Release Notes

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Fe is moving fast. Read up on all the latest improvements.

WARNING: All Fe releases are alpha releases and only meant to share the development progress with developers and enthusiasts. It is NOT yet ready for production usage.

0.19.1-alpha "Sunstone" (2022-07-06)


  • Support returning nested struct.


    pub struct InnerStruct {
        pub inner_s: String<10>
        pub inner_x: i256
    pub struct NestedStruct {
        pub inner: InnerStruct
        pub outer_x: i256
    contract Foo {
        pub fn return_nested_struct() -> NestedStruct {


  • Made some small changes to how the Context object is used.

    • ctx is not required when casting an address to a contract type. Eg let foo: Foo = Foo(address(0))
    • ctx is required when calling an external contract function that requires ctx


    use std::context::Context # see issue #679
    contract Foo {
      pub fn emit_stuff(ctx: Context) {
        emit Stuff(ctx)  # will be `ctx.emit(Stuff{})` someday
    contract Bar {
      pub fn call_foo_emit_stuff(ctx: Context) {
    event Stuff {}


  • Braces! Fe has abandoned python-style significant whitespace in favor of the trusty curly brace.

    In addition, elif is now spelled else if, and the pass statement no longer exists.


    pub struct SomeError {}
    contract Foo {
      x: u8
      y: u16
      pub fn f(a: u8) -> u8 {
        if a > 10 {
          let x: u8 = 5
          return a + x
        } else if a == 0 {
          revert SomeError()
        } else {
          return a * 10
      pub fn noop() {}


  • traits and generic function parameter

    Traits can now be defined, e.g:

    trait Computable {
      fn compute(self, val: u256) -> u256;

    The mechanism to implement a trait is via an impl block e.g:

    struct Linux {
      pub counter: u256
      pub fn get_counter(self) -> u256 {
        return self.counter
      pub fn something_static() -> u256 {
        return 5
    impl Computable for Linux {
      fn compute(self, val: u256) -> u256 {
        return val + Linux::something_static() + self.get_counter()

    Traits can only appear as bounds for generic functions e.g.:

    struct Runner {
      pub fn run<T: Computable>(self, _ val: T) -> u256 {
        return val.compute(val: 1000)

    Only struct functions (not contract functions) can have generic parameters. The run method of Runner can be called with any type that implements Computable e.g.

    contract Example {
      pub fn generic_compute(self) {
        let runner: Runner = Runner();
        assert == 1001
        assert 10)) == 1015


  • Generate artifacts for all contracts of an ingot, not just for contracts that are defined in main.fe (#726)

  • Allow using complex type as array element type.


    contract Foo {
        pub fn bar() -> i256 {
            let my_array: Array<Pair, 3> = [Pair::new(1, 0), Pair::new(2, 0), Pair::new(3, 0)]
            let sum: i256 = 0
            for pair in my_array {
                sum += pair.x
            return sum
    struct Pair {
        pub x: i256
        pub y: i256
        pub fn new(_ x: i256, _ y: i256) -> Pair {
            return Pair(x, y)


  • The fe CLI now has subcommands:

    fe new myproject - creates a new project structure fe check . - analyzes fe source code and prints errors fe build . - builds a fe project (#732)

  • Support passing nested struct types to public functions.


    pub struct InnerStruct {
        pub inner_s: String<10>
        pub inner_x: i256
    pub struct NestedStruct {
        pub inner: InnerStruct
        pub outer_x: i256
    contract Foo {
        pub fn f(arg: NestedStruct) {


  • Added support for repeat expressions ([VALUE; LENGTH]).


    let my_array: Array<bool, 42> = [bool; 42] 

    Also added checks to ensure array and struct types are initialized. These checks are currently performed at the declaration site, but will be loosened in the future. (#747)


  • Fix a bug that incorrect instruction is selected when the operands of a comp instruction are a signed type. (#734)

  • Fix issue where a negative constant leads to an ICE

    E.g. the following code would previously crash the compiler but shouldn't:

    const INIT_VAL: i8 = -1
    contract Foo {
      pub fn init_bar() {
        let x: i8 = INIT_VAL


  • Fix a bug that causes ICE when nested if-statement has multiple exit point.

    E.g. the following code would previously crash the compiler but shouldn't:

     pub fn foo(self) {
        if true {
            if self.something { 
        if true {
            if self.something { 


0.18.0-alpha "Ruby" (2022-05-27)


  • Added support for parsing of attribute calls with generic arguments (e.g.<Baz>()). (#719)


  • Fix a regression where the stateMutability field would not be included in the generated ABI (#722)
  • Fix two regressions introduced in 0.17.0
    • Properly lower right shift operation to yul's sar if operand is signed type
    • Properly lower negate operation to call safe_sub (#723)

0.17.0-alpha "Quartz" (2022-05-26)


  • Support for underscores in numbers to improve readability e.g. 100_000.


        let num: u256 = 1000_000_000_000


  • Optimized access of struct fields in storage (#249)

  • Unit type () is now ABI encodable (#442)

  • Temporary default stateMutability to payable in ABI

    The ABI metadata that the compiler previously generated did not include the stateMutability field. This piece of information is important for tooling such as hardhat because it determines whether a function needs to be called with or without sending a transaction.

    As soon as we have support for mut self and mut ctx we will be able to derive that information from the function signature. In the meantime we now default to payable. (#705)


  • Fixed a crash caused by certain memory to memory assignments.

    E.g. the following code would previously lead to a compiler crash:

    my_struct.x = my_struct.y


  • Reject unary minus operation if the target type is an unsigned integer number.

    Code below should be reject by fe compiler:

    contract Foo:
        pub fn bar(self) -> u32:
            let unsigned: u32 = 1
            return -unsigned
        pub fn foo():
            let a: i32 = 1
            let b: u32 = -a


  • Fixed crash when passing a struct that contains an array

    E.g. the following would previously result in a compiler crash:

    struct MyArray:
        pub x: Array<i32, 2>
    contract Foo:
        pub fn bar(my_arr: MyArray):


  • reject infinite size struct definitions.

