memowe / perlisp

A simple Lisp interpreter, written in Perl

PerLisp

This is a simple statically scoped Lisp interpreter, written in Perl. During the lecture "Structure and Interpretation of Programming Languages", held by Achim Clausing (WWU Münster, Germany), I needed to try out different things, so I wrote this.

DISCLAIMER This is not production ready. I use it to play around and I think there are many bugs lurking around. You have been warned! :-)

How to start?

Install PerLisp like a normal Perl distro:

$ perl Makefile.PL
$ make
$ make test
$ make install
$ make clean

Now you can call the perlisp script which starts a read eval print loop (see more details below):

$ perlisp

If you want to hack PerLisp, view the API docs via

$ perldoc PerLisp

If you don't want to install all that stuff, you can call the perlisp script from the project root directory:

$ cd ~/code/perlisp
$ perl perlisp

And it should just work. Now let's do some Lisp stuff!

PerLisp uses the bind operator to bind values to names, you have a lambda operator to create function values and the convenience operator define to bind a function to a name in one step. With the bound operator you get a list of all bound names. Most of them come from the init.perlisp file, which is loaded while the interpreter starts.

Start typing something like this:

(+ 17 25)
42
(define (square x) (* x x))
Function: (x) -> ( x x)*
(square 42)
1764

You can find more inspirations from PerLisp::Init and the tests, especially t/50-init.t, which shows some use cases for the init functions.

Some interesting stuff

• Like Haskell, PerLisp fully supports autocurryfication, which means a function applied to too few arguments will return a function with parameters for the remaining arguments automagically. It's super-easy to define useful functions using autocurryfication in combination with some well known higher order functions:

(bind product (reduce * 1))
Function: (l) -> (cond (nil? l) 1 ( (car l) (reduce * 1 (cdr l))))*
(product (list 2 7 3))
42

• PerLisp is able to interpret a simple Lisp interpreter written in Lisp which is able to run itself inside PerLisp. Amazing! And there's a test for it in t/61-lisplisp.t. However, to calculate even a simple expression in a three-level nested interpreter chain takes some time so you have to enable the test explicitly by setting the LISPLISP environment variable to a true value.

Interpreter details

The implementation is straight forward. PerLisp uses objects from the following classes to get you started with Lisp:

• PerLisp::Lexer - generates a PerLisp::TokenStream from Lisp strings
• PerLisp::Parser - generates PerLixp::Expr::* objects from a TokenStream
• PerLisp::Expr::* - Lisp expressions
• PerLisp::Context - objects which store value-name bindings
• PerLisp::Operators - perl implementations of built-in operators
• PerLisp::Init - a container package around the init script

View the code for more implementation details, I tried hard to make it very readable (even if you don't know much Perl).

Author, bug reports, license

Copyright (c) Mirko Westermeier, [email protected]

This software is released under the MIT license (view MIT-LICENSE for details).

If you found a bug, please contact me via mail or github. You can also use github's issue tracker. Please provide (failing) tests for your bugs, patches or feature requests.