[assemblyscript/en] Learn AssemblyScript in Y minutes (adambard#4209)

* Learn AssemblyScript in Y minutes

AssemblyScript is based on TypeScript, therefore the tutorial is based on Learn Typescript in Y minutes

* Fixes for compliancy to contrib.md and Quick fix

80 characters line limit
Also return type for function

* Solve suggested changes

Changes were suggested by @SebastianSpeitel

* Adding examples on Array Type inference

As suggested by Max Graey

* Derived class use the extends keyword

Co-authored-by: Sebastian Speitel <sebastian.speitel@outlook.de>

* Fix alert conventional signature

Co-authored-by: Max Graey <maxgraey@gmail.com>

* Removed Type Assertion

Co-authored-by: Sebastian Speitel <sebastian.speitel@outlook.de>

* Fix dangling interfaces and namespace uses

Co-authored-by: Max Graey <maxgraey@gmail.com>
Co-authored-by: Sebastian Speitel <sebastian.speitel@outlook.de>

* Export functions, classes and namespaces

AssemblyScript doesn't tell about errors in a non exported body

Co-authored-by: Max Graey <maxgraey@gmail.com>
Co-authored-by: Sebastian Speitel <sebastian.speitel@outlook.de>

---------

Co-authored-by: Sebastian Speitel <sebastian.speitel@outlook.de>
Co-authored-by: Max Graey <maxgraey@gmail.com>

