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Wing Programming Language Reference

0. Preface

0.1 Motivation

The Wing Programming Language (aka Winglang) is a general purpose programming language designed for building applications for the cloud.

What makes Wing special? Traditional programming languages are designed around the premise of telling a single machine what to do. The output of the compiler is a program that can be executed on that machine. But cloud applications are distributed systems that consist of code running across multiple machines and which intimately use various cloud services to achieve their business goals.

Wing’s goal is to allow developers to express all pieces of a cloud application using the same programming language. This way, we can leverage the power of the compiler to deeply understand the intent of the developer and implement it with the mechanics of the cloud.

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0.2 Design Tenets

  • Developer Experience (DX) is priority #1 for Wing.
  • Syntax design aims to be concise and minimal, while being "batteries included" at the same time in terms of tooling and DX.
  • Developers coming from other mainstream cloud languages (C#, Java, and TS) should feel right at home.
  • Public facing APIs and syntax are designed to be compatible with JSII. Wing Libraries are JSII libraries themselves.
  • All clouds are treated equally.
  • Syntactic sugar comes last.

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0.3 Table of Contents

1. General

1.1 Types

1.1.1 Primitive Types

NameExtra information
voidrepresents the absence of a type
numrepresents numbers (doubles)
strUTF-16 encoded strings
boolrepresents true or false
let x = 1;                  // x is a num
let v = 23.6; // v is a num
let y = "Hello"; // y is a str
let z = true; // z is a bool
let q: num? = nil; // q is an optional num

Numeric literals can be formatted and padded with extra zeroes or underscores to make them easier to read in source code. These don't affect the value of the number or how they are printed:

let price = 0012.34;
let twentyThousand = 20_000;
let aBitMore = 20_000.000_1;

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1.1.2 Container Types

NameExtra information
Array<T>variable size array of a certain type
Map<T>map type (key-value with string keys, keys may be any expression evaluating to a string)
Set<T>set type (unordered collection of unique items)
MutArray<T>mutable array type
MutMap<T>mutable map type
MutSet<T>mutable set type
let y = [1, 2, 3];               // immutable array, Array<num> is inferred
let ym = MutArray<num>[1, 2, 3]; // mutable array
let x = {"a" => 1, "b" => 2}; // immutable map, Map<num> is inferred
let xm = MutMap<num>{}; // mutable map
let z = Set<num>[1, 2, 3]; // immutable set
let zm = MutSet<num>[1, 2, 3]; // mutable set
let w = new SampleClass(); // class instance (mutability unknown)

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1.1.3 Function Types

Function type annotations are written as if they were closure declarations, with the difference that body is replaced with return type annotation.

The inflight modifier indicates that a function is an inflight function.
inflight in Wing implies async in JavaScript.

(arg1: <type1>, arg2: <type2>, ...): <returnType> => <type>
inflight (arg1: <type1>, arg2: <type2>, ...): <returnType> => <type>
// type annotation in wing: (num) => num
let f1 = (x: num): num => { return x + 1; };
// type annotation in wing: inflight (num, str) => void
let f2 = inflight (x: num, s: str) => { /* no-op */ };

Return type is required for function types.

let my_func = (callback: (num): void) => {  };
let my_func2 = (callback: ((num): void): (str): void) => { };

Return type is optional for closures.

let my_func3 = (x: num) => {  };
let my_func4 = (x: num): void => { };
let my_func5 = inflight (x: num) => { };
let my_func6 = inflight (x: num): void => { };

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1.1.4 Json type

🚧 Json support is still a work in progress 🚧
Check out the roadmap section below, to see what parts are still not implemented.

Wing has a primitive data type called Json. This type represents an immutable untyped JSON value, including JSON primitives (string, number, boolean), arrays (both heterogenous and homogeneous) and objects (key-value maps where keys are strings and values can be any other JSON value).

Json objects are immutable and can be referenced across inflight context.

JSON is the "wire protocol of the cloud" and as such Wing offers built-in support for it. However, since Wing is statically-typed (type must be known during compilation) and JSON is dynamically typed (type is only known at runtime), bridging is required between these two models.

Let's look at a quick example:

struct Employee { 
id: str;
name: str;

let response = http.get("/employees");
// returns something like { "items": [ { "id": "12234", "name": "bob" }, ... ] }

let employees = Array<Employee>.fromJson(response.get("items")); //NOTE: Array.fromJson is currently not implemented

for e in employees {
log("hello, {}, your employee id is {}");

In the above example, the http.get function returns a Json object from the server that has a single field items, with a JSON array of JSON objects, each with an id and name fields.

The expression response.get("items") returns a Json array, and we use Array<T>.fromJson to convert this array from Json to an Array<Employee>. Note that by default fromJson will perform schema validation on the array and on each item (based on the declaration of the Employee struct). Literals

Literals can be defined using the Json type initializers:

let jsonString  = Json "hello";
let jsonNumber = Json 123;
let jsonBool = Json true;
let jsonArray = Json [ 1, 2, 3 ];
let jsonObj = Json { boom: 123 };
let jsonMutObj = MutJson {
hello: 123,
world: [ 1, "cat", 3 ], // <-- heterogenous array
"boom boom": { hello: 1233 } // <-- non-symbolic key

The Json keyword can be omitted from Json object literals:

let jsonObj = { boom: 123, bam: [4, 5, 6] };

Every value within a Json array or object also has a type of Json. JSON objects

To access a field within an object, use .get("{field name}"):

let boom: Json = jsonObj.get("boom");

Trying to access a non-existent field will fail at runtime. For example:

// ERROR: Cannot read properties of undefined (reading 'world')

To obtain an array of all the keys, use Json.keys(o):

let j = Json { hello: 123, world: [1, 2, 3] };
assert(Json.keys(j).at(0) == "hello");
assert(Json.keys(j).at(1) == "world");

To obtain an array of all the values, use Json.values(o):

assert(Json.values(j).at(0) == 123);
assert(Json.values(j).at(1) == [1, 2, 3]);

NOTE: values() returns an array inside a Json object because at the moment we cannot represent heterogenous arrays in Wing.