    Fe structs having infinite size due to recursive definitions were not rejected earlier and would cause ICE in the analyzer since they were not properly handled. Now structs having infinite size are properly identified by detecting cycles in the dependency graph of the struct field definitions and an error is thrown by the analyzer. (#682)

  • Return instead of revert when contract is called without data.

    If a contract is called without data so that no function is invoked, we would previously revert but that would leave us without a way to send ETH to a contract so instead it will cause a return now. (#694)

  • Resolve compiler crash when using certain reserved YUL words as struct field names.

    E.g. the following would previously lead to a compiler crash because numer is a reserved keyword in YUL.

    struct Foo:
      pub number: u256
    contract Meh:
      pub fn yay() -> Foo:
        return Foo(number:2)


0.16.0-alpha (2022-05-05)


  • Change static function call syntax from to Bar::foo() (#241)
  • Added support for retrieving the base fee via ctx.base_fee() (#503)


  • Resolve functions on structs via path (e.g. bi::ba::bums()) (#241)

0.15.0-alpha (2022-04-04)


  • Labels are now required on function arguments. Labels can be omitted if the argument is a variable with a name that matches the label, or if the function definition specifies that an argument should have no label. Functions often take several arguments of the same type; compiler-checked labels can help prevent accidentally providing arguments in the wrong order.


    contract CoolCoin:
      balance: Map<address, i256>
      loans: Map<(address, address), i256>
      pub fn demo(self, ann: address, bob: address):
        let is_loan: bool = false
        self.give(from: ann, to: bob, 100, is_loan)
      fn transfer(self, from sender: address, to recipient: address, _ val: u256, is_loan: bool):
        self.cred[sender] -= val
        self.cred[recipient] += val
        if is_loan:
[(sender, recipient)] += val

    Note that arguments must be provided in the order specified in the function definition.

    A parameter's label defaults to the parameter name, but can be changed by specifying a different label to the left of the parameter name. Labels should be clear and convenient for the caller, while parameter names are only used in the function body, and can thus be longer and more descriptive. In the example above, we choose to use sender and recipient as identifiers in the body of fn transfer, but use labels from: and to:.

    In cases where it's ideal to not have labels, e.g. if a function takes a single argument, or if types are sufficient to differentiate between arguments, use _ to specify that a given parameter has no label. It's also fine to require labels for some arguments, but not others.


    fn add(_ x: u256, _ y: u256) -> u256:
      return x + y
    contract Foo:
      fn transfer(self, _ to: address, wei: u256):
      pub fn demo(self):
        transfer(address(0), wei: add(1000, 42))
    ``` ([#397](


  • The region of memory used to compute the slot of a storage map value was not being allocated. (#684)

0.14.0-alpha (2022-03-02)


  • Events can now be defined outside of contracts.


    event Transfer:
        idx sender: address
        idx receiver: address
        value: u256
    contract Foo:
        fn transferFoo(to: address, value: u256):
            emit Transfer(sender: msg.sender, receiver: to, value)
    contract Bar:
        fn transferBar(to: address, value: u256):
            emit Transfer(sender: msg.sender, receiver: to, value)
    ``` ([#80](
  • The Fe standard library now includes a std::evm module, which provides functions that perform low-level evm operations. Many of these are marked unsafe, and thus can only be used inside of an unsafe function or an unsafe block.


    use std::evm::{mstore, mload}
    fn memory_shenanigans():
        mstore(0x20, 42)
        let x: u256 = mload(0x20)
        assert x == 42

    The global functions balance and balance_of have been removed; these can now be called as std::evm::balance(), etc. The global function send_value has been ported to Fe, and is now available as std::send_value. (#629)

  • Support structs that have non-base type fields in storage.


    struct Point:
        pub x: u256
        pub y: u256
    struct Bar:
        pub name: String<3>
        pub numbers: Array<u256, 2>
        pub point: Point
        pub something: (u256, bool)
    contract Foo:
        my_bar: Bar
        pub fn complex_struct_in_storage(self) -> String<3>:
            self.my_bar = Bar(
                name: "foo",
                numbers: [1, 2],
                point: Point(x: 100, y: 200),
                something: (1, true),
            # Asserting the values as they were set initially
            assert self.my_bar.numbers[0] == 1
            assert self.my_bar.numbers[1] == 2
            assert self.my_bar.point.x == 100
            assert self.my_bar.point.y == 200
            assert self.my_bar.something.item0 == 1
            assert self.my_bar.something.item1
            # We can change the values of the array
            self.my_bar.numbers[0] = 10
            self.my_bar.numbers[1] = 20
            assert self.my_bar.numbers[0] == 10
            assert self.my_bar.numbers[1] == 20
            # We can set the array itself
            self.my_bar.numbers = [1, 2]
            assert self.my_bar.numbers[0] == 1
            assert self.my_bar.numbers[1] == 2
            # We can change the values of the Point
            self.my_bar.point.x = 1000
            self.my_bar.point.y = 2000
            assert self.my_bar.point.x == 1000
            assert self.my_bar.point.y == 2000
            # We can set the point itself
            self.my_bar.point = Point(x=100, y=200)
            assert self.my_bar.point.x == 100
            assert self.my_bar.point.y == 200
            # We can change the value of the tuple
            self.my_bar.something.item0 = 10
            self.my_bar.something.item1 = false
            assert self.my_bar.something.item0 == 10
            assert not self.my_bar.something.item1
            # We can set the tuple itself
            self.my_bar.something = (1, true)
            assert self.my_bar.something.item0 == 1
            assert self.my_bar.something.item1
    ``` ([#636](
  • Features that read and modify state outside of contracts are now implemented on a struct named "Context". Context is included in the standard library and can be imported with use std::context::Context. Instances of Context are created by calls to public functions that declare it in the signature or by unsafe code.

