Language Guide

Last updated 2 months ago

The Flint Programming Language Guide gives a high-level overview of the language, it's unique features and helps you getting started with smart contract development in Flint.




The Flint compiler and its dependencies can be installed using Docker:

Docker pull franklinsch/flint
docker run -i -t franklinsch/flint

Example smart contracts are available in /flint/examples/valid/.

Binary Packages and Building from Source



The Flint compiler is written in Swift, and requires the Swift compiler to be installed, either by:

  • Mac only: Installing Xcode (recommended)

  • Mac/Linux: Using swiftenv

    1. Install swiftenv: brew install kylef/formulae/swiftenv

    2. Run swiftenv install 4.1


Flint also requires the Solidity compiler to be installed:


brew update
brew upgrade
brew tap ethereum/ethereum
brew install solidity


sudo add-apt-repository ppa:ethereum/ethereum
sudo apt-get update
sudo apt-get install solc

Binary Packages

Flint is compatible on macOS and Linux platforms, and can be installed by downloading a built binary directly.

The latest releases are available at

Building From Source

The best way to start contributing to the Flint compiler, flintc, is to clone the GitHub repository and build the project from source.

Once you have the swift command line tool installed, you can build flintc.

git clone
cd flint

The built binary is available at .build/debug/flintc.

Add flintc to your PATH using:

export PATH=$

Using Xcode

If you have Xcode on your Mac, you can use Xcode to contribute to the compiler.

You can generate an Xcode project using:

swift package generate-xcodeproj
open flintc.xcodeproj

Syntax Highlighting

Syntax highlighting for Flint source files can be obtained through several editors, including:


By running the following command in the flint repository vim will now have syntax highlighting.

ditto utils/vim ~/.vim


The language-flint package can be downloaded to have syntax highlighting for flint files.


Flint compiles to EVM bytecode, which can be deployed to the Ethereum blockchain using a standard client, or Truffle.

For testing purposes, the recommended way of running a contract is by using the Remix IDE.

Using Remix

Remix is an online IDE for testing Solidity smart contracts. Flint contracts can also be tested in Remix, by compiling Flint to Solidity.

In this example, we are going to compile and run the Counter contract, available to download here.


A Flint source file named counter.flint containing a contract Counter can be compiled to a Solidity file using:

flintc main.flint --emit-ir

You can view the generate code, embedded as a Solidity program:

cat bin/main/Counter.sol

Example smart contracts are available in the repository, under examples/valid.

Interacting with contract in Remix

To run the generated Solidity file on Remix:

  1. Copy the contents of bin/main/Counter.sol and paste the code in Remix.

  2. Press the red Create button under the Run tab in the right sidebar.

  3. You should now see your deployed contract below. Click on the copy button on the right of Counter to copy the contract's address.

  4. Select from the dropdown right above the Create button and select _InterfaceMyContract.

  5. Paste in the contract's address in the "Load contract from Address" field, and press the At Address button.

  6. You should now see the public functions declared by your contract (getValue, set, and increment). Red buttons indicate the functions are mutating, whereas blue indicated non-mutating.

You should now be able to call the contract's functions.

Programming In Flint

A smart contract’s state is represented by its fields, or state properties. Its behaviour is characterised by its functions, which can mutate the contract’s state. Public functions can be called by external Ethereum users. Flint’s syntax is focused on allowing programmers to write and reason about smart contracts easily. Providing an intuitive and familiar syntax is essential for programmers to express their smart contract naturally. As the source code of a smart contract is publicly available, it should be easily understandable by its readers. The syntax is inspired by the Swift Programming Language’s syntax.

When developing Flint, we focused on the novel features aimed at smart contract security. For this reason, some features which developers might expect from a programming language and its compiler such as recursive data types or type inference have not yet been implemented in the compiler.

Constants and Variables

Constants and Variables associate a name with a value of a particular type. The value of a constant can't be changed once it's set, whereas a variable can be set to a different value in the future.

Variables can be state properties - public contract specific data stored in EVM Storage - or local variables - transaction specific data stored temporarily in EVM Memory.

Declaring Constants and Variables

You declare constants with the let keyword and variables with the var keyword.

Flint is a typed language and as such every variable and constant should be annotated with a type. A type annotation places a colon after the name. To see what types are built-in see Types.

let maxValues: Int = 10
var currentValue: Int = 0

A colon can be thought of as "... of type ...", so the code above can be read as:

Declare a constant called maxValues that is of type Int with the value 10.

You can change the value of an existing variable to another value of a compatible type. For example the currentValue can be incremented.

var currentValue: Int = 4
currentValue = 0

Unlike a variable, the value of a constant can't be changed after it's set.

let languageName: String = "Flint"
languageName = "f" // This a compile-time error


Literals represent fixed values in source code that can be assigned to constants and variables.

  • Integer (Int) literals can be written as decimal numbers e.g. 42.

  • Address (Address) literals are written as 40 hexadecimal digits prefixed by a 0x

  • Boolean (Bool) literals are simply true and false.

  • List ([T] or T[n]) literals are of the form:

    • [] which defines a new empty List

    • In the future, we will have lists defined by [x, y, z, ...] where x, y, z etc. are literals of type T

  • Dictionary ([T: U]) literals are of the form:

    • [:] which defines a new empty Dictionary

    • In the future, we will have dictionary literals defined by [x: a, y: b, z: c, ...] where x, y, z are literals of type T and a, b, c are literals of type U


Comments can be used to exclude text from your code. Comments are ignored by the compiler when compiled.

// This is a comment


An operator is a special symbol used to check, change, or combine values. Flint supports most standard C operators and attempts to eliminate common coding errors.

Arithmetic Operators

Flint supports the following arithmetic operators for all number types:

  • Addition (+)

  • Subtraction (-)

  • Multiplication (*)

  • Division (/)

  • Power (**)

1 + 2 // equals 3
5 - 3 // equals 2
2 * 3 // equals 6
10 / 2 // equals 5

Flint has unique Safe Arithmetic

Assignment Operators

The assignment operator (a = b) initializes or updates the value of a with the value of b:

let b: Int = 10
var a: Int = 5
a = b
// a is equal to 10

Compound Assignment Operators

Flint provides compound assignment operators that combine assignment (=) with another operator. Namely:

  • Addition Compound (+=)

  • Subtraction Compound (-=)

  • Times Compound (*=)

  • Division Compound (/=)

Boolean Operators

These operators all result in Bool

  • Equal to (==)

  • Not equal to (!=)

  • OR (||)

  • AND (&&)

  • Less than (<)

  • Less than or equal to (<=)

  • Greater than (>)

  • Greater than or equal to (>=)

1 == 1 // true because 1 is equal to 1
2 != 1 // true because 2 is not equal to 1
2 > 1 // true because 2 is greater than 1
1 < 2 // true because 1 is less than 2
1 >= 1 // true because 1 is greater than or equal to 1
2 <= 1 // false because 2 is not less than or equal to 1

true || false // true because one of true and false is true true && false // false because one of true and false is false


Contracts lie at the heart of Flint. These are the core building blocks of your program's code. Inside of contracts you define constants and variables to be stored on the Chain.

Contracts are declared using the keyword contract followed by the contract name that will be used as the identifier.

contract Bank {
// Variable Declarations
var owner: Address
let name: String = "Bank"


Smart contracts can remain in activity for a large number of years, during which a large number of state mutations can occur. To aid with reasoning, Flint functions cannot mutate smart contracts’ state by default. This helps avoid accidental state mutations when writing the code, and allows readers to easily draw their attention to the mutating functions of the smart contract.

Visibility Modifiers

Variables declared in the contract can have modifiers in front of their declaration which control the automatic synthesis of variable accessors and mutators. By the nature of smart contracts all storage is visible already, but providing accessors makes that process easier.

public var value: Int
visible var name: String = Bank
visible var owner: Address
  • public access synthesises an accessor and a mutator so that the storage variable can be viewed and changed by anyone.

  • visible access synthesises an accessor to the storage variable which allows it to be viewed by anyone.

