (as well as the VM and dart2js) are right.
With the current syntax there is no way to express a (informal) contract that a field of a class will always be a final field. For example, I could change the value-field to be a getter instead of a field. The interface-contract of the class definitely allows me to do that, because I never told anybody that I would keep "value" as a field. However if I did that it would break every program that relied on the existence of this final field.
As a consequence the current behavior is very unlikely to change.
However: in theory it would be possible to improve the Dart language so that you could use "const" instead of "final" for local fields, and initialize them with initializer lists. And in this case accessing the field could be considered a constant operation. I currently don't see any downsides to this behavior and it would be backwards-compatible.
// WARNING: What follows DOES NOT WORK, just a potential example
class InterpolationType {
const value; // Note the "const" instead of "final".
const InterpolationType._(this.value);
}
The language is already pretty stable but you can open a bug at http://dartbug.com/ and suggest this behavior. It's not very likely that the feature-request would be accepted, but it's definitely worth a try.