    Basic example:

    use std::context::Context
    contract Foo:
        my_num: u256
        pub fn baz(ctx: Context) -> u256:
            return ctx.block_number()
        pub fn bing(self, new_num: u256) -> u256:
            self.my_num = new_num
            return self.my_num
    contract Bar:
        pub fn call_baz(ctx: Context, foo_addr: address) -> u256:
            # future syntax: `let foo = ctx.load<Foo>(foo_addr)`
            let foo: Foo = Foo(ctx, foo_addr)
            return foo.baz()
        pub fn call_bing(ctx: Context) -> u256:
            # future syntax: `let foo = ctx.create<Foo>(0)`
            let foo: Foo = Foo.create(ctx, 0)

    Example with __call__ and unsafe block:

    use std::context::Context
    use std::evm
    contract Foo:
        pub fn __call__():
                # creating an instance of `Context` is unsafe
                let ctx: Context = Context()
                let value: u256 = u256(bar(ctx))
                # return `value`
                evm::mstore(0, value)
                evm::return_mem(0, 32)
        fn bar(ctx: Context) -> address:
            return ctx.self_address()
    ``` ([#638](
  • Features

    Support local constant


    contract Foo:
        pub fn bar():
            const LOCAL_CONST: i32 = 1

    Support constant expression


    const GLOBAL: i32 = 8
    contract Foo:
        pub fn bar():
            const LOCAL: i32 = GLOBAL * 8

    Support constant generics expression


    const GLOBAL: u256= 8
    const USE_GLOBAL: bool = false
    type MY_ARRAY = Array<i32, { GLOBAL / 4 }>
    contract Foo:
        pub fn bar():
            let my_array: Array<i32, { GLOBAL if USE_GLOBAL else 4 }>

    Bug fixes

    Fix ICE when constant type is mismatch


    const GLOBAL: i32 = "FOO"
    contract Foo:
        pub fn bar():
            let FOO: i32 = GLOBAL

    Fix ICE when assigning value to constant twice


    const BAR: i32 = 1
    contract FOO:
        pub fn bar():
            BAR = 10
    ``` ([#649](
  • Argument label syntax now uses : instead of =. Example:

    struct Foo:
      x: u256
      y: u256
    let x: MyStruct = MyStruct(x: 10, y: 11)
    # previously:     MyStruct(x = 10, y = 11)
    ``` ([#665](
  • Support module-level pub modifier, now default visibility of items in a module is private.


    # This constant can be used outside of the module.
    pub const PUBLIC:i32 = 1
    # This constant can NOT be used outside of the module.
    const PRIVATE: i32 = 1
    ``` ([#677](

Internal Changes - for Fe Contributors

    • Source files are now managed by a (salsa) SourceDb. A SourceFileId now corresponds to a salsa-interned File with a path. File content is a salsa input function. This is mostly so that the future (LSP) language server can update file content when the user types or saves, which will trigger a re-analysis of anything that changed.
    • An ingot's set of modules and dependencies are also salsa inputs, so that when the user adds/removes a file or dependency, analysis is rerun.
    • Standalone modules (eg a module compiled with fe fee.fe) now have a fake ingot parent. Each Ingot has an IngotMode (Lib, Main, StandaloneModule), which is used to disallow ingot::whatever paths in standalone modules, and to determine the correct root module file.
    • parse_module now always returns an ast::Module, and thus a ModuleId will always exist for a source file, even if it contains fatal parse errors. If the parsing fails, the body will end with a ModuleStmt::ParseError node. The parsing will stop at all but the simplest of syntax errors, but this at least allows partial analysis of source file with bad syntax.
    • ModuleId::ast(db) is now a query that parses the module's file on demand, rather than the AST being interned into salsa. This makes handling parse diagnostics cleaner, and removes the up-front parsing of every module at ingot creation time. (#628)

0.13.0-alpha (2022-01-31)## 0.13.0-alpha (2022-01-31)


  • Support private fields on structs

    Public fields now need to be declared with the pub modifier, otherwise they default to private fields. If a struct contains private fields it can not be constructed directly except from within the struct itself. The recommended way is to implement a method new(...) as demonstrated in the following example.

    struct House:
        pub price: u256
        pub size: u256
        vacant: bool
        pub fn new(price: u256, size: u256) -> House
          return House(price=price, size=size, vacant=true)
    contract Manager:
      house: House
      pub fn create_house(price: u256, size: u256): = House::new(price, size)
        let can_access_price: u256 =
        # can not access `` because the field is private
    ``` ([#214](
  • Support non-base type fields in structs

    Support is currently limited in two ways:

    • Structs with complex fields can not be returned from public functions
    • Structs with complex fields can not be stored in storage (#343)
  • Addresses can now be explicitly cast to u256. For example:

    fn f(addr: address) -> u256:
      return u256(addr)
    ``` ([#621](
  • A special function named __call__ can now be defined in contracts.

    The body of this function will execute in place of the standard dispatcher when the contract is called.

    example (with intrinsics):

    contract Foo:
        pub fn __call__(self):
                if __calldataload(0) == 1:
                    __revert(0, 0)
                    __return(0, 0)
    ``` ([#622](


  • Fixed a crash that happend when using a certain unprintable ASCII char (#551)

  • The argument to revert wasn't being lowered by the compiler, meaning that some revert calls would cause a compiler panic in later stages. For example:

    const BAD_MOJO: u256 = 0xdeaddead
    struct Error:
      code: u256
    fn fail():
      revert Error(code = BAD_MOJO)
    ``` ([#619](
  • Fixed a regression where an empty list expression ([]) would lead to a compiler crash. (#623)

  • Fixed a bug where int array elements were not sign extended in their ABI encodings. (#633)

0.12.0-alpha (2021-12-31)## 0.12.0-alpha (2021-12-31)


  • Added unsafe low-level "intrinsic" functions, that perform raw evm operations. For example:

    fn foo():
        __mtore(0, 5000)
        assert __mload(0) == 5000

    The functions available are exactly those defined in yul's "evm dialect": but with a double-underscore prefix. Eg selfdestruct -> __selfdestruct.

    These are intended to be used for implementing basic standard library functionality, and shouldn't typically be needed in normal contract code.