  • private access means that nothing is synthesised but both accessors and mutators can be manually specified.

Control Flow

For-In Loops

for-in loops can be used to iterate over a sequence (currently contract instance arrays and dictionary values, and ranges)

contract X {
var names: [String]
var fixedLen: Int[10]
var dictionary: [Int: String]

For example, we can iterate over a variable length array names and bind the current iteration's value to the constant name of type: String.

for let name: String in names {}

We can also bind the iteration's value to a variable i.e. i. This will allow the modification of the variable i inside the body - which gets reset on each loop.

for var i: Int in fixedLen {}
for let value: String in dictionary {}

If Statements

The if statement has a single condition i.e. x == 2. It executes a set of statements only if that condition evaluates to true

if x == 2 {
// ...

Else Clauses

The if statement can also provide an alternative set of statements known as an else clause which gets executed when the condition gets evaluated to false.

if x == 2 {
// ...
} else {
// ,,,


Functions are self-contained blocks of code that perform a specific task, which is called using its identifier. In Flint functions cannot be overloaded and are contained within Protection Blocks.

They are defined with the keyword func followed by the identifier and the set of parameters and optional return type:

func identifier() {
// ...

In Flint all functions are private by default and as such can only be accessed from within the contract body.

Function Parameters

Functions can also take parameters which can be used within the function. Below is a function that mutates the dictionary of peoples names to add the Key, Value pair of caller's address and name. For more information about callers, see Caller Bindings:

contract AddressBook {
var people: [Address: String]
AddressBook :: caller <- (any) {
mutating func remember(name: String) {
people[caller] = name

Return Values

You can indicate the function's return type with the return arrow ->, which is followed by the return type.

AddressBook :: (any) {
func name(address: Address) -> String {
return people[address]

Calling Functions

Functions can then be called from within a contract behaviour block with the same identifier. For instance if we wanted to get the caller's name we can reuse the name function:

AddressBook :: caller <- (any) {
func myName() -> String {
return name(caller)

Function Attributes

Attributes annotate functions as having special properties, currently the only example of this is @payable. For more information, see Payable.

Function Modifiers

Functions can have any number of modifiers in front of their declaration - although conflicting modifiers will raise a compile-error and duplicated modifiers will be ignored.

Access Modifiers

  • public access enables functions to be used within their contract and exposes the function to the interface of the contract as a whole when compiled.

  • private access (default and not a keyword that is explicitly set) only enables functions to be used within their contract

    Mutating Modifier

    The mutating modifier specifies that the function mutates the contract storage. This means that the function changes the contract storage.

AddressBook :: caller <- (any) {
func remember(name: String) { // compile-time error
people[caller] = name // This statement is mutating
AddressBook :: caller <- (any) {
mutating func myName() -> String { // compile-time error as this function isn't mutating
return name(caller)


Flint is a statically-typed language with a simple type system, with basic support for subtyping through traits. Type inference is a planned feature.

Flint is a type-safe language. A type safe language encourages clarity about the type of values your code can work with.

It performs type checks when compiling code and flags any mismatched types as errors. This enables you to catch and fix errors as early as possible in the development process.

Basic Types




160-bit Ethereum Address


256-bit integer


Boolean value


String value (currently max of 256 bits)


Void value

Dynamic Types




Dynamic-size array. [T] is an array of element type T

Fixed-size Array

Fixed-size memory block containing elements of the same type. T[n] is an array of size n, of element type T.


Dynamic-size mappings from one key type to a value type [K: V] is a a dictionary of key type K and value type V.


Struct values, including Wei, are considered to be of dynamic type.


Flint includes two range types, which are shortcuts for expressing ranges of values. In particular it is designed to be used with for-in loops.

Half-Open Range

The half-open range operator (a..<b) defines a range that runs from a to b, but doesn't include b.

for let i: Int in (0..<count) {
// ...

Open Range

The open range operator (a..,b) defines a range that runs from a to b and does include b.

for let i: Int in (0..<count) {
// ...


