Language::Bel

A Perl 5 implementation of Paul Graham's Bel. Bel is a self-hosting Lisp dialect, released October 2019.

There are many Lisp dialects in the world, but Bel distinguishes itself by defining a complete stack of features on a metacircular foundation. That includes the evaluator, the reader and printer, but also I/O, error handling, and a numeric tower; these are all specified as Bel code in the language itself. Bel is built on top of Bel; that's true of the language's specification, and increasingly true of this implementation.

Getting started

You need Perl installed.

Right now, the way to install and run Language::Bel is via GitHub.

$ git clone https://github.com/masak/bel.git
Cloning into 'bel'...
[...]
done.

At some point soon, I'll also upload this distribution to CPAN. Then you'll be able to install it using a CPAN installer, such as cpanm.

Usage

After downloading Bel, you can run it like this:

$ perl -Ilib bin/bel
Language::Bel 0.64 -- darwin.
>
> ;; loops
> (set n (len (apply append prims)))
16
> (each word `(there are ,n primitives) (prn word))
there
are
16
primitives
(there are 16 primitives)
>
> ;; sorting and de-duplication
> (sort < '(3 2 8 6 18 12 2 19 13 19))
(2 2 3 6 8 12 13 18 19 19)
> (dedup (sort < '(3 2 8 6 18 12 2 19 13 19)))
(2 3 6 8 12 13 18 19)
>
> ;; templates and places
> (tem vec2d x 0 y 0)
((x lit clo nil nil 0) (y lit clo nil nil 0))
> (set robot (make vec2d))
(lit tab (x . 0) (y . 0))
> robot!y
0
> (zap [+ 5 _] robot!y)
5
> robot!y
5
> (++ robot!y)
6
> ((of list robot) 'x 'y)
(0 6)
>
> ;; arrays
> (set I (array '(3 3) 0))
(lit arr (lit arr 0 0 0) (lit arr 0 0 0) (lit arr 0 0 0))
> (def diag (m n) (m n n))
> (for n 1 3 (set (diag I n) 1))
nil
> I
(lit arr (lit arr 1 0 0) (lit arr 0 1 0) (lit arr 0 0 1))

But is it feature-complete?

Language::Bel implements all of the global definitions from the specification.

However, there are still some non-negotiable features still waiting to be completed:

• Unicode. The built-in chars global needs to be able to recognize unicode codepoints, not just ASCII. Source files and strings ought to be treated as being UTF-8-encoded by default.

• Error messages. A runtime error can usually be traced back to a specific point in source code, and given ample context and explanation. The goal is to give an account of what went wrong, in full sentences, and also to suggest what can be done to fix the problem. (Elm serves as the inspiration here.) Currently, an error consists of only one symbol, such as mistype.

• Debugging. The turnaround between authoring and running a program should be made as small as possible; specifically, when an error does happen, the REPL should put itself in a debugging mode where it's not only easy to inspect the current state of the program (including local variables and the stack) but also to modify things and resume without having to restart the whole program execution.

• Printer bootstrap. The Perl printer should be replaced by the Bel printer. When globals associated with the Bel printer are replaced, the effects on printing should be immediate.

• Reader bootstrap. The Perl reader should be replaced by the Bel reader. When globals associated with the Bel reader are replaced, the effects on reading should be immediate.

• Evaluator boostrap. The Perl code that evaluates Bel programs (that is, large parts of the module Language::Bel itself) should be replaced by the Bel evaluator bel from the Bel globals. By default, this new evaluator should be able to run at speeds comparable to the current Perl evaluator. When globals associated with the Bel evaluator are replaced, the effects on evaluation should be evident as soon as a (bootstrap) call is made.

• Performance. Everything needs to be faster. As a first step, there should be a set of concrete performance goals to aim for, maybe ten or so. These will almost certainly be achieved via compilation to a dedicated bytecode format.

• Documentation. The language needs to be thoroughly described, both at a high level with general concepts and techniques, and at a lower level with a complete reference of all the built-in functions and types. Some tradeoffs and design decisions of this particular implementation (yes, there are a few) need to be explained as well.

Contributing

If you'd like to contribute, please fork the repository and make changes as you'd like. Pull requests are warmly welcome.

License and copyright

This software is Copyright (c) 2019-2024 by Carl Mäsak.

This project is licensed under the GNU GPL 3.0. For details, see LICENSE.