<html> <head> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> </head> <body bgcolor="#FFFFFF" text="#000000"> Thanks for the explanation. I suspected it was for efficiency reasons, but as I've never implemented a "real" scheme, I don't know the trade off. I wonder how bad it is. Way back, they invented lisp. Then they said it was too slow for real stuff. Now they say other languages are too weak for real stuff, and lisp's relatively little slowness is made up for by its power. And here we are, saying a powerful feature would slow it down!<br> <br> Is it 2x, 3x, 4x, 10x, infinityx slower to implement some kind of lvalue system? And wouldn't that system be necessary only in code that uses the mutation?<br> <br> <div class="moz-cite-prefix">On 06/09/2013 08:18 AM, Carl Eastlund wrote:<br> </div> <blockquote cite="mid:CAEOPtY3WjAFvJketQjW=Baz8A07iyk06ap4q4a5hc0rA5+Kf=A@mail.gmail.com" type="cite"> <div dir="ltr"> <div>Sean,<br> <br> </div> Not every Scheme uses an interpreter with an eval function as its primary method of execution, or even at all. Racket uses a bytecode interpreter and a JIT native-code compiler; the eval function simply triggers compilation to bytecode. These give a great deal more efficiency than running via eval, and supporting multiple modes of execution would be significantly more expensive. Evaluating to values by default, rather than to addresses, also gives the compiler a great deal of flexibility. It doesn't need to keep track of the addresses where it found things and refer to them there in case they are about to be mutated; once they have been "found" via evaluation, they can be copied to register and the original address can be forgotten, if that's most expedient. I'm not a compiler implementer myself, so I'm sure others can probably give more specific details. In the meantime, I hope this explanation is helpful.<br> <div class="gmail_extra"><br clear="all"> <div>Carl Eastlund</div> <br> <div class="gmail_quote">On Thu, Jun 6, 2013 at 4:12 PM, Sean Kanaley <span dir="ltr"><<a moz-do-not-send="true" href="mailto:skanaley@gmail.com" target="_blank">skanaley@gmail.com</a>></span> wrote:<br> <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> <div dir="ltr"> <div> <div> <div> <div> <div>Hello all,<br> <br> </div> I was curious why Scheme and now Racket does not inherently support a generic set!. I found an SRFI <a moz-do-not-send="true" href="http://srfi.schemers.org/srfi-17/srfi-17.html" target="_blank">http://srfi.schemers.org/srfi-17/srfi-17.html</a> that suggests a generic method solution requiring a lookup for the "real" setter (and so needing a special setter for every data type. What is the disadvantage of simply changing eval to take a "fetch?" parameter that decides whether to ultimately resolve addresses to values? Then set! is evaluated with this as #f and can operate on whatever that address is. I have implemented this in a toy interpreter with the bare minimum of forms plus vectors to test. The vector-ref type of function gets applied as usual to the vector and index arguments, except that if it's within a set! as the left argument where the fetch? is #f and the final fetching of the address given by vector-ref never happens.<br> <br> </div> Here's the critical pieces:<br> <br> <div>1. setting<br> </div> <div>"update" is what changes the store<br> </div> set! is of course a clause within eval<br> </div> <div>the last parameter to eval in the first line says don't fetch<br> </div> <div><br> [(set! addr-x x) (match-let* ([(v*s addr s) (eval addr-x e s #f)]<br> [(v*s v s) (eval x e s)])<br> (update addr v s))]<br> <br> <div>2. evaluating symbols (another clause)<br> </div> <div>the symbol only returns its address with fetching off<br> </div> <div><br> [(sym y) (let* ([addr (lookup y e)]<br> [val (if fetch? (fetch addr s) addr)])<br> (v*s val s))]<br> </div> <br> </div> 3. the "built-in" (part of environment) vector-ref called vec@<br> </div> "fetch?" will be false if (vec@ ...) is the first argument to set!<br> </div> "a" is the base address of the vector<br> <div> <div><br> <div> <div>(define (vec@-f e s fetch? v i)<br> </div> <div>...unimportant stuff...<br> </div> <div> (let ([val (if fetch? (fetch (+ a i) s) (+ a i))])<br> (v*s val s)))))<br> <br> </div> <div>So long as all built-in types have this conditional fetcher, then every user type built on top of them won't need a special setter. And since this would work just as well for inc! types funtions, from now on<br> <br> (vector-set! v i (add1 (vector-ref v i))<br> </div> <div>is<br> </div> <div>(inc! (vec@ v i))<br> </div> <div><br> </div> <div>I assume this has already been thought of and therefore discarded, but why?<br> </div> </div> </div> </div> </div> <br> ____________________<br> Racket Users list:<br> <a moz-do-not-send="true" href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a><br> <br> </blockquote> </div> <br> </div> </div> </blockquote> <br> </body> </html>