Structures in Flint are general-purpose constructs that group state and functions that can be used as self-contained blocks. They use the same syntax as defining constants and variables for properties. Structure functions are not protected as they can only be called by (protected) contract functions, and are required to be annotated mutating if they mutate the struct's state.


You introduce structures with the struct keyword and their definition is contained within a pair of braces:

struct SomeStructure {
// Structure definition goes here

Here is an example of a structure definition:

struct Rectangle {
var width: Int = 0
var height: Int = 0
func area() -> Int {
return width * height


The Rectangle structure definition describes only what a Rectangle would look like and what functionality they contain. They don't describe specific rectangles. To do that you create an instance of that structure.

let someRectangle: Rectangle = Rectangle()

When you create an instance, you initialize that structure with it's initial values - in this case a width and height of 0. This process can also be done manually using the init special declaration. You can access the properties of the current structure with the special keyword self

struct Rectangle {
// Same definition as above with:
public init(width: Int, height: Int){
self.width = width
self.height = height
let bigRectangle = Rectangle(400, 10000)

Accessing Properties/Functions

You can access properties/functions of an instance using dot syntax. In dot syntax, you write the property name immediately after the instance name, separated by a period (.), without any spaces.

someRectangle.width // evaluates to 0
someRectangle.area() // evaluates to 0 by calling the function area on the Rectangle instance someRectangle

In Flint, you can also access the functions of a struct using this syntax without creating an instance - as long as they don't contain instance properties. (In the future we will support the static keyword to do this)

struct Square {
public shapeName() -> String {
return "Square"
Square.shapeName() // Evaluates to "Square"

State Properties or Local Variables

Struct values can be declared as state properties or local variables/constants through their intialisers. When stored as a state property, the struct's data resides in EVM storage. When stored as a local variable, it resides in EVM memory, and a pointer is allocated on the EVM stack.

Structs as function arguments

When struct values are passed as function arguments, they are passed by value by default. That is, on function entry, a struct is copied in storage or memory (depending on whether it is a state property or a local variable).

Structs can be passed by reference using the inout keyword. The struct is then treated as an implicit reference to the value in the caller.

func foo() {
let s = S(8)
byValue(s) // s.x == 8
byReference(s) // s.x == 10
func byValue(s: S) {
s.x = 10
func byReference(s: inout S) {


An enumeration defines a common group of values with the same type and enables working with those values in a type-safe way within Flint code.


enum CompassPoint: Int {
case north
case south
case east
case west

The values defined in an enumeration (such as north, south, east and west) are its enumeration cases. They are declared using the case keyword.

Each enumeration defines a new type:

var direction: CompassPoint
direction = CompassPoint.north

Associated Values

enum Numbers: Int {
case one = 1
case two = 2
case three
case four

You can assign so called raw values to enumeration cases.

Flint will infer the raw value of cases by default based upon the last declared enumeration cases raw value. Numbers.three will equal 3 due to this.

Flint Specific Features

Safe Arithmetic

Safe arithmetic operators are also provided. The +, -, * and ** operators throw an exception and abort execution of the smart contract when an overflow occurs. The / operator implements integer division. No underflows can occur as we do not support floating point types yet. The performance overhead of our safe operators are low.

In rare cases, allowing overflows is the programmer’s intended behaviour. Flint also supports overflowing operators, &+, &-, and &*, which will not crash on overflows.


Numerous attacks targeting smart contracts, such as ones relating to reentrancy calls (see TheDAO), allow hackers to steal a contract’s Ether. These happen because smart contracts encode Ether values as integers, making it is easy to make mistakes when performing Ether transfers between variables, or to forget to record Ether arriving or leaving the smart contract.

Flint supports special safe operations when handling assets, such as Ether. They help ensure the contract's state consistently represents its Ether value, preventing attacks such as TheDAO.