    Note: some intrinsic functions don't return a value (eg __log0); using these functions in a context that assumes a return value of unit type (eg let x: () = __log0(a, b)) will currently result in a compiler panic in the yul compilation phase. (#603)

  • Added an out of bounds check for accessing array items. If an array index is retrieved at an index that is not within the bounds of the array it now reverts with Panic(0x32). (#606)


  • Ensure ternary expression short circuit.


    contract Foo:
        pub fn bar(input: u256) -> u256:
            return 1 if input > 5 else revert_me()
        fn revert_me() -> u256:
            return 0

    Previous to this change, the code above would always revert no matter which branch of the ternary expressions it would resolve to. That is because both sides were evaluated and then one side was discarded. With this change, only the branch that doesn't get picked won't get evaluated at all.

    The same is true for the boolean operations and and or. (#488)

Internal Changes - for Fe Contributors

  • Added a globally available dummy std lib.

    This library contains a single get_42 function, which can be called using std::get_42(). Once low-level intrinsics have been added to the language, we can delete get_42 and start adding useful code. (#601)

0.11.0-alpha "Karlite" (2021-12-02)


  • Added support for multi-file inputs.

    Implementation details:

    Mostly copied Rust's crate system, but use the the term ingot instead of crate.

    Below is an example of an ingot's file tree, as supported by the current implementation.

    `-- basic_ingot
        `-- src
            |-- bar
            |   `-- baz.fe
            |-- bing.fe
            |-- ding
            |   |-- dang.fe
            |   `-- dong.fe
            `-- main.fe

    There are still a few features that will be worked on over the coming months:

    • source files accompanying each directory module (e.g. my_mod.fe)
    • configuration files and the ability to create library ingots
    • test directories
    • module-level pub modifier (all items in a module are public)
    • mod statements (all fe files in the input tree are public modules)

    These things will be implemented in order of importance over the next few months. (#562)

  • The syntax for array types has changed to match other generic types. For example, u8[4] is now written Array<u8, 4>. (#571)

  • Functions can now be defined on struct types. Example:

    struct Point:
      x: u64
      y: u64
      # Doesn't take `self`. Callable as `Point.origin()`.
      # Note that the syntax for this will soon be changed to `Point::origin()`.
      pub fn origin() -> Point:
        return Point(x=0, y=0)
      # Takes `self`. Callable on a value of type `Point`.
      pub fn translate(self, x: u64, y: u64):
        self.x += x
        self.y += y
      pub fn add(self, other: Point) -> Point:
        let x: u64 = self.x + other.x
        let y: u64 = self.y + other.y
        return Point(x, y)
      pub fn hash(self) -> u256:
        return keccak256(self.abi_encode())
    pub fn do_pointy_things():
      let p1: Point = Point.origin()
      p1.translate(5, 10)
      let p2: Point = Point(x=1, y=2)
      let p3: Point = p1.add(p2)
      assert p3.x == 6 and p3.y == 12
    ``` ([#577](


  • Fixed a rare compiler crash.


    let my_array: i256[1] = [-1 << 1] 

    Previous to this fix, the given example would lead to an ICE. (#550)

  • Contracts can now create an instance of a contract defined later in a file. This issue was caused by a weakness in the way we generated yul. (#596)

Internal Changes - for Fe Contributors

  • File IDs are now attached to Spans. (#587)

  • The fe analyzer now builds a dependency graph of source code "items" (functions, contracts, structs, etc). This is used in the yulgen phase to determine which items are needed in the yul (intermediate representation) output. Note that the yul output is still cluttered with utility functions that may or may not be needed by a given contract. These utility functions are defined in the yulgen phase and aren't tracked in the dependency graph, so it's not yet possible to filter out the unused functions. We plan to move the definition of many of these utility functions into fe; when this happens they'll become part of the dependency graph and will only be included in the yul output when needed.

    The dependency graph will also enable future analyzer warnings about unused code. (#596)

0.10.0-alpha (2021-10-31)


  • Support for module level constants for base types


    const TEN = 10
      pub fn do_moon_math(self) -> u256:
        return 4711 * TEN

    The values of base type constants are always inlined. (#192)

  • Encode revert errors for ABI decoding as Error(0x103) not Panic(0x99) (#492)

  • Replaced import statements with use statements.


    use foo::{bar::*, baz as baz26}

    Note: this only adds support for parsing use statements. (#547)

  • Functions can no be defined outside of contracts. Example:

    fn add_bonus(x: u256) -> u256:
        return x + 10
    contract PointTracker:
        points: Map<address, u256>
        pub fn add_points(self, user: address, val: u256):
            self.points[user] += add_bonus(val)
    ``` ([#566](
  • Implemented a send_value(to: address, value_in_wei: u256) function.

    The function is similar to the sendValue function by OpenZeppelin with the differences being that:

    1. It reverts with Error(0x100) instead of Error("Address: insufficient balance") to safe more gas.

    2. It uses selfbalance() instead of balance(address()) to safe more gas

    3. It reverts with Error(0x101) instead of Error("Address: unable to send value, recipient may have reverted") also to safe more gas. (#567)

  • Added support for unsafe functions and unsafe blocks within functions. Note that there's currently no functionality within Fe that requires the use of unsafe, but we plan to add built-in unsafe functions that perform raw evm operations which will only callable within an unsafe block or function. (#569)

  • Added balance() and balance_of(account: address) methods. (#572)

  • Added support for explicit casting between numeric types.


    let a: i8 = i8(-1)
    let a1: i16 = i16(a)
    let a2: u16 = u16(a1)
    assert a2 == u16(65535)
    let b: i8 = i8(-1)
    let b1: u8 = u8(b)
    let b2: u16 = u16(b1)
    assert b2 == u16(255)

    Notice that Fe allows casting between any two numeric types but does not allow to change both the sign and the size of the type in one step as that would leave room for ambiguity as the example above demonstrates. (#576)


  • Adjust numeric values loaded from memory or storage

    Previous to this fix numeric values that were loaded from either memory or storage were not properly loaded on the stack which could result in numeric values not treated as intended.


    contract Foo:
        pub fn bar() -> i8:
            let in_memory: i8[1] = [-3]
            return in_memory[0]

    In the example above bar() would not return -3 but 253 instead. (#524)

  • Propagate reverts from external contract calls.

