A self-infecting datatype in Lisp

http://okmij.org/ftp/Scheme/circular-list-infections.txt

by gnosis • 15 years ago

52 8 15 years ago

okmij.org

guard-of-terra• 15 years ago

This looks ubercool until you realise that Clojure does this for a living with its seq protocol.

Then it's just neat.

lispm• 15 years ago

Nested DEFUNs is not the best idea...

AUmrysh• 15 years ago

As a CL newbie, what would you suggest be used instead? Also, why would you not use nested defuns?

gumbo72• 15 years ago

Also interested in this

draven2• 15 years ago

labels? http://www.lispworks.com/documentation/HyperSpec/Body/s_flet...

My guess: the nested defun is not only defined in the enclosing defun scope as it may seem, but in the whole current package.

http://www.lispworks.com/documentation/HyperSpec/Body/m_defu... "Defines a new function named function-name in the global environment"

Is it only a bad idea because it's bad style or is there more to it?

draven2• 15 years ago

I'm learning Haskell ATM and it looks similar to lazy lists in WHNF, as at each step you get an element and a "thunk" that can be used to compute the next step.

The code for the cycle Haskell function is really simple: http://hackage.haskell.org/packages/archive/base/latest/doc/...

eridius• 15 years ago

This is similar to one of my favorite Haskell "tricks": Tying the Knot. It's kind of a mindfuck the first time you do it, but it also feels really really cool.

http://www.haskell.org/haskellwiki/Tying_the_Knot

SeanLuke• 15 years ago

Since he mentioned self-infecting Lisp functions (which seems to me to be unrelated to his main topic but whatever), I should mention that I put together a Lisp self-infecting "virus" (so to speak) a long time ago. My original version -- which was beautiful and I have conveniently forgotten -- used a quine to deliver the payload. But here's a version I whipped up for HN a few weeks back which gives you the idea.

    ;; run this on SBCL
    (sb-ext:unlock-package :common-lisp)
    (let (syms) (do-symbols (s :common-lisp) (if (fboundp s) (push s syms)))
      (labels ((worm (sym)
    	     (let ((next (elt syms (random (length syms))))
    		   (old (symbol-function sym)))
    	       (format t "~%Worm attacks ~a, next target is ~a!~%" sym next)
    	       (setf (symbol-function sym)
    		     (lambda (&rest args) 
    		       (when next (worm next) (setf next nil)) (apply old args))))))
      (worm '+)))
    ;; now try adding some numbers (and so on)

A self-infecting datatype in Lisp

Related Stories