An simple use of Assets:

contract Wallet {
var owner: Address
var contents: Wei = Wei(0)
Wallet :: caller <- (any) {
public init() {
owner = caller
public mutating func deposit(implicit value: Wei) {
// Record the Wei received into the contents state property.
// Value is passed by reference.
Wallet :: (owner) {
public mutating func withdraw(value: Int) {
// Transfer an amount of Wei into a local variable. This
// removes Wei from the contents state property.
var w: Wei = Wei(&contents, value)
// Send Wei to the owner's Ethereum address.
send(owner, &w)
public func getContents() -> Int {
return contents.getRawValue()

Another example which uses Assets is the Bank example.

contract Wallet {
var beneficiaries: [Address: Wei]
var weights: [Address: Int]
var bonus: Wei
var owner: Address
Wallet :: (any) {
mutating func receiveBonus(implicit newBonus: inout Wei) {
Wallet :: (owner) {
mutating func distribute(amount: Int) {
let beneficiaryBonus = bonus.getRawValue() / beneficiaries.count
for let person: Address in beneficiaries {
var allocation = Wei(from: &balance, amount: amount * weights[person])
allocation.transfer(from: &bonus, amount: beneficiaryBonus)
send(beneficiaries[i], &allocation)

Protection Blocks

Flint introduces the concept of protection blocks. While traditional computer programs have an entry point (the main function), smart contracts do not. After a contract is deployed on the blockchain, its code does not run until an Ethereum transaction is received. Smart contracts are in fact more akin to RESTful web services presenting API endpoints. It is important to prevent unauthorized parties from calling sensitive functions.

In Flint, functions of a contract are declared within protection blocks, which restrict when the enclosed functions are allowed to be called.

There are two elements to protection blocks, the caller protection and the optional type state protection (see Type States for more detail). A protection block is minimally declared as the contract name (Bank) followed by a :: and a caller group ((manager)).

Solidity uses function modifiers to insert dynamic checks in functions, which can for instance abort unauthorised calls. However, it is easy to forget to specify these checks, as the language does not require programmers to write them. Having a language construct which protects functions from invalid calls could require programmers to systematically think about which parties should be able to call the functions they are about to define.

In Flint, functions of a contract are declared within protection blocks, which protect the functions from invalid access.

Caller Group

Caller Groups consist of a list of Caller Members enclosed in parentheses. These Caller Members can be a function of type Address -> Bool or () -> Address, a contract state property of the types Address, [Address], [T: Address] or Address[n], or the special any keyword that denotes all addresses. For example, (any), (admin), (owners, manager) are all valid Caller Groups where admin and manager refer to a state property of type Address and owners refers to a state property of type [Address]

Functions inside these caller groups can only be called by an Ethereum Address (aka the caller address) that satisfies at least one of these Caller Members. The satisfaction comes in different forms depending on the member.



Address -> Bool

The function is called with the caller as input, satisfied if returns true

() -> Address

The returned address must match the caller address


The address property must match the caller address


The caller address must be contained within the list of addresses

[T: Address]

The caller address must be contained with in the values of the dictionary


Always satisfied

The Ethereum address of the caller of a function is unforgeable. It is not possible to impersonate another user, as a consequence of Ethereum’s mechanism which generates public addresses from private keys. Transactions are signed using a private key, and determine the public key of the caller. Stealing a caller capability would hence require stealing a private key. The recommended way for Ethereum users to transfer their ability to call functions is to either change the backing address of the caller capability they have (the smart contract must have a function which allows this), or to securely send their private key to the new owner, outside of the Ethereum platform.

Calls to Flint functions are validated both at compile-time and runtime, with runtime checks only being added where necessary.

Static checking

In a Flint function, if a function call to another Flint function is performed, the compiler checks that the caller meets the caller protection.

Consider the following example.

Bank :: (any) {
func foo() {
// Error: Protection "any" cannot be used to perform a call to a
// function for "manager"
Bank :: (manager) {
func bar() {}

Within the context of foo, the caller is regarded as any. It is not certain that the caller also satisfies the manager protection, so the compiler rejects the call.

Dynamic checking

Attempt function calls

It is still possible for foo to satisfy the protections of the function bar.

For these cases, two additional language constructs exist:

  • try? bar(): The function bar's body is executed if, at runtime, the protections are satisfied (i.e. the caller satisfies the caller protection and the state of the contract satisfies the type state protection). The expression try? bar() returns a boolean if successful.

