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Bosque Programming Language

Licensed under the MIT License PR's Welcome Build Health

The Bosque Project

The Bosque Programming Language project is a ground up language & tooling co-design effort focused on is investigating the theoretical and the practical implications of:

  1. Explicitly designing a code intermediate representation language (bytecode) that enables deep automated code reasoning and the deployment of next-generation development tools, compilers, and runtime systems.
  2. Leveraging the power of the intermediate representation to provide a programming language that is both easily accessible to modern developers and that provides a rich set of useful language features for developing high reliability & high performance applications.
  3. Taking a cloud-development first perspective on programming to address emerging challenges as we move into a distributed cloud development model based around microservices, serverless, and RESTful architectures.

The Role of Intermediate Representations

Compiler intermediate representations (IRs) are traditionally thought of, and designed with, a specific source language (or languages) in mind. Their historical use has primarily been as an intermediate step in the process of lowering a source language program, with all of the associated syntactic sugar, into a final executable binary. However, over time they have become increasingly important in supporting program analysis and IDE tooling tasks. In these scenarios choices which did not matter in the context of the compilation workflow can have major negative impacts.

In the Bosque project we ask the question of what happens if the IR is designed explicitly to support the rich needs of automated code reasoning, IDE tooling, etc. With this novel IR first perspective we are exploring a new way to think about and build a language intermediate representation and tools that utilize it. Our initial experiments show that this empowers a range of next-generation experiences including symbolic-testing, enhanced fuzzing, soft-realtime compilation with stable GC support, API auto-marshaling, and more!

Regularized Programming

Many features that make the Bosque IR amenable for automated reasoning involve simplifying and removing sources of irregularity in the semantics. These regularizations also simplify the task of understanding and writing code for the human developer. Inspired by this idea the Bosque project is building a new regularized programming language that takes advantage of the features of the IR.

The Bosque Programming Language builds on the strengths of classical Functional Programming, modern TypeScript/Node.js, and the power of the new IR. The result is a language that simultaneously supports the kind of high productivity development experience available to modern developers while also providing a resource efficient and predictable runtime, scaling from small IoT up to heavily loaded cloud services. In addition to bringing all the expressive power expected from a modern language, the Bosque language also introduces several novel features like Typed Strings and API Types, that directly address challenges faced by developers working in a distributed cloud based world.

Cloud First Development

The move into cloud based development, with architectures based around microservices, serverless functions, and RESTful APIs, brings new challenges for development. In this environment an program may interoperate with many other (remote) services which are maintained by different teams (and maybe implemented in different languages). This forces APIs to use least-common denominator types for interop and creates the need for extensive serialize/deserialize/validate logic. Further, issues like cold-service startups, 95% response times, resiliency and diagnostics, all become critical but have not been design considerations in most traditional languages.

The Bosque project takes a cloud and IoT first view of programming languages. Thus, it includes features like API Types to simplify the construction and deployment of REST style APIs. Application initialization design provides 0-cost loading for lighting fast (cold) startup. Choices like fully determinized language semantics, keys and ordering, and memory behavior result in a runtime with minimal performance variability and enable ultra-low overhead tracing.

Powering the Future of Programming

An overarching theme of the Bosque project is increasing the ability of automated tools to reason about and transform code. This mechanization is a foundational part of unlocking the future of using AI and Synthesis in the development pipeline. The ability to mechanically reason about the semantics of a program via symbolic means is a key enabler to synthesizing novel and useful code components using examples or conditions. Bosque’s fully determinized and loop free design can also help facilitate the development and application of automated program differentiation. These are open problems but, just as we saw how Bosque unlocks value in classical tooling/compilation scenarios, we are excited to see what it can do to power the AI and Synthesis programming revolution.

Note: This repository and code are currently still experimental. This means that the language is subject to revision, there are bugs and missing functionality, and the performance is limited.


June 2021 Pushing version 0.8.0 as new master -- this update obsoletes the previous prototype version and lays the ground work for an eventual stable release. Major rework was done in the language, type-checker, validator implementation, and runtime. More details will be posted soon but the headline is exciting improvements in all areas.

However, this also means many many things are broken. Fixes and tests are coming online continuously and I will be opening a number of issues that are suitable for community contributions. Looking forward to seeing Bosque move from an exciting concept and toward a practical language!

