On Feb 19, 1:12 pm, Chris Perkins <chrisperkin...@gmail.com> wrote:
> On Feb 19, 4:32 am, timc <timgcl...@gmail.com> wrote:
> > Is #= an undocumented reader macro character?
>
> Interesting - I had never heard of it either. It appears to allow you
> to execute code at read-time.
>
> user=> (read-string "(foo (+ 2 3) bar)")
> (foo (+ 2 3) bar)
> user=> (read-string "(foo #=(+ 2 3) bar)")
> (foo 5 bar)
>
> I guess the ability to disable it with *read-eval* is something to do
> with reading from an untrusted source.
>
> I can't, off the top of my head, think of how I would use this. I
> thought the ability to run code at macro-expansion time was cool
> enough - now I find out I can do it at read-time too!
One use was easy serialization. That might not be so obvious in
Clojure; but in Common Lisp, objects are often left machine-
unreadable. Take how a 3D point might print:
CL: #<point (1,2,3)>
Clojure: {:x 1, :y 2, :z 3}
(Anyone who's unwittingly printed a gigantic xml-zip at the repl might
see the merits in the CL approach. ;)
So for cheap serialization, you'd write it out to stream like this, to
later read it back in:
;; in CL it's #. and not #=
;; http://www.pentaside.org/paper/persistence-lemmens.txt
(format stream "#.(MAKE-INSTANCE 'POINT :X ~S :Y ~S :Z ~S)" x y z)
Another use is optimization... you could compute (or partially
compute) a big ol' table of info at read-time.
I believe *read-eval*'s (aka "read-evil") default value of true is a
gaping security hole, though perhaps I'm missing a good justification
for it.
All the best,
Tj
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