(define ...) inside a default value

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(define ...) inside a default value

Postby Zabolekar » Fri Jul 25, 2014 4:31 am

I have no reason to use something like that in real code, so this is just pure curiosity. Nevertheless, I would like to understand why this piece of code does what it does.

In newLisp, the following code

Code: Select all
(define (f a (b (define (f a) "D"))) a)
(println (f "A" 1) (f "B") (f "C"))


will print AnilD. I don't quite understand where nil comes from. I would expect either ABD or, less intuitive but still understandable, ABC. It gets even more confusing. When I do

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(define (f a (b (define (f a) "D"))) (println "") a)
(f "A")


the interpreter tells me:

Code: Select all
--- cannot execute: restart shell ---


And, indeed, in this case I have to restart the shell to make it work again.

In Common Lisp, Clojure and Ruby the following pieces of code seem to be equivalent:

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(defun f (a &optional (b (defun f (a) "D"))) a)
(format t "~a~a~a" (f "A" 1) (f "B") (f "C"))


Code: Select all
(defn f ([a] (defn f [a] "D") a) ([a b] a))
(println (str (f "A" 1) (f "B") (f "C")))


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def f(a, b = def f(a); "D"; end); a; end
p f("A", 1) + f("B") + f("C")


All of them print ABD.
Zabolekar
 
Posts: 3
Joined: Fri Jul 25, 2014 3:42 am

Re: (define ...) inside a default value

Postby Lutz » Fri Jul 25, 2014 1:21 pm

It does literally what you tell it to do. newLISP is a fully self reflective language which can introspect or modify itself at run-time. In your example the function f is redefining itself while executing. The old discarded definition is collected by memory management and execution runs into invalid memory causing unexpected results or crashing.

When functions modify themselves, care must be taken to not replace code parts executing at the same moment.

See about self-modifying code in newLISP on this page: http://www.newlisp.org/index.cgi?Closures the paragraphs Introspection and Stateful functions using in-place modification.
Lutz
 
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Joined: Thu Sep 26, 2002 4:45 pm
Location: Pasadena, California

Re: (define ...) inside a default value

Postby Zabolekar » Fri Jul 25, 2014 2:03 pm

Thanks for the answer! So the old function definition is collected right after we redefine the function? That makes sense for me. Then, as far as I understand, modifying the function without actually redefining it should be safe (is it?):

Code: Select all
(define (f a (b (setf (nth 1 f) "D"))) a)
(println (f "A" 1) (f "B") (f "C"))


It prints ADD, as expected.

Is there a way to self-modify a function so that the changes only take place after the function returns?
Zabolekar
 
Posts: 3
Joined: Fri Jul 25, 2014 3:42 am

Re: (define ...) inside a default value

Postby Lutz » Sat Jul 26, 2014 1:44 pm

Some functions in newLISP delay memory cleanup, but not applicable in this case.
Lutz
 
Posts: 5279
Joined: Thu Sep 26, 2002 4:45 pm
Location: Pasadena, California

Re: (define ...) inside a default value

Postby Zabolekar » Sat Jul 26, 2014 5:54 pm

Thanks.
Zabolekar
 
Posts: 3
Joined: Fri Jul 25, 2014 3:42 am


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