To obtain an array of all key/value pairs, use Json.entries(o):

assert(Json.entries(j).at(0).getAt(0) == "hello");
assert(Json.entries(j).at(0).getAt(1) == 123);
assert(Json.entries(j).at(1).getAt(0) == "world");
assert(Json.entries(j).at(1).getAt(1) == [1, 2, 3]);

NOTE: entries() returns an array inside a Json object because at the moment we cannot represent heterogenous arrays in Wing. Assignment from native types

It is also possible to assign the native str, num, bool and Array<T> values and they will implicitly be casted to Json:

let myStr: str = "hello";
let myNum: num = 183;
let myBool: bool = true;
let myArr: Array<num> = [1,2,3];

let jsonObj = Json {
a: myString,
b: myNum,
c: myBool,
d: myArr
}; Assignment to native types

If the Json object is statically known to structurally match a certain type, it is possible to assign it to a variable of that type with no runtime cost:

let j = Json "hello";
let s: str = j;

struct J2 { a: num; }
let j2: J2 = { a: 2 }

This can only be done when the Json literal is present in the program. Otherwise, we cannot guarantee safety.

let response = http.get("/employees");
let s: str = response;
// ^ cannot assign `Json` to `str`.

To dynamically assign a Json to a strong-type variable, use the fromJson() static method on the target type:

let myStr = str.fromJson(jsonString);
let myNumber = num.fromJson(jsonNumber); Schema validation

All fromJson() methods will validate that the runtime type is compatible with the target type in order to ensure type safety (at a runtime cost):

str.fromJson(jsonNumber);      // RUNTIME ERROR: unable to parse number `123` as a string.
num.fromJson(Json "\"hello\""); // RUNTIME ERROR: unable to parse string "hello" as a number

For each fromJson(), there is a tryFromJson() method which returns an optional T? which indicates if parsing was successful or not:

let s = str.tryFromJson(myJson) ?? "invalid string";

Use unsafe: true to disable this check at your own risk:

let trustMe = 123;
let x = num.fromJson(trustMe, unsafe: true);
assert(x == 123); Mutability

To define a mutable JSON container, use the MutJson type:

let myObj = MutJson { hello: "dear" };

Now you can mutate the contents by assigning values:

let myObj = MutJson { hello: "dear" };
let fooNum = 123;
myObj.set("world", "world");
myObj.set("dang", [1,2,3,4]);
myObj.set("subObject", MutJson {});
myObj.get("subObject").set("arr", MutJson [1,"hello","world"]);
myObj.set("foo", fooNum);

For the sake of completeness, it is possible to also define primitives using MutJson but that's not very interesting because there is no way to mutate them:

let foo = MutJson "hello";
// ok what now?

Use the Json.deepCopyMut(MutJson json) method to get an mutable deep copy of a Json object. Use the MutJson.deepCopy(Json json) method to get an immutable deep copy of a MutJson object:

let mutObj = MutJson { hello: 123 };
let immutObj = MutJson.deepCopy(mutObj);
mutObj.set("hello", 999);
assert(immutObj.get("hello") == 123);

To delete a key from an object, use the Json.delete() method:

let myObj = MutJson { hello: 123, world: 555 };
Json.delete(myObj, "world");

let immutObj = Json { hello: 123 };
Json.delete(immutObj, "hello");
// ^^^^^^^^^ expected `JsonMut` Assignment to user-defined structs

All structs also have a fromJson() method that can be used to parse Json into a struct:

struct Contact {
first: str;
last: str;
phone: str?;

let j = Json { first: "Wing", last: "Lyly" };
let myContact = Contact.fromJson(j);
assert(myContact.first == "Wing");

When a Json is parsed into a struct, the schema will be validated to ensure the result is type-safe:

let p = Json { first: "Wing", phone: 1234 };
// RUNTIME ERROR: unable to parse Contact:
// - field "last" is required and missing
// - field "phone" is expected to be a string, got number.

Same as with primitives, it is possible to opt-out of validation using unsafe: true:

let p = Json { first: "Wing", phone: 1234 };
let x = Contact.fromJson(p, unsafe: true);
assert(x.last.len > 0); // RUNTIME ERROR Serialization

The Json.stringify(j: Json): str static method can be used to serialize a Json as a string (JSON.stringify):

let jsonString  = Json "hello";
let jsonObj = Json { boom: 123 };
assert(Json.stringify(jsonString) == "\"hello\"");
assert(Json.stringify(jsonObj) == "{\"boom\":123}");

The Json.parse(s: str): Json static method can be used to parse a string into a Json:

let j = Json.parse("{ \"boom\": 123 }");
let boom = num.fromJson(j.get("boom"));

Json.tryParse returns an optional:

let o = Json.tryParse("xxx") ?? Json [1,2,3]; Logging

A Json value can be logged using log(), in which case it will be pretty-formatted:

log("my object is: {jsonObj}");
// is equivalent to
log("my object is: {Json.stringify(jsonObj)}");

This will output:

my object is: {
boom: 123
} Roadmap

The following features are not yet implemented, but we are planning to add them in the future:

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1.1.5 Duration

The Duration (alias duration) type represents a time duration.

Duration literals are numbers with m, s, h suffixes:

let oneMinute = 1m;
let twoSeconds = 2s;
let threeHours = 3h;
let halfMinute: duration = 0.5m;


assert(oneMinute.seconds == 60);
assert(halfMinute.seconds == 30);
assert(threeHours.minutes == 180);

Duration objects are immutable and can be referenced across inflight context.

1.1.6 Datetime

The Datetime (alias datetime) type represents a single moment in time in a platform-independent format. Datetime objects are immutable and can be referenced across inflight context. Here is the initial API for the Datetime type:

struct DatetimeComponents {
year: num;
month: num;
day: num;
hour: num;
min: num;
sec: num;
ms: num;
tz: num; // timezone offset in minutes from UTC

class Datetime {
static utcNow(): Datetime; // returns the current time in UTC timezone
static systemNow(): Datetime; // returns the current time in system timezone
static fromIso(iso: str): Datetime; // creates an instance from an ISO-8601 string, represented in UTC timezone
static fromComponents(c: DatetimeComponents): Datetime;

timestamp: num; // Date.valueOf()/1000 (non-leap seconds since epoch)
timestampMs: num; // Date.valueOf() (non-leap milliseconds since epoch)

hours: num; // Date.getHours()
min: num; // Date.getMinutes()
sec: num; // Date.getSeconds()
ms: num; // Date.getMilliseconds()
dayOfMonth: num; // Date.getDate()
dayOfWeek: num; // Date.getDay()
month: num; // Date.getMonth()
year: num; // Date.getFullYear()

timezone: num; // (offset in minutes from UTC)
utc: Datetime; // returns the same time in UTC timezone

toIso(): str; // returns ISO-8601 string

A few examples:

let now = Datetime.utcNow();
log("It is now {now.month}/{now.dayOfMonth}/{now.year} at {now.hours}:{now.min}:{now.sec})");
assert(now.timezone == 0); // UTC

let t1 = DateTime.fromIso("2023-02-09T06:20:17.573Z");
log("Timezone is GMT{d.timezone() / 60}"); // output: Timezone is GMT-2
log("UTC: {t1.utc.toIso())}"); // output: 2023-02-09T06:21:03.000Z

1.2 Utility Functions

NameExtra information
loglogs str
assertchecks a condition and throws if evaluated to false
unsafeCastcast a value into a different type
nodeofobtain the tree node of a preflight object
log("Hello {name}");
assert(x > 0);

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1.3 Phase Modifiers

In Wing, we differentiate between code that executes during compilation and code that executes after the application has been deployed by referring to them as preflight and inflight code respectively.

