Bella

A simple programming language, used for a compiler course.

let dozen = 12;
print(dozen % 3 ** 1);
function gcd(x, y) = return y == 0 ? x : gcd(y, x % y);
while (dozen >= 3 || (gcd(1, 10) != 5)) {
  let y = 0;
  dozen = dozen - 2.75E+19 ** 1 ** 3;
}

Language Specification

The language is specified at its home page.

Building

Nodejs is required to build and run this project. Make sure you have a recent version of Node, since the source code uses a fair amount of very modern JavaScript.

Clone the repo, then run npm install.

You can then run npm test.

Usage

To run from the command line:

node src/bella.js <filename> <outputType>

The outputType indicates what you wish to print to standard output:

Option	Description
parsed	A message indicating the syntax is ok
analyzed	The program representation (AST-like)
optimized	The optimized representation
js	The translation of the program to JavaScript

Example runs:

$ cat examples/small.bella
let x = sqrt(9);
function f(x) = 3 * x;
while (true) {
  x = 3;
  print(0 ? f(x) : 2);
}

$ node src/bella.js examples/small.bella parsed
Syntax is ok

$ node src/bella.js examples/small.bella analyzed
   1 | Program statements=[#2,#6,#10]
   2 | VariableDeclaration variable=#3 initializer=#4
   3 | Variable name='x' readOnly=false
   4 | Call callee=#5 args=[9]
   5 | Function name='sqrt' paramCount=1
   6 | FunctionDeclaration fun=#7 params=[#8] body=#9
   7 | Function name='f' paramCount=1
   8 | Variable name='x' readOnly=true
   9 | BinaryExpression op='*' left=3 right=#8
  10 | WhileStatement test=true body=[#11,#12]
  11 | Assignment target=#3 source=3
  12 | PrintStatement argument=#13
  13 | Conditional test=0 consequent=#14 alternate=2
  14 | Call callee=#7 args=[#3]

$ node src/bella.js examples/small.bella optimized
   1 | Program statements=[#2,#4,#8]
   2 | VariableDeclaration variable=#3 initializer=3
   3 | Variable name='x' readOnly=false
   4 | FunctionDeclaration fun=#5 params=[#6] body=#7
   5 | Function name='f' paramCount=1
   6 | Variable name='x' readOnly=true
   7 | BinaryExpression op='*' left=3 right=#6
   8 | WhileStatement test=true body=[#9,#10]
   9 | Assignment target=#3 source=3
  10 | PrintStatement argument=2

$ node src/bella.js examples/small.bella js
let x_1 = 3;
function f_3(x_2) {
return (3 * x_2);
}
while (true) {
x_1 = 3;
console.log(2);
}

Errors are displayed with a little bit of context:

 node src/bella.js examples/bad.bella js
Error: Line 2, col 20:
  1 | let x = 5;
> 2 | function f(x, y) = z + 1;
                         ^
  3 | print f(1, x);
Identifier z not declared

Contributing

I’m happy to take PRs. As usual, be nice when filing issues and contributing. Do remember the idea is to keep the language tiny; if you’d like to extend the language, you’re probably better forking into a new project. However, I would love to see any improvements you might have for the implementation or the pedagogy.

Make sure to run npm test before submitting the PR.

Thanks

This project uses Ohm for much of the front end. Ohm is maintained by Patrick Dubroy.

Marco Berardini chose the language name and took this picture of Bella: