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

Perhaps you want something like this: 

Welcome to Racket v6.0.0.1.
> (current-print (lambda (x) (if (procedure? x) (printf "~a" (object-name x)) (print x)) (newline)))
#<void>
> +
+
> 10
10




On Jan 4, 2014, at 6:40 PM, Rian Shams wrote:

> Sorry for my lack of clarity.
> 
> My question is about the output that is displayed from calling the function. For example, evaluating just
> 
> >$+
> #<procedure:$+>
> 
> The result I am looking for is 
> 
> >$+
> $+
> 
> as opposed to 
> #<procedure:$+>
> 
> I am not sure whether this is just a formatting or renaming problem, or if it is possible to change what the interpreter displays in Dr.Racket? 
> 
> I am fairly new to Racket and programming in general and so I really appreciate your responses and patience.
> 
> 
> On Sat, Jan 4, 2014 at 5:49 PM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
> 
> Yes, you need two functions: one that gives you the name of a procedure and one that picks a procedure. I was merely hinting at how to do the former. -- Matthias
> 
> 
> 
> On Jan 4, 2014, at 4:57 PM, Rian Shams wrote:
> 
>> 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
> 
> 
> 
> 
> -- 
> Rian Shams

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