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The Flint Programming Language Build Status

Flint is a new type-safe, contract-oriented programming language specifically designed for writing robust smart contracts on Ethereum.

Flint is still in alpha development, and is not ready to be used in production yet.

Medium articles: Write Safer Smart Contracts and Flint: A New Language for Safe Smart Contracts on Ethereum

Programming 2018! paper: Writing Safe Smart Contracts in Flint

Current working paper: Flint for Safer Smart Contracts

Flint has been developed as part of projects and summer work at Imperial College Department of Computing under the supervision of Professors Susan Eisenbach and Sophia Drossopoulou. Its original developer was Franklin Schrans for his MEng thesis and then continued. The following people have contributed to Flint: Matteo Bilardi, Aurel Bily, Mohammad Chowdhury, Catalin Craciun, Calin Farcas, Ioannis Gabrielides, Daniel Hails, Alexander Harkness, Constantin Mueller, Yicheng Leo, Matthew Ross Rachar, Franklin Schrans, Niklas Vangerow, and Manshu Wang.

The documentation (reports and presentations) can be accessed here and the codebase is here. We are very pleased to have support from the Ethereum Foundation for this work.

Language Overview

The Flint Programming Language Guide (website, local) gives a high-level overview of the language, and helps you getting started with smart contract development in Flint.

Flint is still under active development and proposes a variety of novel contract-oriented features.

Caller Protections

Caller protections require programmers to think about who should be able to call the contracts sensitive functions. Protections are checked statically for internal calls (unlike Solidity modifiers), and at runtime for calls originating from external contracts.


// State declaration
contract Bank {
  var manager: Address

// Functions are declared in protection blocks,
// which specify which users are allowed to call them.
Bank :: (manager) { // manager is a state property.

  // Only `manager` of the Bank can call `clear`.
  func clear(address: Address) {
    // body

// Anyone can initialize the contract.
Bank :: (any) {
  public init(manager: Address) {
    self.manager = manager

Type States

Type States integrate a design pattern of stateful contracts into the language itself, which both require programmers to think about what state a function can be called in but also to prevent vulnerabilities (e.g. Parity Multi-Sig wallet) from mistakes with respect to administrating state. States are checked statically for internal calls (unlike Solidity modifiers), and at runtime for calls originating from external contracts.


// Enumeration of states.
contract Auction (Preparing, InProgress) {}

Auction @(Preparing, InProgress) :: caller <- (any) {
  public init() {
    // ...
    become Preparing

Auction @(Preparing) :: (beneficiary) {
  public func setBeneficiary(beneficiary: Address) mutates (beneficiary) {
    self.beneficiary = beneficiary

  func openAuction() -> Bool {
    // ...
    become InProgress

Immutability by default

Restricting writes to state in functions helps programmers more easily reason about the smart contract. A function which writes to the contracts state needs to be annotated with the mutates (...) keyword, giving the list of variables that are mutated.


Bank :: (any) {
  func incrementCount() mutates (count) {
    // count is a state property
    count += 1

  func getCount() -> Int {
    return count

  func decrementCount() {
    // error: Use of mutating statement in a nonmutating function
    // count -= 1

Asset types

Assets, such as Ether, are often at the center of smart contracts. Flint puts assets at the forefront through the special Asset trait.

Flint's Asset type ensure a contract's state always truthfully represents its Ether value, preventing attacks such as TheDAO.

A restricted set of atomic operations can be performed on Assets. It is impossible to create, duplicate, or lose Assets (such as Ether) in unprivileged code. This prevents attacks relating to double-spending and re-entrancy.

Example use:

Bank :: account <- (balances.keys) {
   func deposit(implicit value: inout Wei) mutates (balances) {
    // Omitting this line causes a compiler warning: the value received should be recorded.

  func withdraw() mutates(account, balances) {
    // balances[account] is automatically set to 0 before transferring.
    send(account, &balances[account])

The Asset feature is still in development. The FIP-0001: Introduce the Asset trait proposal includes more details.

Safer semantics

In the spirit of reducing vulnerabilities relating to unexpected language semantics, such as wrap-arounds due to integer overflows, Flint aims to provide safer operations. For instance, arithmetic operations on Int are safe by default: an overflow/underflow causes the Ethereum transaction to be reverted.


For detailed installation instructions, see the language guide, however, to install Flint on Ubuntu 18.04 LTS, you can simply run:

bash <(curl -s

which will install Flint into "~/.flint". An older version of the Flint compiler and its dependencies can be installed straight from Docker Hub:

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

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


Contributions to Flint are highly welcomed! Contribution Guide The Issues page tracks the tasks which have yet to be completed.

Flint Improvement Proposals (FIPs) track the design and implementation of larger new features for Flint or the Flint compiler. An example is FIP-0001: Introduce the Asset trait.

Cloning Repo

For more information, see Building from source in the language guide

Assuming you have all the prerequisites, you should be able to build flint by running

git clone --recurse-submodules $HOME/.flint
cd $HOME/.flint
make release
echo "export PATH=$HOME/.flint/.build/release/:$PATH" >> ~/.bashrc
source ~/.bashrc

Ubuntu 18.04 LTS

This assumes a standard Ubuntu build with apt, wget, curl, gnupg, ca-certificates and git installed. If you don't have one of them installed, you should be notified during the process. If you have any kind of error, try installing them. Note Ubuntu 16.04 has different installation procedures when using apt and installing Mono, thus the process would need to be done manually. You may find the 18.04 install script a good place to start on what you need to install it.

bash <(curl -s

Future plans

Future plans for Flint are numerous, and include:

  1. Gas estimation: provide estimates about the gas execution cost of a function. Gas upper bounds are emitted as part of the contract's interface, making it possible to obtain the estimation of a call to an external Flint function.
  2. Formalization: specify well-defined semantics for the language.
  3. The Flint Package Manager: create a package manager which runs as a Flint smart contract on Ethereum. It will store contract APIs as well as safety and gas cost information of dependencies.
  4. Tooling: build novel tools around smart contract development, such as new ways of simulating and visualizing different transaction orderings.


The Flint project is available under the MIT license. See the LICENSE file for more information.

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