boxx

Small-step interpreted programming language, written in rust.

Prerequisites

• Install nightly Rust

Running

Access the REPL

cargo run

Run integration tests

cargo test

Variable binding

var x = 1; // => Undefined
x = 2; // => Undefined
x // => 2

Immutable reference binding

let x = 1; // => Undefined
x = 2; // => Error: Cannot assign Int(2) to const x
x // => 1

Recursive functions

fn fib(n) {
  n == 0 ? 0 : (n == 1 ? 1 : fib(n - 1) + fib(n - 2))
};

fib(8) // => 21

Since functions inherit bindings from the outer scope, we can also write:

var fib = fn(n) {
   n == 0 ? 0 : (n == 1 ? 1 : fib(n - 1) + fib(n - 2))
};

fib(8) // => 21

Control flow

var i = 0;

while (i < 10) {
   if (i % 2 == 0) {
      i = i + 1
   } else {
      i = i + 3
   }
};
i // => 12

No Type Coercion

1 + false // => Error: Invalid type conversion. Expected int and found Bool(false)

fn foo(x) { x + 1 }; // => Undefined
foo + 4 // => Error: Invalid type conversion. Expected int and found Func(Some(Var("foo")), Bop(Plus, Var("x"), Int(1)), [Var("x")])
foo(1) + 4 // => Int(6)

For more examples and planned features, see the integration tests.

Implementation

Expressions are evaluated using a small step interpreter.

boxx uses a hand-rolled recursive descent parser rather than a parser generator in order to have more control of the implementation and avoid dependencies. This is probably the messiest part of the code and source of the trickiest bugs.

boxx liberally uses Rust's Box module for heap allocation. This lets us worry a bit less about lifetimes, since the contents are freed when they go out of scope. As a result, the full AST is copied around way more than necessary. This inspired the temporary project name.

Features in progress

• Human-readable errors for parsing and evaluation. Most cases are covered, but the copy could be cleaned up.
  • TODO: error handling for division by zero
• Handle floating point numbers
• Data Types and runtime type checking
• Ability to read in code files. For now, the language only works as a repl.