README.md

Unnamed

My testbed for implementing a very simple programming language. Tokenizer and recursive descent parser handwritten. Simple interpreter.

Features

Whitespace, Statement Termination, etc.

Whitespace is insignificant. Expressions greedily parse as much as possible. There is no statement terminator like semicolon.

// this works just fine
var x = 1 data Container { content: Str } print(x) var c = Container("foo") print(c.content)

Comments

There are only single-line comments. Comments are introduced with // and go until the end of the line

// prepare for a variable
var x = 1 // sets x to 1
// variable x exists now

Variables

Variables are mutable. New bindings are introduced with the var keyword.

var x = 1
var y = "my x: " + x
x = 2
x = "foo" // currently not forbidden but hopefully eventually

Arrays

Arrays have a fixed length. If an initializer list is passed, the length is optional. The runner will crash if accessing out of bounds.

var a = []int{}         // empty
var b = [5]int{}        // length 5
var c = []int{1, 2, 3}  // length 3
var d = [3]int{1, 2, 3} // redundant but fine
var e = [5]int{1, 2, 3} // error, length must match initializer list

// access and assign
c[0] = c[1] + c[2]

Blocks

A block is a grouping mechanism and allowed everywhere a statement or expression is allowed. The value of the block is the value of the last statement executed.

var x = {
    var a = 1
    var b = 2
    print("returning a + b now")
    a + b
}
// x is 3 now

As the block is effectively treated like a value, it is a 100% replacement for parenthesesis used for grouping of expressions. Blocks are just more versatile as they allow for more than just a simple expression.

// these are equivalent
var x = (1 + 2) * 3
var y = { 1 + 2 } * 3

Control Flow

If

The if statement is an expression and evaluates to the path taken. Else is optional. If else is missing and the condition is false, the expression evaluates to null.

var x = 1
if (x == 1) print("x is indeed 1")
else print("for some reason x is different: " + x)

For

Currently only supports the Go-style short version (otherwise known as a while loop). It evaluates to the last iteration or null if the condition is immediately false.

var i = 0
for i < 5 {
    import io
    print(i)
    i = i + 1
}

The braces are not part of the syntax, it's a regular block.

def main() = {
    var looper = Looper(5, 0, print)
    for looper.isTrue() looper.invoke()
}

data Looper {
    iterations: int
    cur: int
    function: F
}

def Looper.invoke() = {
    // TODO: function execution does not work from fields right now
    var f = this.function
    f(this.cur)
    this.cur = this.cur + 1
}

def Looper.isTrue() = this.cur < this.iterations

Custom Data Types

Custom data types are created with the data keyword. They automatically register a creation function with the same name as the type that can be used to instantiate the type. Fields are accessed with dot notation.

data Dog { name: Str }

var fido = Dog("Fido")
print(fido.name)

Functions

Functions are declared with def. The body of a function is "assigned" with the equals sign =. The body can be any statement. The return value of the function is the return value of the statement.

def add(a, b) = a + b
def noisyAdd(a, b) = {
    print("going to add " + a + " and " + b)
    a + b
}

Functions are called like in any other C-like language.

print(add(1, 2))

var result = noisyAdd(3, 4)
print(result)

The commas in the signature are optional. I haven't made up my mind which way should be preferred.

// this also works
def add(a: Int b: Int) = a + b

Functions are normal variables and can be given to other functions as such. This includes creation functions.

def add(a, b) = a + b
def invokeFn(fn, a, b) = fn(a, b)
print(add(1, 2))
print(invokeFn(add, 1, 2))

data Dog { age: Int friends: Int }
print(invokeFn(Dog, 8, 5))

Methods

Methods are just function definitions with an explicit receiver. They have access to the receiver via the this keyword.

data Dog { name: Str }
def Dog.noise() = this.name + " bark"

var dog = Dog("Fido")
print(dog.noise())

For declared methods only there is support for UFCS (uniform function call syntax). This is not useful right now but should allow handing over method references in the future.

data Dog {}
def Dog.noise() = "bark"

var dog = Dog()
print(dog.noise())
print(Dog.noise(dog))

Traits

A trait defines behavior. It can be used as an input for a function.

data Dog { name: Str }
trait Named { def getName(): Str }
impl Dog is Named { def getName(): Str = this.name }

def describe(named: Named) =
    print("the thing with the name is named " + named.getName())

var fido = Dog("Fido")
describe(fido)

Technically this works by collecting all implementing functions at the callsite and registering them in the scope of the callee. That is why traits currently cannot be used as members.

Under Construction 👷‍♂️

Types

Types are currently mostly optional and resolved at runtime. I want to introduce a type inference step that completes all missing type declarations though. The only exception for now are the traits which require passing implementing functions to the callee.

Function Name Resolution

Currently function resolution is hardcoded very specifically for function evaluation. However, I'd like to make this a bit more generic where they are looked up like ordinary variables and thus can be used in data structures and as function parameters.