  • try! bar(): If at runtime bar's protections are not satisfied an exception is thrown and the body doesn't get executed. Otherwise, it does.

Calls from Ethereum users or non-Flint smart contracts

Functions to contracts on the Blockchain can also be called by users directly, through an Ethereum client, or another non-Flint smart contract.

For those cases, Flint checks at runtime whether the caller has the appropriate protections to perform the call, and throws an exception if not.

Multiple protections

A contract behavior declaration can be restricted by multiple caller protections.

Consider the following contract behavior declaration:

Bank :: (manager, accounts) {
func forManagerOrCustomers() {}

The function forManagerOrCustomers can only be called by either the manager, or any of the accounts registered in the bank.

Calls to functions of multiple protections are accepted if each of the protections of the enclosing function are compatible with any of the target function's protections.

Consider the following examples:

Insufficient protections

Bank :: (manager, accounts) {
func forManagerOrCustomers() {
// Error: "accounts" is not compatible with "manager"
Bank :: (manager) {
func forManager() {}

Sufficient protections

Bank :: (manager, accounts) {
func forManagerOrCustomers() {
// Valid: "manager" is compatible with "manager", and "accounts" is
// compatible with "accounts"
Bank :: (accounts, manager) {
func forManagerOrCustomers2() {}

any is compatible with any caller protection

Bank :: (manager, accounts) {
func forManagerOrCustomers() {
// Valid: "manager" is compatible with "manager" (and "any", too), and "accounts"
// is compatible with "any"
// The caller protection "manager" has no effect: "any" is compatible with any caller protection
Bank :: (manager, any) {
func forManagerOrCustomers2() {}

Static and dynamic checking

Just like single-member protection definitions, multi-member protections are checked both at compile-time and runtime.

Caller Binding

Callers can be bound to temporary variables.

Consider the following example.

Bank :: account <- (accounts) {
// This function is non-mutating
public func getBalance() -> Int {
return balances[account]
public mutating func transfer(amount: Int, destination: Address) {
balances[account] -= amount
balances[destination] += amount

withdraw can be called by any caller which has an account in the bank. The caller's address is bound to the variable account, which can then be used in the body of the functions.

Extended Example

contract ChildCollegeFund {
var parent: Address
var child: Address
var canWithdraw: Bool = false
var target: Int
var contents: Wei = Wei(0)
// Anyone can initialise the contract.
// The contract can be deployed by anyone,
// and is initialised during contract deployment.
// This initialisation allows you to set the contract address.
ChildCollegeFund :: (any) {
public init(parent: Address, child: Address, target: Int) {
self.parent = parent
self.child = child = target
// Functions both the parent and child can call.
ChildCollegeFund :: (parent, child) {
public func getTarget() -> Int {
return target
// Trying to withdraw() here would fail as it
// requires the caller to be 'child' statically.
// At runtime we can only know the caller has one
// of the capabilities.
// Functions only the parent can call.
ChildCollegeFund :: (parent) {
// The parent can deposit money
public mutating func deposit(implicit value: Wei) {
public mutating func allowWithdrawaL() {
self.canWithdraw = true
public func getContents() -> Int {
return contents.getRawValue()
public func getDistanceFromGoal() -> Int {
// The caller of this function is known to be 'parent'
// Therefore, the calls to getTarget and getContents can be performed
return getTarget() - getContents() // OK
// Functions only the child can call
ChildCollegeFund :: (child) {
public mutating func withdraw() {
assert(canWithdraw == false)
send(child, &contents)


When a user creates a transaction to call a function, they can attach Ether to send to the contract. Functions which expect Ether to be attached when called must be annotated with the @payable annotation otherwise the transaction will revert when Ether is attached.

When adding the annotation, a parameter marked implicit of type Wei must be declared. implicit parameters are a mechanism to expose information from the Ethereum transaction to the developer of the smart contract, without using globally accessible variables defined by the language, such as msg.value in Solidity. This mechanism allows developers to name implicit variables themselves, and do not need to remember the name of a global variable.

