[racket] racket newbie question on quoting and the built in procedure-arity function

Thanks Matthias,

If I use your definition:

(define (select-random-safe-function)
  (object-name
   (list-ref safe-function-set (random (length safe-function-set)))))

 then,

>(procedure-arity (select-random-safe-function))
error:procedure-arity: contract violation
  expected: procedure?
  given: '$+


I believe I need two things:

1. (select-random-safe-function) should return an unquoted procedure,
similar to

(define (select-random-safe-function) ;without object-name
   (list-ref safe-function-set (random (length safe-function-set))))
>(select-random-safe-function)
#<procedure:$+>

preferably without #<procedure: $+ >


*so my problem is that I want the output to display *
*$+*
*instead of*
*#<procedure:$+>*

2. The reason the procedure needs to be unquoted is because
(procedure-arity (select-random-safe-function)) will produce an error if
the function is quoted.




Later on I use both (procedure-arity (select-random-safe-function)) and
(select-random-safe-function) in a main function:

;;Generate Initial Population
;population size is a positive integer and the auxiliary-function,
generate-chromsome generates a tree using functions from safe-function-set
and terms (numbers and symbols) from a term-set

(define (generate-initial-population population-size)
  (cond [(zero? population-size) empty]
           [else (cons (generate-chromosome initial-chromosome-depth)
;initial-chromosome-depth is 3
                            (generate-initial-population (sub1
population-size)))]))

> (generate-initial-population 1)
'((#<procedure:$/> (#<procedure:$-> x .234) (#<procedure:$+> .576 x)))

this gets me my desired result, but is hard for me to read especially as
the depth and population size grows (typically I will be working with
populations ranging in the hundreds or thousands and each chromosome/tree's
depth could easily exceed 10) so
* it would be more convenient here if my solution displays *


*'(($/ ($- x .234) ($+ .576 x)))  *

*instead of*

*'((#<procedure:$/> (#<procedure:$-> x **.234**) (#<procedure:$+> **.576*
* x)))*

*and I am not sure how I would go about doing this?*

On Sat, Jan 4, 2014 at 8:45 AM, Matthias Felleisen <matthias at ccs.neu.edu>wrote:

>
> If you really just want the name, use object-name:
>
> (define (select-random-safe-function)
>   (object-name
>    (list-ref safe-function-set (random (length safe-function-set)))))
>
> Welcome to DrRacket, version 6.0.0.1--2013-12-29(bbb0c5f6/d) [3m].
> Language: racket.
> > (select-random-safe-function)
> '$*
>
>
>
>
> On Jan 4, 2014, at 2:48 AM, Rian Shams wrote:
>
> If I do this I get the results I need for (procedure-arity
> (select-random-safe-function)), but when I call
> (safe-function-set) or (select-random-safe-function) or any other function
> that returns a safe function I get for example:
>
> >(safe-function-set)
> '(#<procedure:$+> #<procedure:$-> #<procedure:$*> #<procedure:$/>)
>
> >(select-random-safe-function)
> #<procedure:$->
>
> whereas when it was defined using quote as opposed to list I get:
>
> >(safe-function-set)
> '($+ $- $* $/)
>
> >(select-random-safe-function)
> '$-
>
> *How would I get rid of this #<procedure:$-> part for readability, keeping
> only the $- part which is the actual function I defined, while still
> keeping it a function/procedure that gives me the function arity
> using (procedure-arity (select-random-safe-function))?*
>
> Best,
>
>
> On Fri, Jan 3, 2014 at 5:31 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu>wrote:
>
>> Try replacing `safe-function-set` with:
>>
>> (define safe-function-set (list $+ $- $* $/))
>>
>> Sam
>>
>> On Fri, Jan 3, 2014 at 5:28 PM, Rian Shams <rian.shams at gmail.com> wrote:
>> > Hello,
>> >
>> > I am working with functions that I have defined to only take 1 or 2
>> operands
>> > (called safe-functions) for the implementation of a genetic program.
>> >
>> > (define ($+ augend addend) ;operation is addition
>> >   (+ augend addend)) ; the result is the sum
>> > (define ($- minuend subtrahend) ;operation is subtraction
>> >   (- minuend subtrahend)) ;result is difference
>> > (define ($* multiplicand multiplier) ;operation is multiplication
>> >   (* multiplicand multiplier)) ;result is product
>> > (define ($/ dividend divisor) ;operation is division
>> >   (/ dividend divisor)) ;result is quotient
>> >
>> > (define (infinity? x) (or (eq? x +Inf.0) (eq? x -Inf.0)))
>> > (define ($sin x) (if (infinity? x) (* (sgn x) +Inf.0) (sin x)))
>> > (define ($cos x) (if (infinity? x) (* (sgn x) +Inf.0) (cos x)))
>> >
>> > (define safe-function-set
>> >   '($+
>> >     $-
>> >     $*
>> >     $/
>> >     ;$sin
>> >     ;$cos))
>> >
>> > (define (select-random-safe-function)
>> >   (list-ref safe-function-set (random (length safe-function-set))))
>> >
>> > I would like to use procedure-arity (or a similar function) to
>> determine the
>> > arity of a randomly selected safe function but I get this error:
>> >
>> >>(procedure-arity (select-random-safe-function))
>> > error: procedure-arity: contract violation
>> >   expected: procedure?
>> >   given: '$+
>> >
>> > I think the problem is that the safe-functions are passed to
>> procedure-arity
>> > quoted. Is there a way I can unquote the functions, or adjust
>> > procedure-arity to make (procedure-arity (select-random-safe-function))
>> > work?
>> >
>> > Thanks,
>> > --
>> > Rian Shams
>> >
>> > ____________________
>> >   Racket Users list:
>> >   http://lists.racket-lang.org/users
>> >
>>
>
>
>
> --
> Rian Shams
> ____________________
>  Racket Users list:
>  http://lists.racket-lang.org/users
>
>
>


-- 
Rian Shams
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