    Before this fix the following code to should_revert() or should_revert2() would succeed even though it clearly should not.

    contract A:
      contract_b: B
      pub fn __init__(contract_b: address):
        self.contract_b = B(contract_b)
      pub fn should_revert():
      pub fn should_revert2():
    struct SomeError:
    contract B:
      pub fn fail():
      pub fn fail_with_custom_error():
        revert SomeError()

    With this fix the revert errors are properly passed upwards the call hierachy. (#574)

  • Fixed bug in left shift operation.


    Let's consider the value 1 as an u8 which is represented as the following 256 bit item on the EVM stack 00..|00000001|. A left shift of 8 bits (val << 8) turns that into 00..01|00000000|.

    Previous to this fix this resulted in the compiler taking 256 as the value for the u8 when clearly 256 is not even in the range of u8 anymore. With this fix the left shift operations was fixed to properly "clean up" the result of the shift so that 00..01|00000000| turns into 00..00|00000000|. (#575)

  • Ensure negation is checked and reverts with over/underflow if needed.


    The minimum value for an i8 is -128 but the maximum value of an i8 is 127 which means that negating -128 should lead to an overflow since 128 does not fit into an i8. Before this fix, negation operations where not checked for over/underflow resulting in returning the oversized value. (#578)

Internal Changes - for Fe Contributors

  • In the analysis stage, all name resolution (of variable names, function names, type names, etc used in code) now happens via a single resolve_name pathway, so we can catch more cases of name collisions and log more helpful error messages. (#555)

  • Added a new category of tests: differential contract testing.

    Each of these tests is pased on a pair of contracts where one implementation is written in Fe and the other one is written in Solidity. The implementations should have the same public APIs and are assumed to always return identical results given equal inputs. The inputs are randomly generated using proptest and hence are expected to discover unknown bugs. (#578)

0.9.0-alpha (2021-09-29)


  • The self variable is no longer implicitly defined in code blocks. It must now be declared as the first parameter in a function signature.


    contract Foo:
        my_stored_num: u256
        pub fn bar(self, my_num: u256):
            self.my_stored_num = my_num
        pub fn baz(self):
        pub fn my_pure_func() -> u256:
            return 42 + 26
    ``` ([#520](
  • The analyzer now disallows defining a type, variable, or function whose name conflicts with a built-in type, function, or object.


    error: type name conflicts with built-in type
    ┌─ compile_errors/shadow_builtin_type.fe:1:6
    1 │ type u256 = u8
    │      ^^^^ `u256` is a built-in type
    ``` ([#539](


  • Fixed cases where the analyzer would correctly reject code, but would panic instead of logging an error message. (#534)
  • Non-fatal parser errors (eg missing parentheses when defining a function that takes no arguments: fn foo:) are no longer ignored if the semantic analysis stage succeeds. (#535)
  • Fixed issue #531 by adding a $ to the front of lowered tuple names. (#546)

Internal Changes - for Fe Contributors

  • Implemented pretty printing of Fe AST. (#540)

0.8.0-alpha "Haxonite" (2021-08-31)


  • Support quotes, tabs and carriage returns in string literals and otherwise restrict string literals to the printable subset of the ASCII table. (#329)

  • The analyzer now uses a query-based system, which fixes some shortcomings of the previous implementation.

    • Types can now refer to other types defined later in the file. Example:
    type Posts = Map<PostId, PostBody>
    type PostId = u256
    type PostBody = String<140>
    • Duplicate definition errors now show the location of the original definition.
    • The analysis of each function, type definition, etc happens independently, so an error in one doesn't stop the analysis pass. This means fe can report more user errors in a single run of the compiler. (#468)
  • Function definitions are now denoted with the keyword fn instead of def. (#496)

  • Variable declarations are now preceded by the let keyword. Example: let x: u8 = 1. (#509)

  • Implemented support for numeric unary invert operator (~) (#526)


  • Calling self.__init__() now results in a nice error instead of a panic in the yul compilation stage. (#468)

  • Fixed an issue where certain expressions were not being moved to the correct location. (#493)

  • Fixed an issue with a missing return statement not properly detected.

    Previous to this fix, the following code compiles but it should not:

    contract Foo:
        pub fn bar(val: u256) -> u256:
            if val > 1:
                return 5

    With this change, the compiler rightfully detects that the code is missing a return or revert statement after the if statement since it is not guaranteed that the path of execution always follows the arm of the if statement. (#497)

  • Fixed a bug in the analyzer which allowed tuple item accessor names with a leading 0, resulting in an internal compiler error in a later pass. Example: my_tuple.item001. These are now rejected with an error message. (#510)

  • Check call argument labels for function calls.

    Previously the compiler would not check any labels that were used when making function calls on self or external contracts.

    This can be especially problematic if gives developers the impression that they could apply function arguments in any order as long as they are named which is not the case.

    contract Foo:
        pub fn baz():
  , doesnt_even_exist=2)
        pub fn bar(val1: u256, val2: u256):

    Code as the one above is now rightfully rejected by the compiler. (#517)

Improved Documentation

  • Various improvements and bug fixes to both the content and layout of the specification. (#489)

  • Document all remaining statements and expressions in the spec.

    Also added a CI check to ensure code examples in the documentation are validated against the latest compiler. (#514)

Internal Changes - for Fe Contributors

  • Separated Fe type traits between crates. (#485)

0.7.0-alpha "Galaxite" (2021-07-27)


  • Enable the optimizer by default. The optimizer can still be disabled by supplying --optimize=false as an argument. (#439)

  • The following checks are now performed while decoding data:

    • The size of the encoded data fits within the size range known at compile-time.
    • Values are correctly padded.
      • unsigned integers, addresses, and bools are checked to have correct left zero padding
      • the size of signed integers are checked
      • bytes and strings are checked to have correct right padding
    • Data section offsets are consistent with the size of preceding values in the data section.
    • The dynamic size of strings does not exceed their maximum size.
    • The dynamic size of byte arrays (u8[n]) is equal to the size of the array. (#440)
  • Type aliases can now include tuples. Example:

    type InternetPoints = (address, u256)


  • Revert with custom errors


    struct PlatformError:
      code: u256
    pub fn do_something():
      revert PlatformError(code=4711)

    Error encoding follows Solidity which is based on EIP-838. This means that custom errors returned from Fe are fully compatible with Solidity. (#464)

    • The builtin value msg.sig now has type u256.
    • Removed the bytes[n] type. The type u8[n] can be used in its placed and will be encoded as a dynamically-sized, but checked, bytes component. (#472)
  • Encode certain reverts as panics.

