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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 serverless and microservice architectures.

The Bosque Language is a novel hybrid of functional programming language semantics and an ergonomic block & assignment-based syntax. This allows developers to organize code into familiar/natural blocks and compositions while, simultaneously, benefiting from the correctness and simplicity of a functional programming model (see code examples below). The language also provides a range of ergonomic features for writing high reliability code, such as Typed Strings, unit typedecls for primitives, and first-class assertions/pre-post conditions/invariants.

The Bosque Testing Framework provides a built-in unit testing system, a powerful new SMT powered property-based testing system, and the ability to symbolically search for errors that can be triggered by user inputs in the entrypoints to the program (see the bosque command section below). These tests and checks can find compact debuggable inputs that trigger and error or failing test and, in many cases, can also prove that there will never be a case with a “small repro” that triggers the error!

The Bosque Runtime is a novel pathology free design that focuses on predictable latency, pauses, and 99th percentile behavior. This starts with a new garbage collector that is guaranteed to never need a stop-the-world collection, that only uses live-heap + a small constant in memory to run, and (will eventually) supports background external defragmentation! Beyond the GC behavior the runtime design excludes pathological regex behavior, dynamic execution bailout overload, and catastrophic amortized operation behaviors such as repeated rehashing (instead using slower but stable log time persistent structures).

The Bosque API Types provide a way to specify an application interface in a clean manner that is independent of the specifics of the Bosque type system. This enables the auto-generation of input validation and encoding logic. We currently support a universal JSON encoding but more efficient representations are possible. This design ensures that Bosque APIs can easily be integrated into polyglot systems (e.g. microservice architectures) or integrated into existing codebases (e.g. Node.js or C++).

The Bosque Package Manager (see the bosque command section) provides a centralized way to organize, test, and build an application – either as a standalone command or to integrate into other applications via JSON APIs. This manager is designed to take advantage of the checking capabilities of Bosque and will enable developers to (1) test against imported code using auto-generated mocks and (2) check that package updates do not (intentionally or maliciously) change the package behavior, introduce new data outputs, or expose sensitive data to unintended outputs!


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.

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=0i): Int {
    var y: Int;

    if(x == 0i) {
        y = 0i;
    else {
        y = (x > 0i) ? 1i : -1i;

    return y;

sign(5i)    //1
sign(-5i)   //-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{"bob"}.sayHello()      //"hello bob"

(Algebraic Data Types)++ and Union Types

datatype BoolOp provides APIType using {
    line: Nat
} of
LConst { val: Bool }
| NotOp { arg: BoolOp }
| AndOp { larg: BoolOp, rarg: BoolOp }
| OrOp { larg: BoolOp, rarg: BoolOp }
& {
    recursive method evaluate(): Bool {
        match(this) {
            LConst                  => return this.val;
            | NotOp                 => return !this.arg.evaluate[recursive]();
            | AndOp{_, larg, rarg} => return larg.evaluate[recursive]() && rarg.evaluate[recursive]();
            | OrOp{_, larg, rarg}  => return larg.evaluate[recursive]() || rarg.evaluate[recursive]();

AndOp{2, LConst{1, true}, LConst{1, false}}.evaluate[recursive]() //false
OrOp{2, LConst{1, true}, LConst{1, false}}.evaluate[recursive]()  //true

function printType(x: Bool | Int | String | None ): String {
    return match(x) {|
        Bool     => "b"
        | Int    => "i"
        | String => "s"
        | _        => "n"

printType(1.0f) //type error
printType(true) //"b"
printType(none) //"n"

Validated and Typed Strings:

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

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

ZipcodeUS::accepts("98052-0000") //true
ZipcodeUS::accepts("1234")       //false

is3pt("12")              //type error not a StringOf<CSSpt>
is3pt('98052'_ZipcodeUS) //type error not a StringOf<CSSpt>

is3pt('3pt'_CSSpt) //true
is3pt('4pt'_CSSpt) //false
entity StatusCode provides Parsable {
    field code: Int;
    field name: String;

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

    function accepts(name: String): Bool {
        return name === "IO" || name === "Network";

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

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

isIOCode('Network'_StatusCode)               //false
isIOCode('IO'_StatusCode)                    //true

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

Numeric Types

typedecl Fahrenheit = Int;
typedecl Celsius = Int;

typedecl Percentage = Nat & {
    invariant $value <= 100n;

32_Fahrenheit + 0_Celsius //type error different numeric types
101_Percentage            //invariant error

function isFreezing(temp: Celsius): Bool {
    return temp <= 0_Celsius;

isFreezing(5)          //type error not a celsius number
isFreezing(5_Celsius)  //false
isFreezing(-5_Celsius) //true

The bosque Command

The bosque command is the primary tool for building, testing, and managing bosque packages and applications. The bosque command can be run on sets of files or, preferably, used in conjunction with Bosque packages which are defined with a package.json format.

