[racket] set!: eval-ing to lvalues

From: Sean Kanaley (skanaley at gmail.com)
Date: Sun Jun 9 12:28:51 EDT 2013

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!

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?

On 06/09/2013 08:18 AM, Carl Eastlund wrote:
> Sean,
> 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.
> Carl Eastlund
> On Thu, Jun 6, 2013 at 4:12 PM, Sean Kanaley <skanaley at gmail.com 
> <mailto:skanaley at gmail.com>> wrote:
>     Hello all,
>     I was curious why Scheme and now Racket does not inherently
>     support a generic set!.  I found an SRFI
>     http://srfi.schemers.org/srfi-17/srfi-17.html 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.
>     Here's the critical pieces:
>     1. setting
>     "update" is what changes the store
>     set! is of course a clause within eval
>     the last parameter to eval in the first line says don't fetch
>     [(set! addr-x x) (match-let* ([(v*s addr s) (eval addr-x e s #f)]
>                                       [(v*s v s) (eval x e s)])
>                             (update addr v s))]
>     2. evaluating symbols (another clause)
>     the symbol only returns its address with fetching off
>     [(sym y) (let* ([addr (lookup y e)]
>                         [val (if fetch? (fetch addr s) addr)])
>                    (v*s val s))]
>     3. the "built-in" (part of environment) vector-ref called vec@
>     "fetch?" will be false if (vec@ ...) is the first argument to set!
>     "a" is the base address of the vector
>     (define (vec at -f e s fetch? v i)
>     ...unimportant stuff...
>             (let ([val (if fetch? (fetch (+ a i) s) (+ a i))])
>               (v*s val s)))))
>     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
>     (vector-set! v i (add1 (vector-ref v i))
>     is
>     (inc! (vec@ v i))
>     I assume this has already been thought of and therefore discarded,
>     but why?
>     ____________________
>       Racket Users list:
>     http://lists.racket-lang.org/users

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