Started November 06, 2017

Molten is a programming language which borrows from the ML family of languages, as well as from Rust and Python. The compiler is written in Rust and uses LLVM to generate IR which can be compiled to machine code.

I originally started this project in order to learn Rust. It is intended to be a high level language with a full object system that facilitates both functional and object-oriented programming. Some syntax elements have been changed from typical ML languages to follow conventions found in more common languages, such as C++, Rust, and Python (eg. parenthesis-delimited blocks, conventional class definitions, generics/type parameters with angle brackets, etc). For more info on the internals, see An Overview Of Molten Internals


You will need rustc and cargo installed. It's recommended that you use rustup to install these. I've most recently tested it with rustc version 1.52. You will also need LLVM 11 installed, as well as libgc (Boehm-Demers-Weiser's Garbage Collector), and clang for linking, although clang can be replace with gcc by editing the molten python script.

On Debian/Ubuntu, run: sudo apt-get install llvm-11 llvm-11-runtime llvm-11-dev clang libgc-dev

On macOS, run: brew install llvm@11

You may need to add /usr/local/opt/llvm@11/bin to your path, and you will probably need to install libgc separately


The molten script helps with compiling and linking IR files. To run an example:

./molten run examples/fac.mol

This will run cargo to build the compiler if needed, then compile the fac.mol file, as well as all of its dependencies (in this case, the libcore.mol library), link them together using clang, along with libgc, and then run the binary. It can also compile to LLVM IR, and run LLVM bitcode by using the -S flag. The resulting .bc file can be run using lli-11.


fn fac(x) {
    if x < 1 then
        x * fac(x - 1)



()                  // unit type
Char                // UCS-4 character
(Int, Int) -> Int   // function type
'a                  // universal type variable
Array<Int>          // array of integers
(Int, Real)         // tuple type
{ a: Int, b: Real } // record type


let foo = 0
let bar: String = "Hey"


fn foo(x, y) => x + y           // named inline function

fn foo(x, y) { x + y }          // named block function

let foo = fn x, y => x + y      // anonymous function

fn foo(x: Int, y) -> Int { x + y }  // with optional type annotations

Invoking Functions

Unlike in ML, the brackets of a function call are not elidable. This is a design decision to improve readability of the code and to make the parser simpler and more predictable.

foo(1, 2)


A block is a collection of expressions which return the result of the last expression in the block. They can be used in place of a single expression. They do not create their own local scope, at least at the moment, so variables defined inside blocks will appear in the parent scope (usually the function the block is in). Each expression in the block must end in a newline or semi-colon character (or be the last expression in the block). This applies to the top level.

let is_zero = if self.x <= 0 then {
    self.x = 0
} else {


Infix operators are evaluated using order of operations. Both sides of an infix operation must be on the same line, or a \ character can be used to continue the line.

5 + 12 * 2      // equals 29

42 * 4 \
   % 5          // equals 3

And / Or

The keyword operators and and or have side-effects and will not execute the second expression if the result can be determined from the first expression. The expressions must have the same type, and the returned value is the first expression that returns a non-zero value.

let is_cat = true
let result = is_cat and println("It's a cat") == ()

Since is_cat is Bool, both sides of the and must be Bool. Since the println() function returns Unit, we compare it with itself which will always be true. The println() will only execute if is_cat is true, and result will be true if is_cat is true, or false if is_cat is false.


let tup = (1, "String", 4.5)
println(tup.1)                  // prints "String"


Records are like tuples but with named fields. Record literals use the equals sign ("=") to assign a value to a field. Specifying a record type uses a colon (":") to separate the field name from the type.

let rec = { i = 1, s = "String", r = 4.5 }
println(rec.s)                  // prints "String"

let rec: { i: Int, s: String, r: Real }

Records can be updated, which will copy all fields of the record into a new record, but with some of the fields modified.

let rec = { i = 1, s = "String", r = 4.5 }

let newrec = { rec with s = "Updated" }
println("New Value: " + newrec.s)       // prints "Updated"
println("Old Value: " + rec.s)          // prints "String"


let array1 = [ 1, 3, 6 ]
for x in array1

let array2 = new Array<String>();
array2.insert(0, "Hello")

The Array type is defined in libcore, which must be imported if arrays are used.

