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Version 0.5 | Entry @ | See also: EightebedDieter

This is the reference distribution for Castile, a simple imperative language with union types.

The design of Castile was influenced (in varying degrees) by C, Rust, Eightebed, Python, Ruby, and Erlang. More information on its roots can be found in doc/

The reference implementation can both interpret Castile programs and compile them to a variety of targets — JavaScript, Ruby, C, and a generic stack-based VM (included in this distribution).

A rich test suite in Falderal format, which describes the language with many examples, can be found in tests/

Quick Start

Clone this repository, cd into the repo directory and run

bin/castile eg/hello.castile

Alternately, put the bin subdirectory on your executable search path, so that you can run castile from any directory on your system. castile has no dependencies besides Python (either Python 2 or Python 3.)

Motivating Example

Here are some functions for creating linked lists, written in Castile:

struct list {
  value: integer;
  next: list|void;

fun empty() {
  return null as list|void

fun cons(v: integer, l: list|void) {
  make list(value:v, next:l) as list|void

In this, list|void is a union type. In this case it is expressing the fact that the value can be either a list or void — the moral equivalent of "nullable". In order to access any of the concrete types of a union type, one must use typecase:

fun max(l: list|void) {
  u = l;
  v = u;
  n = 0;
  while true {
    typecase u is void {
    typecase u is list {
      if u.value > n {
        n = u.value
      v =;
    u = v;
  return n

This retains type-safety; the code will never unexpectedly be presented with a null value.

Union types can also encourage the programmer follow a Parse, don't validate approach (which, despite the impression you might get from reading that article, is not restricted to Haskell or even to functional programming). In the above code, cons will never return a void, and max is not defined on empty lists. So ideally, we'd like to tighten their types to exclude those. And we can:


fun cons(v: integer, l: list) {
  make list(value:v, next:l as list|void)

fun singleton(v: integer) {
  make list(value:v, next:null as list|void)

fun max(l: list) {
  u = l as list|void;
  v = u;

Many more examples of Castile programs can be found in tests/