The default (and implicit) execution context in Wing is preflight. This is because in cloud applications, the entrypoint is the definition of the app's cloud architecture, and not the code that runs within a specific machine within this cloud infrastructure.

The phase modifier inflight is allowed in the context of declaring interface and class members (methods, fields and properties). Example code is shown in the preflight classes section.

class Bucket {
// preflight method
allowPublicAccess() {


// inflight method
inflight put(key: str, contents: str): void {


Inflight members can only be accessed from an inflight context (an inflight method or an inflight closure) and preflight members can only be accessed from a preflight context (a preflight method or a preflight closure).

The inflight modifier is allowed when defining function closures or classes. This implies that these types can only be used within inflight context.

let handler = inflight () => {
log("hello, world");

inflight class Foo {
// ...

For example (continuing the Bucket example above):

let bucket = new Bucket();
// OK! We are calling a preflight method from a preflight context
// ERROR: Cannot call into inflight phase while preflight
bucket.put("file.txt", "hello");

let handler = inflight () => {
// now we are in inflight context
// OK! We are calling an inflight methods from an inflight context
bucket.put("file.txt", "hello");

Preflight classes can only be instantiated within preflight context:

class Bar {}

new Bar(); // OK! Bar is a preflight class

let handler2 = inflight() => {
new Bar(); // ERROR: Cannot create preflight class "Bar" in inflight phase

Bridge between preflight and inflight is crossed with the help of immutable data structures, "structs" (user definable and Struct), and the capture mechanism.

Preflight class methods and initializers can receive an inflight function as an argument. This enables preflight classes to define code that will be executed on a cloud compute platform such as lambda functions, docker, virtual machines etc.

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1.4 Storage Modifiers

A storage modifier is a keyword that specifies the placement of a function or variable in the program memory once compiled. Some declarations might have a temporary storage (such as a local closure definition), while others might have a permanent storage (such as a global variable).

Currently the only storage modifier is static. static indicates a definition is only available once per program and for the entire duration of that program. All statics must be defined inline and initialized right away.
Statics are not allowed on structs or interfaces.

Statics are supported in both inflight as well as preflight modes of execution.

A declaration for a static member is a member declaration whose declaration specifiers contain the keyword static. The keyword static must appear before other specifiers. More details in the classes section.

Code samples for static are not shown here. They are shown in the relevant sections below.

To avoid confusion, it is invalid to have a static and a non-static with the same name. Overloading a static is allowed however.
Accessing static is done via the type name and the . operator.

Static class fields are not supported yet, see

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1.5 Access Modifiers (member visibility)

Class members, by default, can only be accessed from within the implementation code of the same class (private). Inner classes or closures can access private members of their containing class.

class Foo {
private_field: num; // This is private by default

new() {this.private_field = 1;}

method() {
log(this.private_field); // We can access `private_field` since we're in Foo

class InnerFoo {
method(f: Foo) {
log(f.private_field); // We can access `private_field` since we're in Foo

Accessing class members of a super class can be done by adding the the protected access modifier.

class Foo {
protected protected_method() {}; // This is a `protected` method

class Bar extends Foo {
method() {
this.protected_method(); // We can access `protected_method` from a subclass

The pub access modifier makes the class member accessible from anywhere. Interface members are always public. Implementing interface members in a class requires explicitly flagging them as pub.

interface FooInterface {
interface_method(): void; // Interface definitions are always implicitly `pub`

class Foo impl FooInterface {
pub public_method() {} // This can be accessed from outside of the class implementation
pub interface_method() {} // This must be explicitly defined as `pub` since it's an interface implementation
let f = new Foo();
f.public_method(); // We can call this method from outside the class - it's public

Access modifier rules apply for both fields and methods of a class. Struct fields are always public and do not have access modifiers.

1.5.1 Method overriding and access modifiers

Private methods cannot be overridden. Overriding a method of a parent class requires the parent class's method to be either pub or protected. The overriding method can have either the same access modifier as the original method or a more permissive one. You cannot "decrease" the access level down the inheritance hierarchy, only "increase" it. In practice this means:

  • protected methods can be overridden by either a protected or a pub method.
  • pub methods can be overridden by a pub method.

Note that method overriding only applies to instance methods. static methods are not treated as part of the inheritance hierarchy.

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1.6 Reassignability

Re-assignment to variables that are defined with let is not allowed in Wing.

Variables can be reassigned to by adding the var modifier:

// wing
let var sum = 0;
for item in [1,2,3] {
sum = sum + item;

To modify a numeric value, it is also possible to use += and -= operators.

// wing
let var x = 0;
x += 5; // x == 5
x -= 10; // x == -5

Re-assignment to class fields is allowed if field is marked with var. Examples in the class section below.

var is available in the body of class declarations. Assigning var to immutables of the same type is allowed. That is similar to assigning non readonlys to readonlys in TypeScript.

By default function closure arguments are non-reassignable. By prefixing var to an argument definition you can make a re-assignable function argument:

// wing
let f = (arg1: num, var arg2: num) => {
if (arg2 > 100) {
// We can reassign a value to arg2 since it's marked `var`
arg2 = 100;

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1.7 Optionality

Nullity is a primary source of bugs in software. Being able to guarantee that a value will never be null makes it easier to write safe code without constantly having to take nullity into account.

In order to allow the compiler to offer stronger guarantees, Wing includes a higher-level concept called "optionality" which requires developers to be more intentional about working with the concept of "lack of value".