    With this change, the following reverts are encoded as Panic(uint256) with the following panic codes:

    • 0x01: An assertion that failed and did not specify an error message
    • 0x11: An arithmetic expression resulted in an over- or underflow
    • 0x12: An arithmetic expression divided or modulo by zero

    The panic codes are aligned with the panic codes that Solidity uses. (#476)


  • Fixed a crash when trying to access an invalid attribute on a string.


    contract Foo:
      pub fn foo():

    The above now yields a proper user error. (#444)

  • Ensure String<N> type is capitalized in error messages (#445)

  • Fixed ICE when using a static string that spans over multiple lines.

    Previous to this fix, the following code would lead to a compiler crash:

    contract Foo:
        pub fn return_with_newline() -> String<16>:
            return "foo

    The above code now works as intended. (#448)

  • Fixed ICE when using a tuple declaration and specifying a non-tuple type. Fixed a second ICE when using a tuple declaration where the number of target items doesn't match the number of items in the declared type. (#469)

Internal Changes - for Fe Contributors

    • Cleaned up ABI encoding internals.
    • Improved yulc panic formatting. (#472)

0.6.0-alpha "Feldspar" (2021-06-10)


  • Support for pragma statement

    Example: pragma ^0.1.0 (#361)

  • Add support for tuple destructuring


    my_tuple: (u256, bool) = (42, true)
    (x, y): (u256, bool) = my_tuple


    1. Call expression can now accept generic arguments
    2. Replace stringN to String<N>


    s: String<10> = String<10>("HI")


    • Many analyzer errors now include helpful messages and underlined code.
    • Event and struct constructor arguments must now be labeled and in the order specified in the definition.
    • The analyzer now verifies that the left-hand side of an assignment is actually assignable. (#398)
  • Types of integer literal are now inferred, rather than defaulting to u256.

    contract C:
      fn f(x: u8) -> u16:
        y: u8 = 100   # had to use u8(100) before
        z: i8 = -129  # "literal out of range" error
        return 1000   # had to use `return u16(1000)` before
      fn g():

    Similar inference is done for empty array literals. Previously, empty array literals caused a compiler crash, because the array element type couldn't be determined.

    contract C:
      fn f(xs: u8[10]):
      fn g():

    (Note that array length mismatch is still a type error, so this code won't actually compile.) (#429)

  • The Map type name is now capitalized. Example:

    contract GuestBook:
        guests: Map<address, String<100>>


  • Convert all remaining errors to use the new advanced error reporting system (#432)

  • Analyzer throws an error if __init__ is not public. (#435)

Internal Changes - for Fe Contributors

  • Refactored front-end "not implemented" errors into analyzer errors and removed questionable variants. Any panic is now considered to be a bug. (#437)

0.5.0-alpha (2021-05-27)


  • Add support for hexadecimal/octal/binary numeric literals.


    value_hex: u256 = 0xff
    value_octal: u256 = 0o77
    value_binary: u256 = 0b11


  • Added support for list expressions.


    values: u256[3] = [10, 20, 30]
    # or anywhere else where expressions can be used such as in a call
    sum: u256 = self.sum([10, 20, 30])


  • Contracts, events, and structs can now be empty.


    event MyEvent:
    contract MyContract:
    struct MyStruct:


  • External calls can now handle dynamically-sized return types. (#415)


  • The analyzer will return an error if a tuple attribute is not of the form item<index>. (#401)

Improved Documentation

  • Created a landing page for Fe at (#394)
  • Provide a Quickstart chapter in Fe Guide (#403)

Internal Changes - for Fe Contributors

  • Using insta to validate Analyzer outputs. (#387)

  • Analyzer now disallows using context.add_ methods to update attributes. (#392)

  • () now represents a distinct type internally called the unit type, instead of an empty tuple.

    The lowering pass now does the following: Valueless return statements are given a () value and functions without a return value are given explicit () returns. (#406)

  • Add CI check to ensure fragment files always end with a new line (#4711)

0.4.0-alpha (2021-04-28)


  • Support for revert messages in assert statements


    assert a == b, "my revert statement"

    The provided string is abi-encoded as if it were a call to a function Error(string). For example, the revert string "Not enough Ether provided." returns the following hexadecimal as error return data:

    0x08c379a0                                                         // Function selector for Error(string)
    0x0000000000000000000000000000000000000000000000000000000000000020 // Data offset
    0x000000000000000000000000000000000000000000000000000000000000001a // String length
    0x4e6f7420656e6f7567682045746865722070726f76696465642e000000000000 // String data


  • Added support for augmented assignments.


    contract Foo:
        pub fn add(a: u256, b: u256) -> u256:
            a += b
            return a
        pub fn sub(a: u256, b: u256) -> u256:
            a -= b
            return a
        pub fn mul(a: u256, b: u256) -> u256:
            a *= b
            return a
        pub fn div(a: u256, b: u256) -> u256:
            a /= b
            return a
        pub fn mod(a: u256, b: u256) -> u256:
            a %= b
            return a
        pub fn pow(a: u256, b: u256) -> u256:
            a **= b
            return a
        pub fn lshift(a: u8, b: u8) -> u8:
            a <<= b
            return a
        pub fn rshift(a: u8, b: u8) -> u8:
            a >>= b
            return a
        pub fn bit_or(a: u8, b: u8) -> u8:
            a |= b
            return a
        pub fn bit_xor(a: u8, b: u8) -> u8:
            a ^= b
            return a
        pub fn bit_and(a: u8, b: u8) -> u8:
            a &= b
            return a


  • A new parser implementation, which provides more helpful error messages with fancy underlines and code context. (#346)

  • Added support for tuples with base type items.


    contract Foo:
        my_num: u256
        pub fn bar(my_num: u256, my_bool: bool) -> (u256, bool):
            my_tuple: (u256, bool) = (my_num, my_bool)
            self.my_num = my_tuple.item0
            return my_tuple



  • Properly reject invalid emit (#211)

  • Properly tokenize numeric literals when they start with 0 (#331)

  • Reject non-string assert reasons as type error (#335)

  • Properly reject code that creates a circular dependency when using create or create2.

