boros

boros (Shortened from Ouroboros) is a language that allows you to write self-modifying programs.

It was developed during the 2021 Lang Jam. The theme of the hackathon was first-class comments.

Language features

boros is a strict, impure, dynamically-typed functional language with ML-inspired syntax.

Syntax

The entire boros script file is one big expression, which is evaluated and printed by the interpreter. (If it evaluates to (), called unit, then nothing is printed)

Variables and functions are declared with let, and multiple mutually-recursive functions can be declared with let ... and.

Here's a simple factorial program:

let fact x =
  if not x then
    1
  else
    x * fact (x - 1)
in

let n = 10 in
fact n

Values in boros are immutable, with the exception of lists and records.

Here's how to define and use them:

let l = [1, 2, 3] in

l.[0] <- "abc";
print l.[0];

let r = { x = 2, y = 3 } in

r.x <- r.x + 1;
r.z <- 4; {- "You can even add new fields to an existing record!"; -}

First-Class Comments

In boros, comments are interpreted as part of the source code. (Can't get more first-class than that, right?)

Furthermore, all comments are accessible (in text form) from within boros scripts via a mutable global list called comments.

When the script terminates, the comments are replaced with the new strings from the comments list, and the script is re-run. This continues until either the script stops modifying the comments, explicitly calls the halt () function, or an exception is thrown.

A good example of this can be found in the Fibonacci example.

Comments are delimited by {- and -}. Currently, there's no support for line comments or nested comments.

Scripts can also have a shebang (#!) on the first line, which is ignored by the interpreter, and is not accessible from within the script.

Other bits of syntax

Custom operators

{- "Safe division"; -}
let (/?) a b =
  if b == 0 then
    0
  else
    a / b
in
2 /? 0

Lambda expressions

let add = x y -> x + y
in add 2 3

{- "Same as the following:"; -}
let add x y = x + y
in add 2 3

let add = (+)
in add 2 3

Sequencing with `;`

a; b will evaluate a and ignore its result, then evaluate and return b. This can be used to sequence (usually side-effectul) expressions.

print 3;
a.[0].x <- 2;
factorial 100

Types and Exceptions

boros is dynamically-typed. Performing operations on values of the wrong type will halt the program with a Runtime error.

You can use the type function to get the "approximate" type of a value, e.g. type 2 == "Num", type [1, "abc"] == "List".

You can also throw exceptions using the throw function. Any value can be provided as parameter, and will be shown in the error message. There is currently no catch/finally mechanism.

VS Code Extension

This repo also includes a VS Code extension for syntax highlighting.

You can install it by running the install-vscode-ext.sh script.

Build instructions

Prerequisites

GHC (>= 9.0.1)
cabal (>= 3.6)

(Can be installed via ghcup)

Build the interpreter

cabal build

Run the interpreter

cabal run . <script file> <args for boros script>

If not already built, cabal run will also build the project.

e.g.

cabal run . Examples/Factorial.brs 100

License

This repository is licensed under the terms of the GNU General Public License v3.

For more details, see the license file.

aionescu/boros