boring-lang/README.md

Boring Lang Proposal

NOTE: This repo is a work in progress as I learn compiler writing, I may abandon this.

The Boring Programming Language (Boring-Lang) is an attempt to create an easy, productive, general purpose programming language that makes as few interesting choices as possible while still being in line with modern concepts in programming languages.

The language (wish list):

• is compiled with a run-time (llvm for convenience + c/rust compatibility)
• is garbage collected
• uses async-await for all IO, with a built-in multi-core scheduler
• supports algebraic data types (Result type for errors, Maybe/Optional type for nullables)
• supports parametric polymorphism (generics)
• uses struct+traits, rather than classes or stuct+interfaces
• has a rich standard library (http server, actor model)
• is immutable by default

It's basically a middle-ground of Rust, Golang, Swift, Typescript, and Python.

Http Server Example

import net.http as http
import logging as log
import json as json

type ExampleResponse struct {
  id: Int32
  name: Str
  email: Str
}

async fn handle(req: http.Request, resp: mut http.Response) {
  log.info("request: ", req.body)
  let response_data = ExampleResponse{id: 4, name: "Steven", email: "swerbenjagermanjensen@example.com"}
  await resp.set_status(200)
  await resp.write(json.encode<ExampleResponse>(response_data))
}

async fn main(args: Array<Str>) Int32 {
    let router = http.Router("").add_route("/myroute", handle)
    http_server = http.Server("localhost", 8080, router)
    let err = await http_server.server_forever()
    await log.info("error serving: ", err)
    return 1
}

Mutability

All variables are immutable by default, to make them mutable use the mut keyword. Once a variable becomes immutable it cannot become mutable again. If you need it to become mutable, either implement the clone trait, or simply create a new one with the same data.

let mut foo = Dict<String, Int32>() // constructors always return a mutable reference
foo.insert("eggs", 12)
foo.insert("bananas", 2)
foo.insert("grapes", 2)

let bar = foo // bar is not mutable

bar.insert("apples", 4) // fails with compiler error

let mut baz = bar.clone()
baz.insert("apples", 4) // fine

Methods on a struct must specify if they mutate the struct.

impl Dict<Key: Hashable, Value> {
  fn insert(self: mut Self, key: Key, value: Value) {
    // mutate self here
  }

  fn get(self: Self, key: Key) Optional<Value> {
    // no need for `mut`
  }
}

Context

Context is an exceptionally useful feature in golang, but a common complaint is that:

1. Because it works as an arbitrary map, it can be used to pass arguments into a function that aren't explicitly stated.
2. It is used for both passing context parameters and cancellation, two fundamentally different tasks that have no reason to be in the same object.

The boring standard library solves this by using parametric polymorphism. Context is by default an empty object passed through the chain, and each function/set of context parameters is an additional trait condition applied at compile time.

type HTTPRequest<Ctx: Context> = async fn(Ctx, http.Request, mut http.Response)

pub fn tracing_middleware<Ctx: Tracing>(handler: HTTPRequest<Ctx>) HTTPRequest {
  return async fn(ctx: Ctx, req: http.Request, resp: mut http.Response) {
    with tracing.Span(ctx, "request_duration") {
      await handler(ctx, req, resp)
    }
  }
}

pub fn auth_middleware<Ctx: Auth>(handler: HTTPRequest<Ctx>, scope: Str) HTTPRequest {
  return async fn(ctx: Ctx, req: http.Request, resp: mut http.Response) {
    if ctx.has_scope(scope) {
      await handler(ctx, req, resp)
    }
    await resp.set_status(403)
    await resp.write("missing scope")
  }
}

pub fn cancel_middleware<Ctx: Cancel>(handler: HTTPRequest<Ctx>) HTTPRequest {
  return async fn(ctx: Ctx, req: http.Request, resp: mut http.Response) {
    if !(await ctx.is_cancelled()) { // check cancel token
      await handler(ctx, req, resp)
    }
    await resp.set_status(400)
    await resp.write("cancelled")
  }
}

for the above examples, you would pass a context type that implements all three traits.

Monadic function modifiers

Boring uses function modifiers to implement functionality like async/await and coroutines. These function by rewriting the code into a state machine prior to compilation. The table below describes the modifiers currently available.

Type	Change To Return Type	Introduces to Scope
async	Promise<ReturnType>	await
coroutine	FirstReturnType,func(Next,Params)...	yield
error<ErrorType>	Result<ReturnType,ErrorType>	?

Import System

Similar to python, folders/files represent the . seperated import path, but relative imports are not supported. Exported values must be marked with pub. All imports take the form:

import package.path as local_name

pub type MyStruct struct {
  id: Int32
}

Basic Statements

if

if is an expression in boring-lang, with the last expression in a block being the return value. If the block ends in a statement rather than an expression, Optional None is returned.

let a = if true {
  4
} else {
  2
}

// a == 4

let b = if false {
  2
}

// b is an Optional<Int32> with value None.

Conditions do not require parenthesis and must evaluate to the Boolean type.

Loops

Boring-lang supports for and while loops, with for having an async variant. while loops require an expression of Boolean type, while for loops require an expression that implements the Iter or AIter traits.

let mut i = 0
while i < 100 {
  i = i + 1
  // do something here
}


for i in range(100) {
  // do something here
}

async for result in paginated_list {
  // do something with result
}

continue and break work similar to other languages.

while true {
  break // do nothing
}

for i in range(100) {
  continue // do nothing
}

with

with and async with blocks are similar to the python statement with the same name. But unlike the python version, with blocks are expressions. with blocks take in an expression that implements the With or AWith trait, and execute a block that may return a result (non-result returns are assumed success).

// commits on success, aborts on error.
// transation.aexit may just return an error as a pass-through after aborting,
// but it may also transform it into another error adding context.

return async with db.transation(ctx) as t {
    await t.insert(ctx, record) // returns result type
}

return

return statements exit a function early, returning the given value. They are purely optional as the last expression in a function will automatically return its value.

match

match expressions provide pattern matching, similar to a C switch statement.

let number = 3
let result = match number {
  1 => 'foo',
  3 => 'bar',
  _ => 'baz',
}

// result = 'bar'

TODO: yield, lambdas,