jakt

The Jakt programming language

Jakt is a memory-safe systems programming language.

It currently transpiles to C++.

NOTE: The language is under heavy development.

NOTE If you're cloning to a Windows PC (not WSL), make sure that your Git client keeps the line endings as \n. You can set this as a global config via git config --global core.autocrlf false.

Usage

The transpilation to C++ requires clang. Make sure you have that installed.

jakt file.jakt
./build/file

Building

See here.

Goals

1. Memory safety
2. Code readability
3. Developer productivity
4. Executable performance
5. Fun!

Memory safety

The following strategies are employed to achieve memory safety:

• Automatic reference counting
• Strong typing
• Bounds checking
• No raw pointers in safe mode

In Jakt, there are three pointer types:

• T (Strong pointer to reference-counted class T.)
• weak T (Weak pointer to reference-counted class T. Becomes empty on pointee destruction.)
• raw T (Raw pointer to arbitrary type T. Only usable in unsafe blocks.)

Null pointers are not possible in safe mode, but pointers can be wrapped in Optional, i.e Optional<T> or T? for short.

Math safety

• Integer overflow (both signed and unsigned) is a runtime error.
• Numeric values are not automatically coerced to int. All casts must be explicit.

For cases where silent integer overflow is desired, there are explicit functions that provide this functionality.

Code readability

Far more time is spent reading code than writing it. For that reason, Jakt puts a high emphasis on readability.

Some of the features that encourage more readable programs:

• Immutable by default.
• Argument labels in call expressions (object.function(width: 10, height: 5))
• Inferred enum scope. (You can say Foo instead of MyEnum::Foo).
• Pattern matching with match.
• Optional chaining (foo?.bar?.baz (fallible) and foo!.bar!.baz (infallible))
• None coalescing for optionals (foo ?? bar yields foo if foo has a value, otherwise bar)
• defer statements.
• Pointers are always dereferenced with . (never ->)
• Trailing closure parameters can be passed outside the call parentheses.
• Error propagation with ErrorOr<T> return type and dedicated try / must keywords.

Code reuse

Jakt is flexible in how a project can be structured with a built-in module system.

import a                                // (1)
import a { use_cool_things }            // (2)
import fn()                             // (3)
import relative foo::bar                // (4)
import relative parent::foo::baz        // (5)
import relative parent(3)::foo::baz     // (6)

1. Import a module from the same directory as the file.
2. Import only use_cool_things() from module a.
3. Imports can be calculated at compile time. See Comptime Imports
4. Import a module using the relative keyword when the module is a sub path of the directory containing the file.
5. Import a module in a parent path one directory up from the directory containing the file.
6. Syntactic sugar for importing a module three parent paths up from the directory containing the file.

The Jakt Standard Library

Jakt has a Standard Library that is accessed using the jakt:: namespace:

import jakt::arguments
import jakt::libc::io { system }

The Jakt Standard Library is in its infancy, so please consider making a contribution!

Function calls

When calling a function, you must specify the name of each argument as you're passing it:

rect.set_size(width: 640, height: 480)

There are two exceptions to this:

• If the parameter in the function declaration is declared as anon, omitting the argument label is allowed.
• When passing a variable with the same name as the parameter.

Structures and classes

There are two main ways to declare a structure in Jakt: struct and class.

struct

Basic syntax:

struct Point {
    x: i64
    y: i64
}

Structs in Jakt have value semantics:

• Variables that contain a struct always have a unique instance of the struct.
• Copying a struct instance always makes a deep copy.

let a = Point(x: 10, y: 5)
let b = a
// "b" is a deep copy of "a", they do not refer to the same Point

Jakt generates a default constructor for structs. It takes all fields by name. For the Point struct above, it looks like this:

Point(x: i64, y: i64)

Struct members are public by default.

class

• basic class support
• private-by-default members
• inheritance
• class-based polymorphism (assign child instance to things requiring the parent type)
• Super type
• Self type

Same basic syntax as struct:

class Size {
    width: i64
    height: i64

    public fn area(this) => .width * .height
}

Classes in Jakt have reference semantics:

• Copying a class instance (aka an "object") copies a reference to the object.
• All objects are reference-counted by default. This ensures that objects don't get accessed after being deleted.

Class members are private by default.

Member functions

Both structs and classes can have member functions.

There are three kinds of member functions:

Static member functions don't require an object to call. They have no this parameter.

class Foo {
    fn func() => println("Hello!")
}

// Foo::func() can be called without an object.
Foo::func()

Non-mutating member functions require an object to be called, but cannot mutate the object. The first parameter is this.

class Foo {
    fn func(this) => println("Hello!")
}

// Foo::func() can only be called on an instance of Foo.
let x = Foo()
x.func()

Mutating member functions require an object to be called, and may modify the object. The first parameter is mut this.

class Foo {
    x: i64

    fn set(mut this, anon x: i64) {
        this.x = x
    }
}

// Foo::set() can only be called on a mut Foo:
mut foo = Foo(x: 3)
foo.set(9)

Shorthand for accessing member variables

To reduce repetitive this. spam in methods, the shorthand .foo expands to this.foo.