assemblyscript.html.markdown

@@ -0,0 +1,202 @@
+---
+language: Assemblyscript
+contributors:
+    - ["Philippe Vlérick", "https://github.com/pvlerick"]
+    - ["Steve Huguenin-Elie", "https://github.com/StEvUgnIn"]
+    - ["Sebastian Speitel", "https://github.com/SebastianSpeitel"]
+    - ["Max Graey", "https://github.com/MaxGraey"]
+filename: learnassemblyscript.ts
+---
+
+__AssemblyScript__ compiles a variant of __TypeScript__ (basically JavaScript with types) to __WebAssembly__ using __Binaryen__. It generates lean and mean WebAssembly modules while being just an `npm install` away.
+
+This article will focus only on AssemblyScript extra syntax, as opposed to [TypeScript](/docs/typescript) and [JavaScript](/docs/javascript).
+
+To test AssemblyScript's compiler, head to the
+[Playground](https://bit.ly/asplayground) where you will be able
+to type code, have auto completion and directly see the emitted WebAssembly.
+
+```ts
+// There are many basic types in AssemblyScript,
+let isDone: boolean = false;
+let name: string = "Anders";
+
+// but integer type come as signed (sized from 8 to 64 bits)
+let lines8: i8 = 42;
+let lines16: i16 = 42;
+let lines32: i32 = 42;
+let lines64: i64 = 42;
+
+// and unsigned (sized from 8 to 64 bits),
+let ulines8: u8 = 42;
+let ulines16: u16 = 42;
+let ulines32: u32 = 42;
+let ulines64: u64 = 42;
+
+// and float has two sizes possible (32/64).
+let rate32: f32 = 1.0
+let rate64: f64 = 1.0
+
+// But you can omit the type annotation if the variables are derived
+// from explicit literals
+let _isDone = false;
+let _lines = 42;
+let _name = "Anders";
+
+// Use const keyword for constants
+const numLivesForCat = 9;
+numLivesForCat = 1; // Error
+
+// For collections, there are typed arrays and generic arrays
+let list1: i8[] = [1, 2, 3];
+// Alternatively, using the generic array type
+let list2: Array<i8> = [1, 2, 3];
+
+// For enumerations:
+enum Color { Red, Green, Blue };
+let c: Color = Color.Green;
+
+// Functions imported from JavaScript need to be declared as external
+// @ts-ignore decorator
+@external("alert")
+declare function alert(message: string): void;
+
+// and you can also import JS functions in a namespace
+declare namespace window {
+  // @ts-ignore decorator
+  @external("window", "alert")
+  function alert(message: string): void;
+}
+
+// Lastly, "void" is used in the special case of a function returning nothing
+export function bigHorribleAlert(): void {
+  alert("I'm a little annoying box!"); // calling JS function here
+}
+
+// Functions are first class citizens, support the lambda "fat arrow" syntax
+
+// The following are equivalent, the compiler does not offer any type
+// inference for functions yet, and same WebAssembly will be emitted.
+export function f1 (i: i32): i32 { return i * i; }
+// "Fat arrow" syntax
+let f2 = (i: i32): i32 => { return i * i; }
+// "Fat arrow" syntax, braceless means no return keyword needed
+let f3 = (i: i32): i32 => i * i;
+
+// Classes - members are public by default
+export class Point {
+  // Properties
+  x: f64;
+
+  // Constructor - the public/private keywords in this context will generate
+  // the boiler plate code for the property and the initialization in the
+  // constructor.
+  // In this example, "y" will be defined just like "x" is, but with less code
+  // Default values are also supported
+
+  constructor(x: f64, public y: f64 = 0) {
+    this.x = x;
+  }
+
+  // Functions
+  dist(): f64 { return Math.sqrt(this.x * this.x + this.y * this.y); }
+
+  // Static members
+  static origin: Point = new Point(0, 0);
+}
+
+// Classes can be explicitly marked as extending a parent class.
+// Any missing properties will then cause an error at compile-time.
+export class PointPerson extends Point {
+  constructor(x: f64, y: f64, public name: string) {
+    super(x, y);
+  }
+  move(): void {}
+}
+
+let p1 = new Point(10, 20);
+let p2 = new Point(25); //y will be 0
+
+// Inheritance
+export class Point3D extends Point {
+  constructor(x: f64, y: f64, public z: f64 = 0) {
+    super(x, y); // Explicit call to the super class constructor is mandatory
+  }
+
+  // Overwrite
+  dist(): f64 {
+    let d = super.dist();
+    return Math.sqrt(d * d + this.z * this.z);
+  }
+}
+
+// Namespaces, "." can be used as separator for sub namespaces
+export namespace Geometry {
+  class Square {
+    constructor(public sideLength: f64 = 0) {
+    }
+    area(): f64 {
+      return Math.pow(this.sideLength, 2);
+    }
+  }
+}
+
+let s1 = new Geometry.Square(5);
+
+// Generics
+// AssemblyScript compiles generics to one concrete method or function per set 
+// of unique contextual type arguments, also known as [monomorphisation]. 
+// Implications are that a module only includes and exports concrete functions 
+// for sets of type arguments actually used and that concrete functions can be 
+// shortcutted with [static type checks] at compile time, which turned out to 
+// be quite useful.
+// Classes
+export class Tuple<T1, T2> {
+  constructor(public item1: T1, public item2: T2) {
+  }
+}
+
+export class Pair<T> {
+  item1: T;
+  item2: T;
+}
+
+// And functions
+export function pairToTuple <T>(p: Pair<T>): Tuple<T, T> {
+  return new Tuple(p.item1, p.item2);
+};
+
+let tuple = pairToTuple<string>({ item1: "hello", item2: "world" });
+
+// Including references to a TypeScript-only definition file:
+/// <reference path="jquery.d.ts" />
+
+// Template Strings (strings that use backticks)
+// String Interpolation with Template Strings
+let name = 'Tyrone';
+let greeting = `Hi ${name}, how are you?`
+// Multiline Strings with Template Strings
+let multiline = `This is an example
+of a multiline string`;
+
+let numbers: Array<i8> = [0, 1, 2, 3, 4];
+let moreNumbers: Array<i8> = numbers;
+moreNumbers[5] = 5; // Error, elements are read-only
+moreNumbers.push(5); // Error, no push method (because it mutates array)
+moreNumbers.length = 3; // Error, length is read-only
+numbers = moreNumbers; // Error, mutating methods are missing
+
+// Type inference in Arrays
+let ints = [0, 1, 2, 3, 4]  // will infer as Array<i32>
+let floats: f32[] = [0, 1, 2, 3, 4]  // will infer as Array<f32>
+let doubles = [0.0, 1.0, 2, 3, 4]  // will infer as Array<f64>
+let bytes1 = [0 as u8, 1, 2, 3, 4]  // will infer as Array<u8>
+let bytes2 = [0, 1, 2, 3, 4]  as u8[] // will infer as Array<u8>
+let bytes3: u8[] = [0, 1, 2, 3, 4] // will infer as Array<u8>
+
+```
+
+## Further Reading
+ * [AssemblyScript Official website] (https://www.assemblyscript.org/)
+ * [AssemblyScript source documentation] https://github.com/AssemblyScript/website/tree/main/src)
+ * [Source Code on GitHub] (https://github.com/AssemblyScript/assemblyscript)