Functions in Flint can be marked as payable using the @payable attribute. The amount of Wei sent is bound to an implicit variable:

public func receiveMoney(implicit value: Wei) {

Payable functions may have an arbitrary amount of parameters, but exactly one needs to be implicit and of a currency type.


JavaScript applications can listen to events emitted by an Ethereum smart contract.

In Flint, events are declared in contract declarations. They use a similar syntax to structs except using the keyword event and only allowing constants. It can be assigned a default value if desired.

Events can then be emitted using the keyword emit followed by an event call. An event call is the identifier followed by the arguments to the event which should be labelled with the correct variable name followed a colon and the value it should take.

contract Bank {
var balances: [Address: Int]
event didCompleteTransfer {
let origin: Address
let destination: Address
let amount: Int
Bank :: caller <- (any) {
mutating func transfer(to: Address, value: Int) {
balances[caller] -= value
balances[to] += value
// A JavaScript client could listen for this event
emit didCompleteTransfer(origin: caller, destination: to, amount: value)

Type States

Flint introduces the concept of type states. Insufficient and incorrect state management in Solidity code have led to security vulnerabilities and unexpected behaviour in widely deployed smart contracts. Avoiding these vulnerabilities by the design of the language is a strong advantage.

In Flint, states of a contract are declared within protection blocks, which protect the enclosed function from invalid calls.

// Ahyone can deposit into the Bank iff the state is Deposit
Bank @(Deposit) :: (any) { // Deposit is a state identifier.
func deposit(address: Address) {
// body

States are identifiers declared in the contract's declaration.

contract Auction (Preparing, InProgress, Terminated) {}
// Preparing, InProgress, Terminated are State Identifiers

Note: The special state identifier any allows execution of the function in the group in any state.

Calls to Flint functions are validated both at compile-time and runtime.

contract Auction (Preparing, InProgress, Terminated) { // Enumeration of states.
var beneficiary: Address
var highestBidder: Address
var highestBid: Wei
Auction @(any) :: caller <- (any) {
public init() {
self.beneficiary = caller
self.highestBidder = caller
self.highestBid = Wei(0)
become Preparing
Auction @(Preparing) :: (beneficiary) {
public mutating func setBeneficiary(beneficiary: Address) {
self.beneficiary = beneficiary
mutating func openAuction() -> Bool {
// ...
return true
become InProgress
Auction @(InProgress) :: (beneficiary) {
mutating func endAuction() {
// ...
become Terminated
Auction @(InProgress) :: (any) {
func bid(implicit value: Wei) -> Bool{
// ...
// State is not explicitly changed.
return Bool


We introduce the concept of ‘traits’ to Flint based in part on Rust Traits. Traits describe the partial behaviour of Contract or Structures which conform to them. For Contracts, traits constitute a collection of functions and function stubs in restriction blocks, and events. For Structures, traits only constitute a collection of functions and function stubs.

Contracts or Structures can conform to multiple traits. The Flint compiler enforces the implementation of function stubs in the trait and allows usage of the functions declared in them. Traits allow a level of abstraction and code reuse for Contracts and Structures. We also plan to have Standard Library Traits that can be inherited which provide common functionality to Contracts (Ownable, Burnable, MultiSig, Pausable, ERC20, ERC721, etc.) and Structures (Transferable, RawValued, Describable etc.). It will also form the basis for allowing end users to access compiler level guarantees and restrictions as in Assets and Numerics.

Structure Traits

Traits can be implemented for structures using the keyword combination of struct trait followed by a unique identifier before the block of trait members.

Structure traits can contain functions, function signatures, initialisers, or initialiser signatures. A signature is simply missing a code block following it.

