KLambda
The means of achieving Shen involved developing a small fast Lisp called Kλ which is rich enough to encode Qi,
but small enough to be easily mapped into other platforms or implemented as a standalone.
In terms of design, Kλ reflects the following design criteria.
1. It should be powerful enough to express Qi.
2. It should be minimal, with very few primitives and so easy to implement. No macros, optional arguments etc.
because Shen can support all these things already.
3. It is not designed as a language to compete against Common Lisp; deliberately. It is a Lisp assembly language
for a higher level language. The minimalism is deliberate.
4. It should be clean; i.e. it should support lexical scoping and tail recursion
optimisation.
Some comparable small Lisps to Kλ are PicoLisp, FemtoLisp, TinyScheme and SmallLisp.
Unlike Common Lisp, Kλ is very portable over different platforms. In place of Common Lisp's 1150 pages of
specification, Kλ requires only 46 system functions. It is much easier to develop a stand-alone implementation
of Kλ than to build a complete version of CL. Even more significantly, it is possible to map Kλ into popular
existing platforms such as Clojure, Python as well as Common Lisp.
The implementation, Shen, attached to this document runs Qi on this reduced instruction set and, in terms of
primitives required, has nearly 1/3 of the footprint of Qi II. Shen is entirely defined in Kλ. Hence Shen is
no more difficult to port than Kλ itself. All that needs to be done is to map the simple reduced instruction
set of Kλ into the chosen platform. Our distribution in fact contains one such mapping - of Kλ to Common Lisp;
and this is done in 250 lines of code.
Kλ misses many of the features of bigger Lisps (like macros) because Shen effectively incorporates many of these
features. Even LIST and QUOTE are missing from Kλ because they are not needed for Shen and Qi has never needed or
missed them. Similarly we do not impose a comment convention on Kλ because we do not consider that people need to
or should write in Kλ. The emphasis is on devising an absolutely efficient minimal set needed to port Qi which is
easy for people to code in other languages.
However anybody who implements Kλ, and wants to write directly in this notation, can add extra features to their
implementation to Kλ. This does not affect the standard; just as a CL which includes (e.g.) threads does not cease
to be a CL even though it incorporates a non-standard feature. However the systems code for Shen will not rely on
extra features outside the standard. It is intentionally minimal. |