Strings

Strings are provided in the language mainly as the type String, which is a reference-counted (and heap-allocated) string type. String literals are written with double quotes, like "Hello, world!".

Overloaded string literals

String literals are of type String by default; however, they can be used to implicitly construct any type that implements the FromStringLiteral (or ThrowingFromStringLiteral) trait. In the language prelude, currently only StringView implements this trait, which can be used only to refer to strings with a static lifetime:

let foo: StringView = "foo" // This string is not allocated on the heap, and foo is only a fat pointer to the static string.

Overloaded string literals can be used by providing a type hint, whether by explicit type annotations, or by passing the literal to a function that expects a specific type:

struct NotString implements(FromStringLiteral) {
    fn from_string_literal(anon string: StringView) -> NotString => NotString()
}

fn test(x: NotString) {}

fn main() {
    let foo: NotString = "foo"
    test(x: "Some string literal")
}

Arrays

Dynamic arrays are provided via a built-in Array<T> type. They can grow and shrink at runtime.

Array is memory safe:

• Out-of-bounds will panic the program with a runtime error.
• Slices of an Array keep the underlying data alive via automatic reference counting.

Declaring arrays

// Function that takes an Array<i64> and returns an Array<String>
fn foo(numbers: [i64]) -> [String] {
    ...
}

Shorthand for creating arrays

// Array<i64> with 256 elements, all initialized to 0.
let values = [0; 256]

// Array<String> with 3 elements: "foo", "bar" and "baz".
let values = ["foo", "bar", "baz"]

Dictionaries

• Creating dictionaries
• Indexing dictionaries
• Assigning into indexes (aka lvalue)

fn main() {
    let dict = ["a": 1, "b": 2]

    println("{}", dict["a"])
}

Declaring dictionaries

// Function that takes a Dictionary<i64, String> and returns an Dictionary<String, bool>
fn foo(numbers: [i64:String]) -> [String:bool] {
    ...
}

Shorthand for creating dictionaries

// Dictionary<String, i64> with 3 entries.
let values = ["foo": 500, "bar": 600, "baz": 700]

Sets

• Creating sets
• Reference semantics

fn main() {
    let set = {1, 2, 3}

    println("{}", set.contains(1))
    println("{}", set.contains(5))
}

Tuples

• Creating tuples
• Index tuples
• Tuple types

fn main() {
    let x = ("a", 2, true)

    println("{}", x.1)
}

Enums and Pattern Matching

• Enums as sum-types
• Generic enums
• Enums as names for values of an underlying type
• match expressions
• Enum scope inference in match arms
• Yielding values from match blocks
• Nested match patterns
• Traits as match patterns
• Support for interop with the ?, ?? and ! operators

…

Generics

• Generic types
• Constant generics (minimal support)
• Constant generics (full support)
• Generic type inference
• Traits

Jakt supports both generic structures and generic functions.

fn id<T>(anon x: T) -> T {
    return x
}

fn main() {
    let y = id(3)

    println("{}", y + 1000)
}

struct Foo<T> {
    x: T
}

fn main() {
    let f = Foo(x: 100)

    println("{}", f.x)
}

struct MyArray<T, comptime U> {
    // NOTE: There is currently no way to access the value 'U', referring to 'U' is only valid as the type at the moment.
    data: [T]
}

Namespaces

• Namespace support for functions and struct/class/enum
• Deep namespace support

namespace Greeters {
    fn greet() {
        println("Well, hello friends")
    }
}

fn main() {
    Greeters::greet()
}

Type casts

There are two built-in casting operators in Jakt.

• as? T: Returns an Optional<T>, empty if the source value isn't convertible to T.
• as! T: Returns a T, aborts the program if the source value isn't convertible to T.

The as cast can do these things (note that the implementation may not agree yet):

• Casts to the same type are infallible and pointless, so might be forbidden in the future.
• If the source type is unknown, the cast is valid as a type assertion.
• If both types are primitive, a safe conversion is done.
  • Integer casts will fail if the value is out of range. This means that promotion casts like i32 -> i64 are infallible.
  • Float -> Integer casts truncate the decimal point (?)
  • Integer -> Float casts resolve to the closest value to the integer representable by the floating-point type (?). If the integer value is too large, they resolve to infinity (?)
  • Any primitive -> bool will create true for any value except 0, which is false.
  • bool -> any primitive will do false -> 0 and true -> 1, even for floats.
• If the types are two different pointer types (see above), the cast is essentially a no-op. A cast to T will increment the reference count as expected; that's the preferred way of creating a strong reference from a weak reference. A cast from and to raw T is unsafe.
• If the types are part of the same type hierarchy (i.e. one is a child type of another):
  • A child can be cast to its parent infallibly.
  • A parent can be cast to a child, but this will check the type at runtime and fail if the object was not of the child type or one of its subtypes.
• If the types are incompatible, a user-defined cast is attempted to be used. The details