LispKit: building custom Lisp interpreters in Squeak

by Stéphane Rollandin
hepta@zogotounga.net


Introduction: LispKernel

The LispKit package provides an easy (and fun !) way to play with Lisp interpreters within Squeak. The class LispKernel implements a Lisp engine completely embedded in Smalltalk, in the same sense as Schelog is a Prolog embedded in Scheme: there is no hard boundary in syntax or evaluation.

Most notably LispKit does not use a specific parser. Lisp code/data is represented by a ConsCell, which can be built from an Array by sending it #asCons. So for example
prog := #(funcall (lambda (x) (sqrt x)) 5) asCons
is a Lisp program. Because it is plain Smalltalk code, if prog is defined as above somewhere in a method, you will find it by looking for the senders of #funcall (using alt-n); more about this below...

Now the usual interface with a Lisp interpreter is a GUI accepting plain text input (scanned and parsed via the regular read function).



The ELisp GUI in Squeak 4.5, with an explorer on a ConsCell
At top we have a workspace, at bottom a read-eval-print loop



LispKernel is a dynamic (variables are not lexically scoped) Lisp-2 (one namespace for values, one namespace for functions) inspired by Emacs Lisp.

LispKernel has been designed for pedagogy (its own author going through the learning process !); it is a tool for understanding what is Lisp and how it works. All the machinery is clearly exposed and hopefully commented well enough.

By subclassing LispKernel one can built an interpreter for its own Lisp dialect.
Currently we have


The Scheme implementation: ULisp


the ULisp GUI in Squeak 4.5
pi.scm is a file from the SCM distribution
You can see the pretty printer from SLIB at work on the code for pi



Current status


ULisp includes the following third-party applications as libraries (all work fine):



Download

The most current code for LispKit is available on SqueakMap: check up the entry LispKit.
LispKit can be installed in any Squeak image from version 3.8 upward (last tested on 4.5)

You can also get it here: LispKit-93.sar (February 9th, 2014), although this may not be the latest version.




What are we talking about here ?

Squeak and Smalltalk links

Lisp links

Scheme links




More (random) details


Documentation

The documentation is to be found in the class comments of the LispKernel hierarchy; start by LispKernel itself.


About Lisp code in Array format

The Array syntax can be cumbersome and has definite limits, hence Lisp code can easily come to look a bit weird, like
#(defun #'my-function' (a b) (list a $'and b))
where my-function needs to be quoted (else it would be parsed in the three symbols #my #- and #function) and the quotation mark needs to be introduced as a character since it is the string delimiter in Smalltalk.


More stuff to come here soon...

...


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