March 2021 Update on road to 1.0 -- this branch now builds and runs a small set of tests for the verifier and model checker! Progress on this work has been slow and hard but it looks to be paying off.

December 2020 Big news on our road to a 1.0 version of Bosque!

While there are many things we liked about the initial experiments with the language there were some features missing that we really wanted to add and some things that, in hindsight, we knew could be done much better. I have been working extensively in a side branch, and while there is still much to do, wanted to get this code into the main repo for anyone interested.

Some highlights of the work include:

  • Operators with static and dynamic dispatch forms (plus overloading of builtins). With support for literal expression (literal Bools, Ints, Typed Ints, and Enums) dispatching this brings powerful ways to express computations -- operator op(cond: Bool==true, data: string) {...} operator op(cond: Bool==false, data: string) {...}.
  • Typed Numbers (aka unit types) to support type safe and expressive code -- let speed = 2.0_MetersPerSecond
  • Reworked Tuples/Records and equality to allow Tuples/Records as key types and simplify the encoding of equality in SMT.
  • Literal Template Types -- typedef Point2D = Vector<2, Float>;
  • Support for derived and default values of fields as well as improved optional type support.
  • Greatly improved type (and template type) inference.
  • Overall design simplifications that allow us to produce simpler SMT encodings and target specialized theories in Z3 as well as to apply more aggressive compiler optimizations.
  • A new and novel reference counting GC design.
  • Binders and default variable introduction -- switch(x.f) { type Int => $match + 1; _ => $match; } or x.{f=$f+1}.

There is still a lot of work to complete all of these changes but I am very excited at what the resulting language looks like and the impact is has on the verification/validation/compilation tooling story. I want to thank everyone who has contributed on this journey either through PRs, issues, comments, new ideas, and other miscellaneous feedback.

May 2020: We will be running a Bosque Webinar with Live Q&A on May 27th (registration is here). An on demand recording will be available as well for those that cannot make the live event.


Small samples of code and unique Bosque tooling are below in the Code Snippets and Tooling sections. A rundown of notable and/or unique features in the Bosque language is provided in the language overview section 0. For a look at how the language works and flows in the large please see the code for a simple tic-tac-toe program.

Detailed Documentation, Tutorials, and Technical Information:

Code Snippets

Add 2 numbers:

function add2(x: Nat, y: Nat): Nat {
    return x + y;

add2(2, 3)     //5
add2(x=2, y=3) //5
add2(y=2, 5)   //7

All positive check using rest parameters and lambda:

function allPositive(...args: List<Int>): Bool {
    return args.allOf(fn(x) => x >= 0i);

allPositive(1, 3, 4) //true

Tuples and Records:

function doit(tup: #[Int, Bool], rec: #{f: String, g: Int}): Int {
    return tup.0 + rec.g;

doit(#[1, false], #{f="ok", g=3}) //4

Sign (with default argument):

function sign(x?: Int=0): Int {
    var y: Int;

    if(x == none || x == 0) {
        y = 0;
    else {
        y = (x > 0) ? 1 : -1;

    return y;

sign(5)    //1
sign(-5)   //-1
sign()     //0

Nominal Types Data Invariants:

concept WithName {
    invariant $name != "";

    field name: String;

concept Greeting {
    abstract method sayHello(): String;

    virtual method sayGoodbye(): String {
        return "goodbye";

entity GenericGreeting provides Greeting {
    const instance: GenericGreeting = GenericGreeting@{};

    override method sayHello(): String {
        return "hello world";

entity NamedGreeting provides WithName, Greeting {
    override method sayHello(): String {
        return String::concat("hello", " ",;

GenericGreeting@{}.sayHello()         //"hello world"
GenericGreeting::instance.sayHello()  //"hello world"

NamedGreeting@{}.sayHello()           //type error no value provided for "name" field
NamedGreeting@{name=""}.sayHello()    //invariant error
NamedGreeting@{name="bob"}.sayHello() //"hello bob"

Validated and Typed Strings:

typedef ZipcodeUS = /[0-9]{5}(-[0-9]{4})?/;
typedef CSSpt = /[0-9]+pt/;

function is3pt(s1: StringOf<CSSpt>): Bool {
    return s1.string() === "3pt";