Here's a quick summary of how optionality works in Wing:

  • x: T? marks x as "optional of T". This means that x can either be nil (without a value) or have a value of type T.
  • To test for a value, the unary expression x? returns a true if x has a value and false otherwise.
  • if let y = x { } else { } is a special control flow statement which binds y inside the first block only if x has a value. Otherwise, the else block will be executed.
  • The x! notation will return the value in x if there is one, otherwise it will throw an error.
  • The x?.y?.z notation can be used to access fields only if they have a value. The type of this expression is Z? (an optional based on the type of the last component).
  • The x ?? y notation will return the value in x if there is one, y otherwise.
  • The keyword nil can be used in assignment scenarios to indicate that an optional doesn't have a value. It cannot be used to test if an optional has a value or not.
  • A type annotation in Wing can always be enclosed in parentheses: num and (num) are the same type. This is useful when you want to denote an optional function type. For example ((str):num)? means an optional function receiving a str and returning a num, while the similarly written (str):num? means a function receiving a str and returning an optional num.

1.7.1 Declaration Struct fields

One of the more common use cases for optionals is to use them in struct declarations.

struct Person {
name: str;
address: str?;

In the Person struct above, the address field is marked as optional using ?. This means that we can initialize without defining the address field:

let david = Person { name: "david" };
let jonathan = Person { name: "jonathan", address: "earth" };
assert(david.address? == false);
assert(jonathan.address? == true); Variables

Use T? to indicate that a variable is optional. To initialize it without a value use = nil.

let var x: num? = 12;
let var y: num? = nil;
assert(y? == false); // y doesn't have a value
assert(x? == true); // x has a value

// ok to reassign another value because `y` is reassignable (`var`)
y = 123;
assert(y? == true);

x = nil;
assert(x? == false); Class fields

Similarly to struct fields, fields of classes can be also defined as optional using T?:

class Foo {
myOpt: num?;
var myVar: str?;

new(opt: num?) {
this.myOpt = opt;
this.myVar = nil; // everything must be initialized, so you can use `nil` to indicate that there is no value

setMyVar(x: str) {
this.myVar = x;
} Function arguments

In the following example, the argument by is optional, so it is possible to call increment() without supplying a value for by:

let increment = (x: num, by: num?): num => {
return x + (by ?? 1);

assert(increment(88) == 89);
assert(increment(88, 2) == 90);

Non-optional arguments can only be used before all optional arguments:

let myFun = (a: str, x: num?, y: str): void => { /* ... */ };
//-----------------------------^^^^^^ ERROR: cannot declare a non-optional argument after an optional

If a function uses a keyword argument struct as the last argument, and there are other optional arguments before, it also has to be declared as optional.

let parseInt = (x: str, radix: num?, opts?: ParseOpts): num { /* ... */ };

The optionality of keyword arguments is determined by the struct field's optionality:

struct Options {
myRequired: str;
myOptional: num?;

let f = (opts: Options) => { };

f(myRequired: "hello");
f(myOptional: 12, myRequired: "dang"); Function return types

If a function returns an optional type, use the return nil; statement to indicate that the value is not defined.

struct Name { 
first: str;
last: str;

let tryParseName = (fullName: str): Name? => {
let parts = fullName.split(" ");
if parts.length < 2 {
return nil;

return Name { first:, last: };

// since result is optional, it needs to be unwrapped in order to be used
if let name = tryParseName("Neo Matrix") {
log("Hello, {name.first}!");

1.7.2 Testing using x?

To test if an optional has a value or not, you can either use x == nil or x != nil or the special syntax x?.

struct MyPerson {
name: str;
address: str?;
let myPerson = MyPerson {name: "John", address: nil};

let isAddressDefined = myPerson.address?; // type is `bool`
let isAddressReallyDefined = myPerson.address != nil; // equivalent

// or within a condition
if myPerson.address? {
log("address is defined but i do not care what it is");

// can be negated
if !myPerson.address? {
log("address is not defined");

if myPerson.address == nil {
log("no address");

1.7.3 Unwrapping using if let

The if let statement (or if let var for a reassignable variable) can be used to test if an optional is defined and unwrap it into a non-optional variable defined inside the block:

if let address = myPerson.address {
log(address); // type of address is `str`

NOTE: if let is not the same as if. For example, we currently don't support specifying multiple conditions, or unwrapping multiple optionals. This is something we might consider in the future.

1.7.4 Unwrapping or default value using ??

The ?? operator can be used to unwrap or provide a default value. This returns a value of T that can safely be used.

let address: str = myPerson.address ?? "Planet Earth";

1.7.5 Optional chaining using ?.

The ?. syntax can be used for optional chaining. Optional chaining returns a value of type T? which must be unwrapped in order to be used.

let ipAddress: str? = options.networking?.ipAddress;

if let ip = ipAddress {
log("the ip address is defined and it is: {ip}");

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1.7.6 Roadmap

The following features are not yet implemented, but we are planning to add them in the future:

1.8 Type Inference

Type can optionally be put between name and the equal sign, using a colon.
Partial type inference is allowed while using the ? keyword immediately after the variable name.

When type annotation is missing, type will be inferred from r-value type.
r-value refers to the right hand side of an assignment here.

All defined symbols are immutable (constant) by default.
Type casting is generally not allowed unless otherwise specified.

Type annotations are required for method arguments and their return value but optional for anonymous closures.

let i = 5;
let m = i;
let arrOpt: MutArray<num>? = MutArray<num> [];
let arr = Array<num>[];
let copy = arr;
let i1: num? = nil;
let i2: num? = i;

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1.9 Error Handling

Exceptions and try/catch/finally are the error mechanism. Mechanics directly translate to JavaScript. You can create a new exception with a throw call.

In the presence of try, both catch and finally are optional but at least one of them must be present. In the presence of catch the variable holding the exception (e in the example below) is optional.

throw is meant to be recoverable error handling.

try {
let x: num? = 1;
throw("hello exception");
} catch e {
} finally {

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Wing recommends the following formatting and naming conventions:

  • Interface names should start with capital letter "I".
  • Class, struct, and interface names should be PascalCased.
  • Members of classes, and interfaces cannot share the same PascalCased representation as the declaring expression itself.
  • Parentheses are optional in expressions. Any Wing expression can be surrounded by parentheses to enforce precedence, which implies that the expression inside an if/for/while statement may be surrounded by parentheses.

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1.11 Memory Management

There is no implicit memory de-allocation function, dynamic memory is managed by Wing and is garbage collected (relying on JSII target GC for the meantime).

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1.12 Execution Model

Execution model currently is delegated to the JSII target. This means if you are targeting JSII with Node, Wing will use the event based loop that Node offers.

In Wing, writing and executing at root block scope level is forbidden except for in entrypoint files (designated by main.w, *.main.w or *.test.w). Root block scope is considered special and compiler generates special instructions to properly assign all preflight classes to their respective scopes recursively down the constructs tree based on entry.

