Author: grant

Lisp is the smartest way to misuse a computer

Lisp has jokingly been called “the most intelligent way to misuse a computer”. I think that description is a great compliment because it transmits the full flavor of liberation: it has assisted a number of our most gifted fellow humans in thinking previously impossible thoughts.

– Edsger Dijkstra, CACM, 15:10

What are syntax-case pattern variables?

I asked here:

syntax-case takes a syntax object as an argument and destructures it via pattern matching into pattern variables.
What are pattern variables composed of?
syntax takes a template, and the pattern variables in it, to produce a syntax object that has the same lexical scoping as that of the pattern variable, so where does that lexical scope get stored?

and then Ryan explained:

The short answer is that syntax objects store scoping information and
that pattern variables have nothing to do with it.
Here’s a longer answer:
A pattern variable is essentially bound to an ellipsis-depth n and syntax object(s). If the pattern variable doesn’t have ellipses after it, then n is 0 and the pattern variable refers to a single syntax object. If it has one ellipsis after it, then n is 1 and there is a list of syntax objects. And so forth.
The ‘syntax’ form is like ‘quasiquote’, except that there is no explicit ‘unquote’; rather, pattern variables are automatically substituted, and the rest of the template becomes syntax object literals (via ‘quote-syntax’) that remember the bindings in scope where they occurred.
So pattern variables, while convenient, are not necessary for writing macros. You could do just the same with ‘quote-syntax’, ‘syntax-e’, ‘datum->syntax’, ‘car’, etc.
Lexical scoping is managed by the syntax objects. The macro arguments already have their lexical contexts embedded in them, and that is preserved as the macro takes them apart and puts them together with new syntax. This new syntax mostly comes from syntax literals.
The essence of the algorithm is explained in “Syntactic Abstraction in Scheme” by Dybvig et al.

A Scheme program that drives wheelchairs

You can read about a Scheme program that drives wheelchairs using machine learning and induction here: 


;; Larceny version.
;; This program is part of a pattern recognition
;; system that drives a wheel-chair for quadriplegic
;; people: www.ciaem.org.br/ciaem.qps/Ref/QUIS-7GJP7J
;; A description of the complete system, written in
;; Scheme by Philippos, will be published in a paper
;; by Marcus Vinicius dos Santos, that can be reached
;; at web.mac.com/marcusvsantos/iWeb/Site/About%20Me.html
;; The wheel chair learns how to recognize EMG signals
;; from facial muscles. A neural network recognizes
;; the coeficients of a polynomial approximation of
;; the signal, and drives the chair accordingly.
;; The chair will be available soon, although the
;; pattern recognition version needs a special
;; order, and user training. This Larceny version
;; is not used in the chair prototype.
;; Store in file larcenet.scm

Already Postive Change for R7RS

The R7RS steering committee will be updating the charter to allow for another two members in the steering committee: Olin Shivers and Chris Hanson. Two more members is a good thing. Additionally, the post hints that the passionate discussion on the R6RS mailing list may have payed off with changes to the process that will better represent all of the community. I look forward to what comes of this.

(via PLT)