    Example, the following code is now rightfully rejected because it tries to create an instance of Foo from within the Foo contract itself.

    contract Foo:
      pub fn bar()->address:
        return address(foo)


Internal Changes - for Fe Contributors

  • AST nodes use Strings instead of &strs. This way we can perform incremental compilation on the AST. (#332)
  • Added support for running tests against solidity fixtures. Also added tests that cover how solidity encodes revert reason strings. (#342)
  • Refactoring of binary operation type checking. (#347)

0.3.0-alpha "Calamine" (2021-03-24)


  • Add over/underflow checks for multiplications of all integers (#271)

  • Add full support for empty Tuples. (#276)

    All functions in Fe implicitly return an empty Tuple if they have no other return value. However, before this change one was not able to use the empty Tuple syntax () explicitly.

    With this change, all of these are treated equally:

    contract Foo:
      pub fn explicit_return_a1():
      pub fn explicit_return_a2():
        return ()
      pub fn explicit_return_b1() ->():
      pub fn explicit_return_b2() ->():
        return ()
      pub fn implicit_a1():
      pub fn implicit_a2() ->():
  • The JSON ABI builder now supports structs as both input and output. (#296)

  • Make subsequently defined contracts visible.

    Before this change:

    # can't see Bar
    contract Foo:
    # can see Foo
    contract Bar:

    With this change the restriction is lifted and the following becomes possible. (#298)

    contract Foo:
        bar: Bar
        pub fn external_bar() -> u256:
    contract Bar:
        foo: Foo
        pub fn external_foo() -> u256:
  • Perform checks for divison operations on integers (#308)

  • Support for msg.sig to read the function identifier. (#311)

  • Perform checks for modulo operations on integers (#312)

  • Perform over/underflow checks for exponentiation operations on integers (#313)


  • Properly reject emit not followed by an event invocation (#212)

  • Properly reject octal number literals (#222)

  • Properly reject code that tries to emit a non-existing event. (#250)

    Example that now produces a compile time error:

    emit DoesNotExist()
  • Contracts that create other contracts can now include __init__ functions.

    See (#304)

  • Prevent multiple types with same name in one module. (#317)

    Examples that now produce compile time errors:

    type bar = u8
    type bar = u16


    struct SomeStruct:
        some_field: u8
    struct SomeStruct:
        other: u8


    contract SomeContract:
        some_field: u8
    contract SomeContract:
        other: u8

    Prevent multiple fields with same name in one struct.

    Example that now produces a compile time error:

    struct SomeStruct:
        some_field: u8
        some_field: u8

    Prevent variable definition in child scope when name already taken in parent scope.

    Example that now produces a compile time error:

    pub fn bar():
        my_array: u256[3]
        sum: u256 = 0
        for i in my_array:
            sum: u256 = 0
  • The CLI was using the overwrite flag to enable Yul optimization.


    # Would both overwrite output files and run the Yul optimizer. 
    $ fe my_contract.fe --overwrite

    Using the overwrite flag now only overwrites and optimization is enabled with the optimize flag. (#320)

  • Ensure analyzer rejects code that uses return values for __init__ functions. (#323)

    An example that now produces a compile time error:

    contract C:
        pub fn __init__() -> i32:
            return 0
  • Properly reject calling an undefined function on an external contract (#324)

Internal Changes - for Fe Contributors

  • Added the Uniswap demo contracts to our testing fixtures and validated their behaviour. (#179)
  • IDs added to AST nodes. (#315)
  • Failures in the Yul generation phase now panic; any failure is a bug. (#327)

0.2.0-alpha "Borax" (2021-02-27)


  • Add support for string literals.


    fn get_ticker_symbol() -> string3:
        return "ETH"

    String literals are stored in and loaded from the compiled bytecode. (#186)

  • The CLI now compiles every contract in a module, not just the first one. (#197)

    Sample compiler output with all targets enabled:

    |-- Bar
    |   |-- Bar.bin
    |   |-- Bar_abi.json
    |   `-- Bar_ir.yul
    |-- Foo
    |   |-- Foo.bin
    |   |-- Foo_abi.json
    |   `-- Foo_ir.yul
    |-- module.ast
    `-- module.tokens
  • Add support for string type casts (#201)


    val: string100 = string100("foo")
  • Add basic support for structs. (#203)


    struct House:
        price: u256
        size: u256
        vacant: bool
    contract City:
        pub fn get_price() -> u256:
            building: House = House(300, 500, true)
            assert building.size == 500
            assert building.price == 300
            assert building.vacant
            return building.price
  • Added support for external contract calls. Contract definitions now add a type to the module scope, which may be used to create contract values with the contract's public functions as callable attributes. (#204)


    contract Foo:
        pub fn build_array(a: u256, b: u256) -> u256[3]:
            my_array: u256[3]
            my_array[0] = a
            my_array[1] = a * b
            my_array[2] = b
            return my_array
    contract FooProxy:
        pub fn call_build_array(
            foo_address: address,
            a: u256,
            b: u256,
        ) -> u256[3]:
            foo: Foo = Foo(foo_address)
            return foo.build_array(a, b)
  • Add support for block, msg, chain, and tx properties: (#208)

    block.coinbase: address
    block.difficulty: u256
    block.number: u256
    block.timestamp: u256 u256
    msg.value: u256
    tx.gas_price: u256
    tx.origin: address

    (Note that msg.sender: address was added previously.)


    fn post_fork() -> bool:
        return block.number > 2675000
  • The CLI now panics if an error is encountered during Yul compilation. (#218)

  • Support for contract creations.