Within the entrypoint file, a root preflight class is made available for all subsequent preflight classes that are initialized and instantiated. The type of the root class is determined by the target being used by the compiler. The root class might be of type aws-cdk-lib.App in AWS CDK or cdktf.TerraformApp in case of CDK for Terraform target.

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1.13 Asynchronous Model

Wing builds upon the asynchronous model of JavaScript currently and expands upon it with new keywords and concepts. The async keyword of JavaScript is replaced with inflight in Wing deliberately to indicate extended functionality.

Main concepts to understand:

  • preflight implies synchronous execution.
  • inflight implies asynchronous execution.

Contrary to JavaScript, any call to an async function is implicitly awaited in Wing.

1.13.1 Roadmap

The following features are not yet implemented, but we are planning to add them in the future:

1.14 Roadmap

2. Statements

2.1 bring

bring statement can be used to import and reuse code from Wing and other JSII supported languages. The statement is detailed in its own section in this document: Module System.

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2.2 break

break statement allows to end execution of a cycle. This includes for and while loops.

for i in 1..10 {
if i > 5 {

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2.3 continue

continue statement allows to skip to the next iteration of a cycle. This includes for and while loops currently.

for i in 1..10 {
if i > 5 {

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2.4 return

return statement allows to return a value or exit from a called context.

class MyClass {
myMethod() {}
myMethod2(): void {}
myMethod3(): void { return; }
myMethod4(): str { return "hi!"; }

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2.5 if

Flow control can be done with if/elif/else statements.
The if statement is optionally followed by elif and else.

// Wing program:
let x = 1;
let y = "sample";
if x == 2 {
log("x is 2");
} elif y != "sample" {
log("y is not sample");
} else {
log("x is 1 and y is sample");

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2.6 for statement is used to iterate over an array, a set or a range. Range is inclusive of the start value and exclusive of the end value. The loop invariant in for loops is implicitly re-assignable (var).

// Wing program:
let arr = [1, 2, 3];
let items = Set<num>[1, 2, 3];
for item in arr {
for item in items {
for item in 0..100 {
log("{item}"); // prints 0 to 99

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2.7 while

The while statement evaluates a condition, and if it is true, a set of statements is repeated until the condition is false.

// Wing program:
while callSomeFunction() {

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2.8 throw

The throw statement raises a user-defined exception, which must be a string expression. Execution of the current function will stop (the statements after throw won't be executed), and control will be passed to the first catch block in the call stack. If no catch block exists among caller functions, the program will terminate. (An uncaught exception in preflight causes a compilation error, while an uncaught exception in inflight causes a runtime error.)

// Wing program:
throw "Username must be at least 3 characters long.";

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3. Declarations

3.1 Structs

Structs are loosely modeled after typed JSON literals in JavaScript.
Structs are defined with the struct keyword.
Structs are "bags" of immutable data.

Structs can only have fields of primitive types, preflight classes, and other structs.
Array, set, and map of above types is also allowed in struct field definition.
Visibility, storage and phase modifiers are not allowed in struct fields.

Structs can inherit from multiple other structs.

// Wing program:
struct MyDataModel1 {
field1: num;
field2: str;
struct MyDataModel2 {
field3: num;
field4: bool?;
struct MyDataModel3 extends MyDataModel1, MyDataModel2 {
field5: str;
let s1 = MyDataModel1 { field1: 1, field2: "sample" };
let s2 = MyDataModel2 { field3: 1, field4: true };
let s3 = MyDataModel2 { field3: 1 };
let s4 = MyDataModel3 {
field1: 12,
field2: "sample",
field3: 11,
field4: false,
field5: "sample"

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3.2 Classes

Similar to other object-oriented programming languages, Wing uses classes as its first-class composition pattern.

Classes consist of fields and methods in any order.
The class system is a single-dispatch class based object-orientated system.
Classes are instantiated with the new keyword.

Classes are associated with a specific execution phase (preflight or inflight). The phase indicates in which scope objects can be instantiated from this class.

If a phase modifier is not specified, the class inherits the phase from the scope in which it is declared. This implies that, if a class is declared at the root scope (e.g. the program's entrypoint), it will be a preflight class. If a class is declared within an inflight scope, it will be implicitly an inflight class.

A method that has the name init is considered to be a class constructor (or initializer).

inflight class Name extends Base impl IMyInterface1, IMyInterface2 {
// class fields
_field1: num;
_field2: str;

new() {
// constructor implementation
// order is up to user
this._field1 = 1;
this._field2 = "sample";

// static method (access with Name.staticMethod(...))
static staticMethod(arg: type, arg: type, ...) { /* impl */ }
// visible to outside the instance
publicMethod(arg:type, arg:type, ...) { /* impl */ }

If no new() is defined, the class will have a default constructor that does nothing.

Implicit default field initialization does not exist in Wing. All member fields must be initialized in the constructor. Absent initialization is a compile error. All field types, including the optional types must be initialized.

class Foo {
x: num;
new() { this.x = 1; }
class Bar {
y: num;
z: Foo;
new() {
this.y = 1;
this.z = new Foo();
this.log(); // OK to call here
pub log() {
let a = new Bar();
a.log(); // logs 1

Overloading methods is currently not allowed. This means functions cannot be overloaded with many signatures only varying in the number of arguments and their unique type order. Inheritance is allowed with the extends keyword. super can be used to access the base class, immediately up the inheritance chain (parent class).

Calling using the member access operator . before calling super in inherited classes is forbidden. The behavior is similar to JavaScript and TypeScript in their "strict" mode.

class Foo {
x: num;
new() { this.x = 0; }
pub method() { }
class Boo extends Foo {
new() {
// this.x = 10; // compile error
this.x = 10; // OK

Classes can inherit and extend other classes using the extends keyword.
Classes can implement interfaces iff the interfaces do not contain inflight.

class Foo {
x: num;
new() { this.x = 0; }
pub method() { }
class Boo extends Foo {
new() { super(); this.x = 10; }

Statics are not inherited. As a result, statics can be overridden mid hierarchy chain. Access to statics is through the class name that originally defined it: <class name>.Foo.

Multiple inheritance is invalid and forbidden.
Multiple implementations of various interfaces is allowed.
Multiple implementations of the same interface is invalid and forbidden.

In methods if return type is missing, : void is assumed.


The following features are not yet implemented, but we are planning to add them in the future:

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3.3 Preflight Classes

Classes declared within a preflight scope (the root scope) are implicitly bound to the preflight phase. These classes can have specific inflight members.

