[plt-scheme] stupid Swindle tricks

Eli Barzilay writes:
 > On May 26, Doug Orleans wrote:
 > > Candidates for swindle/misc.ss, in decreasing order of usefulness:
 > > 
 > >   (defsyntax* set-if!
 > >     (syntax-rules ()
 > >       ((_ condition then-place else-place val)
 > >        (if condition (set! then-place val) (set! else-place val)))))
 > 
 > I thought about adding that at the time, but there was some reason not
 > to do it.  Possibly the double code output in `val'.

Unfortunately, (set! (cond ...) val) doesn't seem to work, even if you
define set-begin! and set-#%top!.  Probably because the macros come
from a different place.  Anyway, this isn't earth-shatteringly useful,
but I thought it was a cute hack.

 > >   (defsyntax* last-of
 > >     (syntax-rules ()
 > >       ((_ successor first clause ...)
 > >        (collect (x first (successor x)) clause ...))))

I think this is useful.  Maybe it needs a better name.  I think it's a
good abstraction of "apply a function repeatedly and return the
last result" (where "last" is defined by the conditions in the other
clauses).  The rest of the stuff I posted is mostly just one
particular use for this, but those are probably too contrived.  (It
was late at night, I was a little loopy...)

--dougorleans at gmail.com