Flow Control

The return value of an if expression is the result of evaluating either the then clause or else clause. The types of both clauses must match. The else clause can be left out as long as the true clause evaluates to Nil.

if x == 5 then
    "It's five"
    "It's not five"

A match expression allows pattern matching, which can unpack refs, tuples, records, and enums. It can bind values to named variables in the pattern and creates a new scope for each arm of the match expression.

match x with
| 1 => "It's one"
| 5 => "It's five"
| num => "It's not one or five, it's " + str(num)

Values can be unpacked with match as well, including records, tuples, refs, and enum variants. A underscore _ will match any value, and an identifier (eg. value) will match anything and bind that value to the name in the process, so that it can be referenced inside the match arm. Here's an example using a record:

match { num = 10, name = "Ten" } with
| { num = 10, name = value } => println(value)
| { num = _, name = value } => println("Something else named " + value)


while true

for i in iter([ 1, 2, 3 ])
    println("counting " + i)

For loops take an instance of Iterator<'item> and calls the .next() method on it, running the body for each Option::Some('item) returned. The iter function is defined for different types to convert them into an appropriate iterator. In the case of arrays, it will return the result of new ArrayInterator<'item>(input_array)


A ref is an indirect reference to some data. It can be passed around as a value, and dereferenced to get or set the data inside of it. The internal value of a reference is always mutable

let r = ref 42
println(str(*r))                // prints 42
*r = 65
println(str(*r))                // prints 65

fn foo(x: ref Int) { }          // ref types look similar to ref constructors

let r = ref { a = 42, b = "The Answer" }
println(*r.b)                   // prints "The Answer"


class Foo {
    // A field with type String
    val mut name: String

    fn new(self, name) { = name

    fn get(self) =>

    fn static(x) => x * 2

class Bar extends Foo {
    fn get(self, title) => + " " + title

let bar = new Bar("Mischief")
bar.get("The Cat")              // returns "Mischief The Cat"

All methods are both closures, and virtual methods, so they can access variables in their parents' scopes and also be overridden by a child class's implementation of the same method, accessible with a reference to the parent class type.

Fields can have an optional type, but not an initializer. If a class has at least one field, it must have at least one "new" constructor, which must assign to each field an initial value. The type can be inferred from this assignment if the optional type is not supplied. If a field is declared as mutable, it can be reassigned to, but if the mut keyword is absent, the field can only be assigned to within the constructor, and will be immutable after the constructor has returned. Every constructor must also call the Super::new method of its parent class, if it has a parent that has a constructor.

Enums (Tagged Unions)

An enum can either have no arguments, or a tuple of arguments. Constructing an enum variant requires using the Resolve (::) notation. Pattern matching is currently the only way to get values out of a variant. Unlike with classes, which allocate memory for a new instance, enums are immediate data types like tuples and records. In order to store it in a memory location, a ref must be used. A ref is required in order to make a recursive enum.

enum Value =
| None
| Integer(Int)
| String(String)
| Pair(String, String)
| Reference(ref Value)          // A recursive reference

let val = Value::String("Hey")

match val with
| Value::String(s) => println(s)
| _ => ()

Methods can be added to enums using a methods body. Currently it can only be used with enums.

methods Value {
    fn is_some(val: Value) {
        match val with
        | Value::None => false
        | _ => true


Traits and Trait Objects

A trait can be defined with method declarations in the body, with the predefined type alias "Self" used to refer to the current trait object

trait Add {
    decl add(Self, Self) -> Self

A trait can then be implemented for a given type. An impl block can only have function definitions that match the trait declarations. Inside the impl block the "Self" type alias will refer to the implementation type. Trait objects that are passed to arguments with type "Self" will automatically be unpacked into their impl type, and if the return type has type "Self", the result will automatically be packed back into a trait object.

impl Add for Int {
    fn add(x: Self, y: Self) -> Self {
        x + y

impl Add for Real {
    fn add(x: Self, y: Self) -> Self {
        x + y

Traits are not an object type, but instead are specified as a constraint on a universal variable using a where clause. If the actual type given does not implement the constrained trait, then an type error will be raised. Currently trait objects can only be created by passing them into a function that takes a universal variable with a constraint. At the moment, only one trait can be specified as a constraint but this will be changed in future.

fn do_some_adding(x: 'a) -> 'a where a: Add {


The above example will output:



All exceptions must be an instance of the Exception class defined in libcore.

try open("file.txt")
with e => println("Exception Occurred: " + e.msg)

try {
    raise new Exception("Problem")
} with
    e => println(e.msg)


A value can be type annotated using a colon followed by the type.

5 : Int
str(i : Int)
(func() : String)


import libcore

External Functions

A function can be declared without being implemented, and functions can also be defined with an ABI specifier so that they are accessible to other languages. Only C support is currently implemented. A C function cannot be a closure.

decl foo(Int) -> Int         // external molten function
decl bar(Int) -> Int / C     // external C function

fn baz(i: Int) / C {
    // molten function that can be called from C

Linking to C

An example of writing a C file, and linking it to a molten program is shown in lib/libccore.c. When imported into a molten file and compiled with the molten script, the library will be compiling using clang and the libccore.cdec (manually maintained) will be copied to libccore.dec. The importing molten program will be able to use declarations from libccore.cdec. Some declarations that can be used in C are in include/molten.h, such as accessing the garbage collected allocator.

Previously Uncompleted

I'd be happy to hear of any additional features ideas or suggestions, if you'd like to leave them under "Issues" on github.

Get the Source

Or clone with:
git clone