For example:

// Wing Code:
class Foo {
// preflight fields
field1: num;
field2: str;
field3: bool;

// re-assignable class fields (preflight, in this case), read about them in the mutability section
var field4: num;
var field5: str;

// inflight fields
inflight field6: num;
inflight field7: str;
inflight field8: bool;

// preflight constructor
new(field1: num, field2: str, field3: bool, field4: num, field5: str) {
/* initialize preflight fields */
this.field1 = field1;
this.field2 = field2;
this.field3 = field3;
this.field4 = field4;
this.field5 = field5;

// inflight constructor
inflight new() {
/* initialize inflight fields */
this.field6 = 123;
this.field7 = "hello";
this.field8 = true;

// preflight methods
foo1(arg: num): num { return arg; }
boo1(): num { return 32; }

// inflight methods
inflight foo2(arg: num): num { return arg; }
inflight boo2(): num { return 32; }

Preflight objects all have a scope and a unique ID. Compiler provides an implicit scope and ID for each object.

The default for scope is this, which means the scope in which the object was defined (instantiated). The implicit ID is the type name of the class iff the type is the only preflight object of this type being used in the current scope. In other words, if there are multiple preflight objects of the same type defined in the same scope, they must all have an explicit id.

Preflight objects instantiated at block scope root level of entrypoint are assigned the root app as their default implicit scope.

Preflight object instantiation syntax uses the let keyword the same way variables are declared in Wing. The as and in keywords can be used to customize the identifier and scope assigned to this preflight object respectively.

let <name>[: <type>] = new <Type>(<args>) [as <id>] [in <scope>];
// Wing Code:
let a = new Foo(); // with default scope and id
let a = new Foo() in scope; // with user-defined scope
let a = new Foo() as "custom-id" in scope; // with user-defined scope and id
let a = new Foo(...) as "custom-id2" in scope; // with constructor arguments

"id" must be of type string. It can also be a string literal with substitution support (normal strings as well as shell strings).
"scope" must be an expression that resolves to a preflight object.

Preflight objects can be captured into inflight scopes and once that happens, inside the capture block only the inflight members are available.

Preflight classes can extend other preflight classes (but not structs) and implement interfaces.

Declaration of fields of the same name with different phases is not allowed due to the requirement of having inflight fields of same name being implicitly initialized by the compiler. Declaration of methods with different phases is not allowed as well.

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3.4 Interfaces

Interfaces represent a contract that a class must fulfill. Interfaces are defined with the interface keyword. Currently, preflight interfaces are allowed, while inflight interfaces are not supported yet (see impl keyword is used to implement an interface or multiple interfaces that are separated with commas.

All methods of an interface are implicitly public and cannot be of any other type of visibility (private, protected, etc.). Return type is required for interface methods.

Interface fields are not supported.

// Wing program:
interface IMyInterface1 {
method1(x: num): str;
inflight method3(): void;

interface IMyInterface2 {
method2(): str;

class MyResource impl IMyInterface1, IMyInterface2 {
field1: num;
field2: str;

new(x: num) {
this.field1 = x;
this.field2 = "sample";
method1(x: num): str {
return "sample: {x}";
inflight method3(): void { }
method2(): str {
return this.field2;

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3.5 Variables

Let let be let. (Elad B. 2022)

let [var] <name>[: <type>] = [<type>] <value>;

Assignment operator is =.
Assignment declaration keyword is let.
Type annotation is optional if a default value is given.
var keyword after let makes a variable mutable.

let n = 10;
let s: str = "hello";
s = "world"; // error: Variable is not reassignable
let var s = "hello";
s = "hello world"; // compiles

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3.6 Functions

3.6.1 Closures

It is possible to create closures.
It is not possible to create named closures.
However, it is possible to create anonymous closures and assign to variables (function literals). Inflight closures are also supported.

// preflight closure:
let f1 = (a: num, b: num) => { log("{a + b}"); };
// inflight closure:
let f2 = inflight (a: num, b: num) => { log("{a + b}"); };
// OR:
// preflight closure:
let f4 = (a: num, b: num): void => { log("{a + b}"); };
// inflight closure:
let f5 = inflight (a: num, b: num): void => { log("{a + b}"); };

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3.6.2 Struct Expansion

If the last argument of a function call is a struct, then the struct in the call is "expandable" with a special : syntax.
In this calling signature, order of struct members do not matter.
Partial struct expansion in terms of supplying less number of arguments than the number of fields on type of the struct expected is not allowed. Omitting nils is allowed with the same rules as explicit initialization in class constructors.

This style of expansion can be thought of as having positional arguments passed in before the final positional argument, which if happens to be a struct, it can be passed as named arguments. As a result of named arguments being passed in, it is safe to omit optional struct fields, or have order of arguments mixed.

struct MyStruct {
field1: num;
field2: num;
let f = (x: num, y: num, z: MyStruct) => {
log("{x + y + z.field1 + z.field2}");
// last arguments are expanded into their struct
f(1, 2, field1: 3, field2: 4);
// f(1, 2, field1: 3); // can't do this, partial expansion is not allowed

3.6.3 Variadic Arguments

When a function signature's final parameter is denoted by ... and annotated as an Array type, then the function accepts typed variadic arguments. Inside the function, these arguments can be accessed using the designated variable name, just as you would with a regular array instance.

let f = (x: num, ...args: Array<num>) => {
log("{x + args.length}");
// last arguments are expanded into their array
f(4, 8, 15, 16, 23, 42); // logs 9

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3.7 Arrays

Arrays are dynamically sized in Wing and are defined with the [] syntax.
Individual array items are accessed using the .at(index: num) method.
Arrays are similar to dynamically sized arrays or vectors in other languages.

let arr1 = [1, 2, 3];
let arr2 = ["a", "b", "c"];
let arr3 = MutArray<str>["a1", "b2", "c3"];
let l = arr1.length + arr2.length + arr3.length +;

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3.8 Enumeration

Enumeration type (enum) is a type that groups a list of named constant members. Enumeration is defined by writing enum, followed by enumeration name and a list of comma-separated constants in a {}. Naming convention for enums is to use "TitleCase" for name and ALL_CAPS for members.

enum SomeEnum { ONE, TWO, THREE }
enum MyFoo {
let x = MyFoo.B;
let y = x; // type is MyFoo

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3.9 Unit tests

Unit tests can be defined in Wing using the built-in test statement. A test statement expects a name and a block of inflight code to execute.

let b = new cloud.Bucket();

test "can add objects" {
b.put("key", "value");
assert(b.get("key") == "value");

The behavior of running tests with wing test CLI command is determined by the cloud.TestRunner resource in the Wing SDK, which can be implemented for any compiler target.

