30 minutes Lisp in Ruby (direct link)

http://gist.github.com/562017

by RyanMcGreal • 16 years ago

31 15 16 years ago

gist.github.com

Lisp's simplicity is one of its most interesting features. As far as I know, Lisp is the only language that can achieve such great power while managing to be so simple.

Lisp has staying power.

kingkilr• 16 years ago

It's kind of cheating to not even include a lexer/parser.

Goladus• 16 years ago

If your platform provides one, why bother?

shaunxcode• 16 years ago

so that you can read lisp source instead of [:+, 6, 7]

However the parsing is probably the easiest part so punting makes sense as you can get down to the stuff that matters and add that in later.

akkartik• 16 years ago

I wish somebody would build a toy lisp with some sort of macro. That's the part of the implementation that is still opaque to me.

Last time this came up: http://news.ycombinator.com/item?id=1468345

EdiX3• 16 years ago

I don't know what's the proper way of implementing macros but I can tell you how I did it in my toy lisp implementation: I compiled defmacro as a normal function but I also added it to a macro table, then in my compile-form function I would use this table to decide if the form is

a) a special form b) a function call c) a macro

(a) and (b) are obvious, (c) means I would call the compiled macro "function" on the rest of the form and the execute compile-form on the result.

It's unhygenic and there's probably some fatal flaw I didn't catch.

bitsai3• 16 years ago

Question about (a):

How do you handle special forms? I wrote a toy Lisp interpreter in Clojure, but couldn't figure out how to implement ', which I assumed should be implemented as a special form.

abecedarius3• 16 years ago

That's the standard way to do it, except DEFMACRO is not a function. So define a function called SET-MACROEXPANDER or something, and define DEFMACRO as a macro using it.

abecedarius3• 16 years ago

http://github.com/darius/awklisp does it the usual way. It's an order of magnitude bigger than the OP, and not focused on How To Do Macros, but on the plus side, it is just a toy.

jcoglan• 16 years ago

I wrote a Scheme in Ruby that has macros last year: http://github.com/jcoglan/heist. It's an interpreter, not a compiler, and is extremely dynamic: all functions, macros, special forms are just treated as function calls by the parser. Macros are just a subtype of functions that accept and return syntax nodes before eval-ing the output, and this is done at runtime while running all your regular code.

30 minutes Lisp in Ruby (direct link)

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