# On Implementing a Lisp

## 0. Background

About a year ago I read McCarthy’s paper “Recursive Functions of Symbolic Expressions and Their Computation by Machine and I thought about implementing the Lisp described in it in Ruby (a language I was learning at the time). The project languished for quite some time until just recently when I went ahead and implemented the language specification laid out in that paper.

The project is yarlisp, it’s name means “Yet Another Ruby Lisp” given an assumption that I could not possibly be the first person to do this sort of thing, mixed with the idea that “Ruby is an acceptable Lisp” and even a bit of “Talk Like a Pirate Day”.

My goal in this project is to try to minimize the amount I need to implement in my ‘assembly language’ (Ruby). There is an idea that Lisp needs to have only seven primitive operations implemented in its assembly language and everything else can be implemented in the Lisp itself. (Obviously this seven primitives thing really only implements the symbol processing parts - things like numbers, IO etc. really need to be implemented in the underlying language). I have not entirely succeeded in this so far - I was more interested in the beginning to implement the functionality; I think I can refactor to more purity.

## 1. Current state of the project

I have implemented ATOM, EQ, CAR, CDR, CONS, COND, QUOTE and EVAL all in Ruby. The features of Ruby I have used are subroutine definition, conditionals, boolean true/false, equality and raising exceptions (for undefined behavior).

The implementation of EVAL required some more helper functions due to their recursive definitions, these too are written with Ruby as I had not yet found a way to implement subroutine definition - especially not recursive subroutine definition.

ATOMs are symbols (e.g. :foo); CONS cells are arrays of length two (e.g. [:a, [:b, :NIL]). Anything not an array is an ATOM. All public methods of the Yarlisp module are named in ALL CAPS; these are the Lisp primitives. Any helper methods are implemented as inner methods and are named in lowercase. If it is not in ALL CAPS it is not a method available in my Lisp language.

Currently the only sexp syntax available is CONS cells; the list shorthand style of sexp is not yet available. This means I need to write lists like [:a, [:b, :NIL]] instead of [:a, :b]. This has become very annoying, very quickly.

One oddity of my implementation is wherever I call a Lisp primitive I do it in an oddly Lisp looking way (odd to see in a Ruby file). For example, when the CAR of the expression passed to EVAL is :CDR it returns: (EVAL (CAR args), env). While it makes the code look odd for Ruby - it brings out the Lispyness of it. I will continue with this as an aesthetic in the code I write in this project.

Because of my choice of Ruby as my ‘assembly language’ I have not needed to implement memory allocation or garbage collection.

## 2. Thoughts

I found this a fun little exercise and want to continue to do it. I think I will also implement Lisp in other languages. It turned out not to be very hard and was something that could be easily be broken up into pieces by the different primitives and clauses in EVAL.

Now that I see all of EVAL implemented I think I can see a way to reimplement more of the language in Lisp itself by using the LABEL and LAMBDA Lisp functions. Also there are things like :NIL and true/false which I could implement by having EVAL append bindings to the environment it is passed. That way there will be default bindings for these things. Or perhaps I’ll just leave that for the caller of EVAL to do.

I ran into an odd issue while finishing my implementation of LABEL. There was a feeling of double evaluation on the arguments the method I was defining. Since my goal at the time was to implement the spec as written I went along with it. However I believe there is an superfluous call to EVAL in the definition as given in the paper. I want to hunt that down and fix it.

What I am struck with overall is the cleanliness and sparseness of it. The logic of simple, regular syntax, and the association list lead to no surprises during implementing. Each step made sense; although LABEL and LAMBDA took longer for me to understand - partly/mostly because the M-expr syntax used in the paper, along with the formatting of the paper, made that section of the specification harder to read. In reviewing my implementation I can see the specification very clearly in it; the definition of Lisp is very clear in its implementation.

I hope to continue working on this project - and others like it. I’ll continue to write about my adventures with it.

## 3. Next steps

The next phase of this project will include the following:

1. Adding the list style sexp shorthand - all these CONS cells are getting hard to write out.
2. Refactoring to higher purity of the language - implementing more of the language in itself - less in Ruby
3. Implementing a reader - which will need to be in Ruby. Although with the more comfortable sexp syntax this may not be so necessary.
4. I want to find a more Lisp way of handling the undefined cases - to remove the raising of exceptions from Ruby code. This is an extension of the purity issue above.

The very next step for me to take is to implement a real example method in my Lisp. Even though I do not have numbers I may implement a Fibonacci or factorial method using Church Numerals. At the very least I will implment the FF method presented in the paper.

lisprubyyarlisp