[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Extend-Syntax for MacScheme

In the book "The Scheme Programming Language", Kent Dybvig describes
a powerful tool for syntactic extension called extend-syntax.  Basically,
it allows you to specify transformations from input representations to
the corresponding code to be executed.  For a quick example, consider
we wish to implement the LET syntactic form.  This could be done with
extend-syntax as follows:

	(extend-syntax (PIGLET)
		((PIGLET ((x v) ...) e1 e2 ...)
		 (andmap symbol? '(x ...))
		 ((lambda (x ...) e1 e2 ...) v ...)))

This would define a new syntactic form (i.e. macro) named PIGLET that 
translates things of the form (PIGLET ((x v) ...) e1 e2 ...) to the
corresponding LAMBDA form after verifying that all the x's are 
symbols.  Notice the powerful pattern matching abilities provided by
using the ellipsis (...).  There is a *LOT* more to extend-syntax than
this example would indicate.  For the full glories, (defstruct in about
20 lines of code, objects (as in OOP) in about the same), see the book.

Well, I coveted the power of extend-syntax and desired it greatly, so
I asked Kent Dybvig if he would part with the necessary incantations...

Behold! I had Mail! (thanks kent!)

I am running MacScheme Version 1.22 and have converted the extend-syntax
code to work under such.  If you would like a copy, send me e-mail to
that effect and I'll see that a copy is sent to you.  If I get swamped
with requests, I might just post the whole thing to here.

Jeff De Vries   (ARPA: jdevries@ads.arpa)
Usual Disclaimers Apply.