ZipcodeUS::accepts("98052-0000") //true
ZipcodeUS::accepts("98052-")     //false
ZipcodeUS::accepts("abc")        //false

is3pt("12")                        //type error String is not a StringOf
is3pt('98052' of ZipcodeUS)        //type error StringOf<ZipcodeUS> not StringOf<CSSpt>

is3pt(StringOf<CSSpt>::from("a"))  //error not a StringOf<CSSpt> value
is3pt('3' of CSSpt)                //type error '3' is incompatible with CSSpt 
is3pt('3pt' of CSSpt)              //true
parsable entity StatusCode {
    field code: Int;
    field name: String;

    function tryParse(name: String): Result<StatusCode, String> {
        return switch(name) {
            case "IO"      => ok(StatusCode@{1, name})
            case "Network" => ok(StatusCode@{2, name})
            case _         => err("Unknown code")

function isIOCode(s: DataString<StatusCode>): Bool {
    return s === 'IO' of StatusCode;

isIOCode("IO");                               //type error not a StringOf<StatusCode>
isIOCode(StatusCode'Input')                   //type error not a valid StatusCode string
isIOCode(StringOf<StatusCode>::from("Input")) //runtime error not a valid StatusCode string

isIOCode(StatusCode'Assert')               //false
isIOCode(StringOf<StatusCode>::from("IO")) //true

let ec: StatusCode = StatusCode@'IO';
assert(ec.code == 1); //true

Structural, Nominal, and Union Types (plus optional arguments)

entity Person {
    field name: String; 

function foo(arg?: {f: Int, n?: String} | String | Person): String {
    if(arg == none) {
        return "Blank";
    else {
        return switch(arg) {
            type Record => arg.n ?| "Blank"
            type String => arg
            type Person =>

foo()                    //"Blank"
foo(none)                //Type error - none not allowed
foo("Bob")               //Bob
foo(Person@{name="Bob"}) //Bob
foo({f=5})               //"Blank"

foo({f=1, n="Bob"})      //"Bob"
foo({g=1, n="Bob"})      //Type error - Missing f property

Pre/Post Conditions

entity Animal {
    invariant $says != "";

    field says: String;

function createAnimal(catchPhrase: String): Animal
    return Animal@{says=catchPhrase};

function createAnimalPre(catchPhrase: String): Animal
    requires catchPhrase != "";
    return Animal@{says=catchPhrase};

function createAnimalPreSafe(catchPhrase: String): Animal
    requires release catchPhrase != "";
    return Animal@{says=catchPhrase};

typedef ErrData = {msg: String, data?: Any};

entrypoint function getSays(animal: String, catchPhrase: String): Result<String, ErrData?>
    validate animal != "" or return err({ msg="Invalid animal" });
    validate catchPhrase != "" or return err({ msg="Invalid catchPhrase" });
    return String::concat("The ", animal, " says ", createAnimal::(catchPhrase).says);

createAnimal("woof woof") //ok always
createAnimal("")          //fails invariant in debug
createAnimalPre("")       //fails precondition in debug *but not* release
createAnimalPreSafe("")   //fails precondition in all build flavors

getSays("dog", "woof") //Ok<String, ErrData>@{value="The dog says woof"}
getSays("", "woof") //Err<String, ErrData>@{error={ msg="Invalid animal" }}
getSays("dog", "") //Err<String, ErrData>@{error={ msg="Invalid catchPhrase" }}

Numeric Types

API Types



Note: If you are running examples from the "Learn Bosque Programming" book please use the LearnBosqueProgramming branch which is sync'd with the version of code used in the book.

Symbolic Testing

Bosque provides a powerful new way to test your applications. Unit-testing is a great way to ensure that code works as expected and to prevent accidental changes to behavior when adding new features or fixing bugs. However, writing and maintaining large numbers of tests can be a tedious and time consuming task. To help with this Bosque provides a symbolic testing harness that augments unit-testing and provides high coverage for bugs that result in runtime failures -- arising either as builtin language errors or from failed user provided pre/post/invariant/assert conditions.

