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

Similarly, you can represent safe functions as applicable structures (so
they can behave like functions) with a custom printing method.

    #lang racket

    (struct safe-function (f name)
            #:property prop:procedure 0 ; first field is the function
            #:methods gen:custom-write
            [(define (write-proc safe-function port mode)
               (display (safe-function-name safe-function) port))])

    (define $+ (safe-function + '$+))

    ($+ 1 2) ; => 3
    $+ ; prints "$+"


Vincent



At Sat, 4 Jan 2014 18:52:55 -0500,
Matthias Felleisen wrote:
> 
> [1  <multipart/alternative (7bit)>]
> [1.1  <text/plain; iso-8859-1 (quoted-printable)>]
> 
> 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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> ____________________
>   Racket Users list:
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