See the Test Concenpt Doc for more details on running tests.

3.10 Roadmap

The following features are not yet implemented, but we are planning to add them in the future:

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4. Module System

The module system in Wing uses the bring expression to reuse code.
bring expression allows code to "import" functions, classes, and variables from other files, to allow reusability.
bring expression is only allowed at the top of the file before any other code. Comments before the first bring expression are valid.

4.1 Imports

To import a built-in module or trusted Wing library, you can use the following syntax:

bring util; // import types from the built-in "util" module
bring cloud; // import types from the built-in "cloud" module
bring containers; // import types from the `@winglibs/containers` trusted library

To use a trusted library, you must install the relevant npm package with npm i @winglibs/containers.

To import a Wing or JSII library under a named import, you may use the following syntax:

bring "cdktf" as cdktf; // from "cdktf" bring * as cdktf;

To import an individual Wing file as a module, you can specify its path relative to the current file:

bring "./my-module.w" as myModule;

It's also possible to import a directory as a module. The module will contain all public types defined in the directory's files. If the directory has subdirectories, they will be available under the corresponding names.

bring "./my-module" as myModule;

// from ./my-module/submodule/my-class.w
new myModule.submodule.MyClass();

The following features are not yet implemented, but we are planning to add them in the future:

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4.2 Exports

Wing currently does not not support exporting symbols from a module - see to track.

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5. Interoperability

5.1 JSII Interoperability

5.1.1 External Libraries

You may import JSII modules in Wing and they are considered preflight classes if their JSII type manifest shows that the JSII module is a construct. Wing is a consumer of JSII modules currently.

bring "aws-cdk-lib" as awscdk;
let bucket = new awscdk.aws_s3.Bucket(
blockPublicAccess: awscdk.aws_s3.BlockPublicAccess.BLOCK_ALL,

5.2 JavaScript

The extern "<commonjs module path>" modifier can be used on method declarations in classes to indicate that a method is backed by an implementation imported from a JavaScript module. The module must be a relative path and will be loaded via require().

In the following example, the static inflight method makeId is implemented in helper.js:

// task-list.w
class TaskList {
// ...

inflight addTask(title: str) {
let id1 = TaskList.makeId();
let id2 = TaskList.v4();
// ...

// Load js helper file
pub extern "./helpers.js" static inflight makeId(): str;

// helpers.js
const uuid = require("uuid");

exports.makeId = function () {
return uuid.v4();

Given a method of name X, the compiler will map the method to the JavaScript export with the matching name (without any case conversion).

Extern methods do not support access to class's members through this, so they must be declared static.

5.2.1 TypeScript

It is possible to use TypeScript to write helpers, but at the moment this is not directly supported by Wing. This means that you will need to setup the TypeScript toolchain to compile your code to JavaScript and then use extern against the JavaScript file.

5.2.2 Type model

The table below shows the mapping between Wing types and JavaScript values, shown with TypeScript types. When calling extern function, the parameter and return types are assumed to be satisfied by the called function.

Built-in Wing typeTypeScript type
Set<T>, MutSet<T>Set<T>
Map<T>, MutMap<T>{ [key: string]: T }
Array<T>, MutArray<T>Array<T>
Json, MutJsonstring ⏐ number ⏐ boolean ⏐ null ⏐ Json[] ⏐ { [key: string]: Json }
User-defined Wing typeTypeScript type
classclass, only with members whose phase is compatible with the function signature
interfaceinterface, only with members whose phase is compatible with the function signature
enumstring-based enum-like Object

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6. Miscellaneous

6.1 Equality

Checking for equality is performed with the == operator. It returns true if the two values are equal, and false otherwise. Similarly, inequality is performed with the != operator.

The main difference between equality in JavaScript and Wing is that == in Wing is not allowed to compare values of different types. For example, 1 == "1" is not allowed in Wing, and will result in a compile-time error.

Equality in Wing is a symmetric and transitive relationship - that is, (1) if a == b, then b == a, and (2) if a == b and b == c, then a == c.

The execution phase (preflight or inflight) that a value was created in does not affect its equality. For example, a value created in preflight can be equal to a value created in inflight.

Some types are compared by value, which means that two values are equal if their contents are equivalent. For example, two str values are equal if they have the same characters in the same order, even if they are stored in different places in memory.

Other types are compared by reference, which means that two values are equal if they point to the same object in memory. For example, two functions are equal if they are the same object, even if they have the same code.

The following is a set of rules for checking equality:

6.1.1 Basic types

Basic types are compared by value.

  1. Two str values are equal if they have the same characters in the same order.
  2. Two num values are equal if they have the same floating-point value. The IEEE 754 standard is used for storing numbers, which means that for example -0 == +0. NaN is not equal to any value, including itself.
  3. Two bool values are equal if they are both true or both false.
  4. Two duration values are equal if they have the same number of milliseconds.
  5. Two T? types (optional T values) are equal if they are both empty (nil) or both non-empty, and if they are both non-empty, their inner values are equal. A value of type T? is never equal to a value of type T.

Note: Equality checking for duration is not fully implemented. See #2941.

6.1.2 Collection types

Wing contains six collection types: Array, MutArray, Map, MutMap, Set, and MutSet. The following rules apply to all of them:

  1. Two collections are equal if they have the same number of elements, and if each element in the first collection is equal to the corresponding element in the second collection (according to the rules of equality of that type). The order of elements only matters for Array and MutArray.
  2. The mutability of a collection does not affect its equality. In other words, a MutArray is equal to an Array with the same elements, and a MutMap is equal to a Map with the same keys and values.
  3. Only collections of the same "kind" can be equal. For example, an Array cannot be equal to a Map, and a MutArray cannot be equal to a MutMap.
assert(Array<num>[1, 2, 3] == Array<num>[1, 2, 3]);
assert(Array<num>[1, 2, 3] != Array<num>[3, 2, 1]);
assert(MutArray<num>[1, 2, 3] == Array<num>[1, 2, 3]);

assert(Map<str>{"a" => "1", "b" => "2"} == Map<str>{"a" => "1", "b" => "2"});
assert(Map<str>{"a" => "1", "b" => "2"} == Map<str>{"b" => "2", "a" => "1"});

assert(Set<num>[1, 2, 3] == Set<num>[1, 2, 3]);
assert(Set<num>[1, 2, 3] == Set<num>[3, 2, 1]);

Note: Collection type equality checking is not fully implemented. See #2867, #2940.