    Example of create2, which takes a value and address salt as parameters.

    contract Foo:
        pub fn get_my_num() -> u256:
            return 42
    contract FooFactory:
        pub fn create2_foo() -> address:
            # value and salt
            foo: Foo = Foo.create2(0, 52)
            return address(foo)

    Example of create, which just takes a value parameter.

    contract Foo:
        pub fn get_my_num() -> u256:
            return 42
    contract FooFactory:
        pub fn create_foo() -> address:
            # value and salt
            foo: Foo = Foo.create(0)
            return address(foo)

    Note: We do not yet support init parameters. (#239)

  • Support updating individual struct fields in storage. (#246)


     pub fn update_house_price(price: u256):
            self.my_house.price = price
  • Implement global keccak256 method. The method expects one parameter of bytes[n] and returns the hash as an u256. In a future version keccak256 will most likely be moved behind an import so that it has to be imported (e.g. from std.crypto import keccak256). (#255)


    pub fn hash_single_byte(val: bytes[1]) -> u256:
        return keccak256(val)
  • Require structs to be initialized using keyword arguments.


    struct House:
        vacant: bool
        price: u256

    Previously, House could be instantiated as House(true, 1000000). With this change it is required to be instantiated like House(vacant=true, price=1000000)

    This ensures property assignment is less prone to get mixed up. It also makes struct initialization visually stand out more from function calls. (#260)

  • Implement support for boolean not operator. (#264)


    if not covid_test.is_positive(person):
  • Do over/underflow checks for additions (SafeMath).

    With this change all additions (e.g x + y) for signed and unsigned integers check for over- and underflows and revert if necessary. (#265)

  • Added a builtin function abi_encode() that can be used to encode structs. The return type is a fixed-size array of bytes that is equal in size to the encoding. The type system does not support dynamically-sized arrays yet, which is why we used fixed. (#266)


    struct House:
        price: u256
        size: u256
        rooms: u8
        vacant: bool
    contract Foo:
        pub fn hashed_house() -> u256:
            house: House = House(
            return keccak256(house.abi_encode())
  • Perform over/underflow checks for subtractions (SafeMath). (#267)

    With this change all subtractions (e.g x - y) for signed and unsigned integers check for over- and underflows and revert if necessary.

  • Support for the boolean operations and and or. (#270)


    contract Foo:
        pub fn bar(x: bool, y: bool) -> bool:
            return x and y
    contract Foo:
        pub fn bar(x: bool, y: bool) -> bool:
            return x or y

    Support for self.address.

    This expression returns the address of the current contract.


    contract Foo:
        pub fn bar() -> address:
            return self.address


  • Perform type checking when calling event constructors

    Previously, the following would not raise an error even though it should:

    contract Foo:
        event MyEvent:
            val_1: string100
            val_2: u8
        pub fn foo():
            emit MyEvent("foo", 1000)

    Wit this change, the code fails with a type error as expected. (#202)

  • Fix bug where compilation of contracts without public functions would result in illegal YUL. (#219)

    E.g without this change, the following doesn't compile to proper YUL

    contract Empty:
      lonely: u256
  • Ensure numeric literals can't exceed 256 bit range. Previously, this would result in a non user friendly error at the YUL compilation stage. With this change it is caught at the analyzer stage and presented to the user as a regular error. (#225)

  • Fix crash when return is used without value.

    These two methods should both be treated as returning ()

      pub fn explicit_return():
      pub fn implicit():

    Without this change, the explicit_return crashes the compiler. (#261)

Internal Changes - for Fe Contributors

  • Renamed the fe-semantics library to fe-analyzer. (#207)
  • Runtime testing utilities. (#243)
  • Values are stored more efficiently in storage. (#251)

0.1.0-alpha "Amethyst" (2021-01-20)

WARNING: This is an alpha version to share the development progress with developers and enthusiasts. It is NOT yet intended to be used for anything serious. At this point Fe is missing a lot of features and has a lot of bugs instead.

This is the first alpha release and kicks off our release schedule which will be one release every month in the future. Since we have just started tracking progress on changes, the following list of changes is incomplete, but will appropriately document progress between releases from now on.


  • Added support for for loop, allows iteration over static arrays. (#134)

  • Enforce bounds on numeric literals in type constructors.

    For instance calling u8(1000) or i8(-250) will give an error because the literals 1000 and -250 do not fit into u8 or i8. (#145)

  • Added builtin copying methods clone() and to_mem() to reference types. (#155)


    # copy a segment of storage into memory and assign the new pointer
    my_mem_array = self.my_sto_array.to_mem()
    # copy a segment of memory into another segment of memory and assign the new pointer
    my_other_mem_array = my_mem_array.clone()
  • Support emitting JSON ABI via --emit abi. The default value of --emit is now abi,bytecode. (#160)

  • Ensure integer type constructor reject all expressions that aren't a numeric literal. For instance, previously the compiler would not reject the following code even though it could not be guaranteed that val would fit into an u16.

    pub fn bar(val: u8) -> u16:
            return u16(val)

    Now such code is rejected and integer type constructor do only work with numeric literals such as 1 or -3. (#163)

  • Support for ABI decoding of all array type. (#172)

  • Support for value assignments in declaration.

    Previously, this code would fail:

    another_reference: u256[10] = my_array

    As a workaround declaration and assignment could be split apart.

    another_reference: u256[10]
    another_reference = my_array

    With this change, the shorter declaration with assignment syntax is supported. (#173)

Improved Documentation

  • Point to examples in the README (#162)
  • Overhaul README page to better reflect the current state of the project. (#177)
  • Added descriptions of the to_mem and clone functions to the spec. (#195)

Internal Changes - for Fe Contributors

  • Updated the Solidity backend to v0.8.0. (#169)
  • Run CI tests on Mac and support creating Mac binaries for releases. (#178)