The symtest tool implements the symbolic testing algorithm and works as follows. Given the application shown below:

namespace NSMain;

global ops: Set<String> = Set<String>@{

function checkIntBounds(arg: Int?): Bool {
    //our calculator is for small numbers -- maybe use BigInt later
    return arg == none || ((-100 <= arg) && (arg <= 100)); 

entrypoint function processOp(op: String, arg1: Int, arg2: Int?): Int 
    requires release NSMain::ops.has(op);
    requires release checkIntBounds(arg1) && checkIntBounds(arg2);
    //requires release (op == "add" || op == "sub") ==> arg2 != none;
    if(op == "negate") {
        return -arg1;
    else {
        assert(arg2 != none);

        if(op == "add") {
            return arg1 + arg2;
        else {
            return arg1 - arg2;

Assuming this code is in a file called process_op.bsq then we can run the following command to check for errors:

> node bin\runtimes\symtest\symtest.js process_op.bsq

Which will report that an error is possible.

Re-running the symbolic tested with model generation on as follows:

> node bin\runtimes\symtest\symtest.js -m division.bsq

Will report that an error is possible when op == "negate" and arg2 == none. Note that the tester was aware of the precondition requires _ops.has(op) and so did not generate any spurious failing test inputs (such as op="").

Un-commenting the second requires line tells the tester that this, and similar errors are excluded by the API definition, and re-running the tester will now report that the code has been verified up to the bound.

Note: we recommend specifying preconditions as always checked, requires release, on entrypoint functions to ensure that these externally exposed API endpdoints are not misused.

More details on this symbolic checker can be found in the readme.

Ahead-of-Time Compilation

To provide the best performance Bosque supports the generation of standalone command-line executables via the ExeGen tool. This tool, and the design of the Bosque runtime, are designed to provide:

  1. Fast cold start response time by precompiling startup logic directly into constant values whenever possible and minimizing the number of operations required to start handling user input.
  2. Stable execution behavior over time and possible inputs.
    • The GC is a novel reference counting with eager free implementation to minimize memory footprint and prevent any indeterminate GC jitter.
    • The runtime itself uses sorted container implementations for Sets/Maps so that the variance between average and worst case costs of operations is minimized and to protect against pathological behaviors (like extreme hash-code collisions).
  3. Safe recursion is available with the [TODO] flag. This compiles recursive functions into a CPS form that uses constant stack space, eliminating any possible Out-of-Stack issues, and allows us to preserve the full call-stack in all debug builds.

A simple example use is to create a file called "max.bsq" with the following code:

namespace NSMain;

entrypoint function main(x: Int, y: Int): Int {
    return (x > y) ? x : y;

Then run the following command to produce the max.exe (on Windows executable) which can then be invoked with:

> node impl\bin\runtimes\exegen\exegen.js -o max.exe max.bsq

Which will create an executable named max.exe in the current directory.

Running this executable:

> max.exe 1 5

Will output 5.

More details on the exeGen tool can be found in the readme.

Using the Bosque Language

The current focus of the Bosque project is core language design. As a result there is no support for packaging, deployment, lifecycle management, etc.

Note: If you are running examples from the "Learn Bosque Programming" book please use the LearnBosqueProgramming branch which is sync'd with the version of code used in the book.


In order to build the language the following are needed:

  • 64 bit Operating System
  • The LTS version of node.js ( According to your OS )
  • Typescript (install with: npm i typescript -g)
  • A C++ compiler -- by default clang on Windows/Linux/Mac

Build & Test

The impl directory contains the reference implementation parser, type checker, interpreter, and command line runner. In this directory, build and test the Bosque reference implementation with:

npm install && npm test

Note: the Z3 theorem prover is provided as a binary dependency in the repo via git LFS. To ensure these are present you will need to have git LFS installed, run git lfs install to setup the needed hooks, and pull.

Visual Studio Code Integration

This repository provides basic Visual Studio Code IDE support for the Bosque language (currently limited to syntax and brace highlighting). The installation requires manually copying the full bosque-language-tools/ folder into your user .vscode/extensions/ directory and restarting VSCode.


This project welcomes community contributions.

This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact with any additional questions or comments.

Please refer to Contribution Guidelines for more details.


Code licensed under the MIT License.


The Bosque programming language is an experiment in regularized design for a machine assisted rapid and reliable software development lifecycle.




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