6.1.3 Function types

Two functions are equal if they are both the same object (by reference). This means that two functions that have the same code are not necessarily equal, since they may have been defined in different places.

let f1 = (x: num): num => { return x + 1; };
let f2 = (x: num): num => { return x + 1; };
let f3 = f1;

assert(f1 != f2);
assert(f1 == f3);

Functions can only be compared if they have the same signature (including its execution phase). For example, a function defined in preflight cannot be compared to a function defined in inflight, even if they have the same code.

let f1 = (x: num): num => { return x + 1; }; // (preflight)
let f2 = inflight (x: num): num => { return x + 1; };

assert(f1 != f2); // compile error (can't compare different types)

6.1.4 Enums

Two enum values are equal if they refer to the same case.

enum PizzaTopping {

let topping1 = PizzaTopping.CHEESE;
let topping2 = PizzaTopping.CHEESE;
let topping3 = PizzaTopping.PINEAPPLE;

assert(topping1 == topping2);
assert(topping1 != topping3);

6.1.5 Classes and interfaces

Two class instances or interface-satisfying objects are equal if they are the same instance (by reference). This means that two class instances, or interface-satisfying objects that have the same data are not necessarily equal, since they may have been created in different places.

class Shop {
hats: num;
new(hats: num) {
this.hats = hats;

let shop1 = new Shop(1) as "Shop1";
let shop2 = new Shop(1) as "Shop2";
let shop3 = shop1;

assert(shop1 != shop2);
assert(shop1 == shop3);

6.1.6 Json

Two Json values are equal if they contain the same structure and values. Another way to think about it is the two Json values are equal if their stringified representation is equal. The following rules apply:

  1. Two Json values are equal if they are both null.
  2. Two Json values are equal if they are both bool values and are equal.
  3. Two Json values are equal if they are both num values and are equal.
  4. Two Json values are equal if they are both str values and are equal.
  5. Two Json values are equal if they are both Array values and are equal.
  6. Two Json values are equal if they are both Map values and are equal.
assert(Json true == Json true);
assert(Json false == Json false);
assert(Json 1 == Json 1);
assert(Json -0.42 == Json -0.42);
assert(Json "foo" == Json "foo");
assert(Json [1, 2, 3] == Json [1, 2, 3]);
assert(Json { "foo": 1, "bar": 2 } == Json { "foo": 1, "bar": 2 });

6.1.7 Structs

Two structs are equal if they have the same type and all of their fields are equal (based on rules of equality of their type).

struct Cat {
name: str;
age: num;
struct Dog {
name: str;
age: num;

let cat1 = Cat { name: "Mittens", age: 3 };
let cat2 = Cat { name: "Mittens", age: 3 };
let cat3 = Cat { name: "Mittens", age: 4 };
let dog = Dog { name: "Mittens", age: 3 };

assert(cat1 == cat2); // fields and types match
assert(cat1 != cat3); // field "age" does not match
assert(cat1 != dog); // compile time error (can't compare different types)

Note: Struct equality is not fully implemented. See #2939.

6.2 Strings

String reference doc is available here. Type of string is UTF-16 internally.
All string declaration variants are multi-line.

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6.2.1 Normal strings "..."

The string inside the double quotes is processed, and all notations of form {<expression>} are substituted from their respective scopes. The behavior is similar to `text ${sub.prop}` notation in JavaScript.
Processing unicode escape sequences happens in these strings.
" and { can be escaped with backslash \ inside string substitutions.

let name = "World";
let s = "Hello, {name}!";
let l = s.length;

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Single line comments start with a // and continue to the end of the line.
Multi-line comments are supported with the /* ... */ syntax.

// comment
/* comment */
multi line comment

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6.4 Operators

Unary operators are not supported except outline below.
Arithmetic assignment operators are not supported.
Ternary or conditional operators are not supported.

6.4.1 Relational Operators

==Checks for equalitya == b
!=Checks for inequalitya != b
>Checks if left is greater than righta > b
<Checks if left less than righta < b
>=Checks if left is greater than or equal to righta >= b
<=Checks if left is less than or equal to righta <= b

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6.4.2 Logical Operators

&&Logical AND operatora && b
\|\|Logical OR operatora \|\| b
!Logical NOT operator!a

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6.4.3 Mathematics Operators

*Multiplicationa * b
/Divisiona / b
\Floor Divisiona \ b
%Modulusa % b
+Additiona + b
-Subtractiona - b
**Powera ** b

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6.4.4 Operator Precedence

-xUnary minus
*, /, \, %Multiplication, Division, Floor division, Modulus
+, -Addition, Subtraction
==, !=, >, >=, <, <=Comparisons, Identity, operators
!Logical NOT
&&Logical AND
||Logical OR

Table above is in descending order of precedence.
= operator in Wing does not return a value so you cannot do let x = y = 1.
Operators of the same row in the table above have precedence from left to right in the expression they appear in (e.g. 4 * 2 \ 3). In other words, order is determined by associativity.

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6.4.5 Short Circuiting

For the built-in logical NOT operators, the result is true if the operand is false. Otherwise, the result is false.

For the built-in logical AND operators, the result is true if both operands are true. Otherwise, the result is false. This operator is short-circuiting if the first operand is false, and the second operand is not evaluated.

For the built-in logical OR operators, the result is true if either the first or the second operand (or both) is true. This operator is short-circuiting if the first operand is true, and the second operand is not evaluated.

Note that bitwise logic operators do not perform short-circuiting.

In conditionals, if an optional type is used as the only r-value expression of the condition statement, it's equivalent to checking it against nil. Note that using a bool? type in this short-circuit is a compile error due to ambiguity. Using a nil? type is also ambiguous and results in a compile error.

let x: num? = 1;
if x? {
// ...

Which is equivalent to:

let x: num? = 1;
if x != nil {
// ...

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6.5 Roadmap

  • Asynchronous Execution Safety Model.
  • Make the language async by default.
  • First class support for regex, glob, and cron types.
  • Support of math operations over date and duration types.
  • More useful enums: Support for Enum Classes and Swift style enums.
  • Reflection: add an extended typeof operator to get type information.
  • Advanced OOP: Support for abstract and private implementations.
  • Enforce naming conventions on public APIs (required by JSII).
  • Develop a conformance test suite for ISO certification.
  • Launch a formal spec site with ECMA standards.
  • Built-in automatic formatter and linter.
  • Distributed concurrency primitives.
  • Distributed data structures.

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6.6 Credits