Calculator Example

To illustrate how packages and the bosque command work we have a simple calculator app in the impl/src/test/apps/readme_calc/ directory (along with more interesting tic-tac-toe and rental apps).

This directory contains a package.json file which defines the package. As expected it has required name/version/description/license fields. The next part of the package definition, the src entry, is a list of source files (or globs) that make up the core logic of the application. Finally, we define two sets of files (or globs) that define the entrypoints of the application that will be exposed to consumers and a set of testfiles that can be used for unit-testing and property-based symbolic checking.

Calculator Source Code, Entrypoints, and Test Definitions

The source code file, calc_impl.bsq, for the calculator has two simple function implementation (sign and abs):

namespace Main;

function abs_impl(arg: BigInt): BigInt {
    var res = arg;

    if(arg < 0I) {
        res = -arg;
    return res;

function sign_impl(arg: BigInt): BigInt {
    return arg > 0I ? 1I : -1I;

These functions are used, along with some direct implementations, to create the external API surface of the package (defined in the entrypoints files with a .bsqapi extension). The calculator exports several functions including div which uses a Result to handle the case of division by zero and uses the pre/post features of the Bosque language (ensures) to document the behavior of the abs and sign methods for the clients of this package.

namespace Main;

//More entrypoint functions ...

entrypoint function div(arg1: BigInt, arg2: BigInt): Result<BigInt> {
    if(arg2 == 0I) {
        return err();
    else {
        return ok(arg1 / arg2);

entrypoint function abs(arg: BigInt): BigInt 
    ensures $return == arg || $return == -arg;
    ensures $return >= 0I;
    return abs_impl(arg);

entrypoint function sign(arg: BigInt): BigInt 
    ensures $return == 1I || $return == -1I;
    return sign_impl(arg);

The run Action

The run action in the bosque command provides a simple interface for invoking the entrypoints from a command line using JSON values. The syntax run [package.json] [--entrypoint Namespace::function] will load the code/api specified in the package (default ./package.json) and find/run the specified function (default Main::main). The arguments can be provided on the command line, --args [...], or via stdin. The image blow shows how to execute the div and sign APIs.

The test Action

The test action handles running unit-tests and property-tests defined in the testfiles (with a .bsqtest extension). All functions that are declared as chktest functions will be run. Functions with 0 arguments are physically executed while functions with arguments are treated as parametric property tests and checked with the SMT solver for small inputs that may violate the desired property (i.e. the test returns false).

namespace Main;

chktest function abs_neg(): Bool {
    return abs_impl(-3I) == 3I;

chktest function sign_pos(): Bool {
    return sign_impl(5I) > 0I;

chktest function sign_neg(): Bool {
    return sign_impl(-4I) < 0I;

chktest function sign_neg_is_minus1(x: BigInt): Bool 
    requires x < 0I;
    return sign_impl(x) == -1I;

chktest function sign_pos_is_1(x: BigInt): Bool 
    requires x >= 0I;
    return sign_impl(x) == 1I;

Running the test action as shown results in 3 tests being identified as unit-tests and physically executed with 2 tests being identified as parametric property tests and checked symbolically. In this app all 3 of the unit-tests pass and the symbolic checker is able to prove that one of the property tests is satisfied for all (small) inputs. However, the other property test does have a violating input, namely when x is 0 when the function sign_impl evaluates to -1 but the expected property is that the sign should be 1.

The apptest Action

The apptest action takes the power of the symbolic checker that Bosque provides and applies it to possible runtime errors, assertion failures, pre/post conditions, and invariants that may be triggered by a client calling an API provided in the package entrypoints. Running the apptest command takes each entrypoint function and checks all possible errors reachable to either (1) find a small repro input that triggers the error or (2) prove that no such small input exists.

This results in 2 checks of postconditions, for sign and abs, and one check for a possible div-by-zero in the div entrypoint. In all three cases the checker is able to prove that there is no input that can trigger any of these errors or violate any of the post-conditions!

The other apps have more interesting code, tests, and errors to experiment with as well.

Installing the Bosque Language (Development)

In order to install/build the project 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)
  • Git and git-lfs setup
  • A C++ compiler -- by default clang on Linux/Mac and cl.exe on Windows

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.

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

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.