In the example below we define an Animal structure trait. The Person structure then conforms to the Animal trait allowing the use of functions within that.

struct trait Animal {
// Must have an empty and named initialiser
public init()
public init(name: String)
// These are signatures that conforming structures must implement
// access properties of the structure
func isNamed() -> Bool
public func name() -> String
public func noise() -> String
// This is a pre-implemented function using the functions already in the trait.
public func speak() -> String {
if isNamed() {
return name()
else {
return noise()
struct Person: Animal {
let name: String
public init() { = "John Doe"
public init(name: String) { = name
// People always have a name, it's just not always known
func isNamed() -> Bool {
return true
// These access the properties of the struct
public func name() -> String {
public func noise() -> String {
return "Huh?"
// Person can also have functions in addition to Animal
public func greet() -> String {
return "Hi"

Contract Traits

Traits can be implemented for structures using the keyword combination of contract trait followed by a unique identifier before the block of trait members.

Contract traits can contain anonymous contract behaviour declarations containing methods or method signatures, or events.

In the example below, we define Ownable, which declares a contract as something that can be owned and transferred. The Ownable trait is then specified by the ToyWallet contract allowing the use of methods in Ownable. This demonstrates how we can expose contract properties:

contract trait Ownable {
event OwnershipRenounced {
let previousOwner: Address
event OwnershipTransfered {
let previousOwner: Address
let newOwner: Address
self :: (any) {
public getOwner() -> Address
self :: (getOwner) {
func setOwner(newOwner: Address)
public func renounceOwnership() {
emit OwnershipRenounced(getOwner())
public func transferOwnership(newOwner: Address) {
assert(newOwner != 0x000...)
emit OwnershipTransfered(getOwner(), newOwner)
contract ToyWallet: Ownable {
visible var owner: Address // visible automatically creates getOwner
// Skipping initialiser as not relevant for this example
ToyWallet :: (getOwner) {
func setOwner(newOwner: Address){
self.owner = newOwner

Standard Library


The standard library defines the Wei type. For full details see Assets

Global Functions


Assertions are checks that happen at runtime. They are used to ensure an essential condition is satisfied before executing any further code. If the Boolean condition evaluates to true then the execution continues as usual. Otherwise the transaction is reverted. It is a global function accessible from any contract or contract group:

assert(x == 2)

In essence an assertion is a shorthand for the longer:

if x == 2 {

Fatal Error

fatalError() is a function exposed that reverts a transaction when called. This means that any contract storage changes are rolledback and no values are returned.


send(address: Address, value: inout Wei) sends the value Wei to the Ethereum address address, and clears the contents of value.


Bank Smart Contract

The following code declares the Bank contract and its functions. More examples are available on GitHub.

// Contract declarations contain only their state properties.
contract Bank {
var manager: Address
var balances: [Address: Wei] = [:]
var accounts: [Address] = []
var lastIndex: Int = 0
var totalDonations: Wei = Wei(0)
var didCompleteTransfer: Event<Address, Address, Int>
// The functions in this block can be called by any user.
Bank :: account <- (any) {
public init(manager: Address) {
self.manager = manager
// Returns the manager's address.
public mutating func register() {
accounts[lastIndex] = account
lastIndex += 1
public func getManager() -> Address {
return manager
public mutating func donate(implicit value: Wei) {
// This will transfer the funds into totalDonations.
// Only the manager can call these functions.
Bank :: (manager) {
// This function needs to be declared "mutating" as its body mutates
// the contract's state.
public mutating func freeDeposit(account: Address, amount: Int) {
var w: Wei = Wei(amount)
public mutating func clear(account: Int) {
balances[account] = Wei(0)
// This function is non-mutating.
public func getDonations() -> Int {
return totalDonations.getRawValue()
// Any user in accounts can call these functions.
// The matching user's address is bound to the variable account.
Bank :: account <- (accounts) {
public func getBalance() -> Int {
return balances[account].getRawValue()
public mutating func transfer(amount: Int, destination: Address) {
// Transfer Wei from one account to another. The balances of the
// originator and the destination are updated atomically.
// Crashes if balances[account] doesn't have enough Wei.
balances[destination].transfer(&balances[account], amount)
// Emit the Ethereum event.
didCompleteTransfer(account, destination, amount)
public mutating func withdraw(amount: Int) {
// Transfer some Wei from balances[account] into a local variable.
let w: Wei = Wei(&balances[account], amount)
// Send the amount back to the Ethereum user.
send(account, &w)