[racket] Code Error Creating List

Todd

Thanks, you are definitely making me think, but I don't see what would 
be wrong with what I am trying. The first thing I tried was to remove 
free-vars and just call free-vars-list directly and change the case of

(var-exp (id) id)

to
(var-exp (id) (list id)) ;; in this example id == x

shouldn't it return '(x)? It always returns (x). If I try (list 'id) I obviously get '(id). Any clarification on why I would get (x) from (list id)?

Thanks for all your help!
Todd D



On 5/17/2011 9:20 PM, Todd O'Bryan wrote:
> I apologize in advance for not actually reading through your code
> beyond the point where it began using constructs that I'm not familiar
> with, but I will give some advice that may or may not be helpful.
>
> You're trying to return '(x). What you're actually returning is
> (x)--an application of the value x with no arguments. It looks to me
> like you're missing a level of quote-ness somewhere, or you're
> accidentally evaluating the return value instead of just returning it.
>
> Todd (but not the same one who asked the question)
>
> On Tue, May 17, 2011 at 8:03 PM, Todd Dobmeyer<dobmeyer.2 at wright.edu>  wrote:
>> I am taking a class at Wright State University and we are working on problem
>> 3.27 from the EOPL 2nd Edition. This problem has us modifying the closure
>> procedure to only bind the free variables in our environment instead of
>> storing all the variables. I am trying to write some code that will create a
>> list of all the variables in my expression. This will then be passed to a
>> difference procedure (that has been provided) to determine which variables
>> are free. Unfortunately, every time I run my procedure, I receive the error:
>> "procedure application: expected procedure, given: x (no arguments)" What I
>> should be returning is the list '(x) as "x" is the only variable in my
>> simple expression. The string I am running is as follows:
>>
>> (define str2 "let x = 15 y = 27
>>               in let f = proc () x
>>                  in let x = 8
>>                     in (f)")
>>
>> I then run the command:
>>
>> (run str2)
>>
>> After it works through its beginning work, it finally hits the proc-exp
>> where it calls closure. My closure function calls me free-vars procedure but
>> it apparently does not return a list like I think it should. I have tried
>> numerous ways and always get this same error. Any ideas on where I am going
>> wrong would be greatly appreciated. I'm sure there are errors inside my
>> logic, but if anybody can help me figure out why I am not returning a list
>> out of (free-vars) that would be great! My code looks like:
>>
>> (define free-vars
>>   (lambda (body)
>>     (list (free-vars-list body))
>>   )
>> )
>>
>> (define free-vars-list
>>   (lambda (body)
>>     (
>>       (if (list? body) (append (free-vars-list (car body)) (free-vars-list
>> (cdr body)))
>>                        (cases expression body
>>                           ;; When we have a var-exp, return the variable
>>                           (var-exp (id) id)
>>                           (primapp-exp (prim rands) (append(free-vars-list
>> (car rands)) (free-vars-list (cdr rands))))
>>                           (if-exp (test-exp true-exp false-exp)
>>                                   (append (free-vars-list test-exp)
>> (free-vars-list true-exp) (free-vars-list false-exp)))
>>                           (begin-exp (exp1 exps) (append (free-vars-list
>> exp1) (free-vars-list exps)))
>>                           (let-exp (ids rands body) (append (free-vars-list
>> body)))
>>                           (proc-exp (ids body) (append (free-vars-list
>> body)))
>>                           (app-exp (rator rands) (append (free-vars-list
>> rator) (free-vars-list rands)))
>>                           (else '())
>>                        )
>>       )
>>     )
>>   )
>> )
>>
>> (define closure
>>   (lambda (ids body env)
>>     (let ((todd (free-vars body)))
>>     (let ((freevars (difference todd ids)))
>>       (let ((saved-env
>>                    (extend-env
>>                          freevars
>>                          (map (lambda (v) (apply-env env v)) freevars)
>>                          (empty-env))
>>            ))
>>          (lambda (args)
>>                   (eval-expression body (extend-env ids args env)))
>>       )
>>     )
>>   ))
>> )
>>
>> My call stack looks like:
>>
>> procedure application: expected procedure, given: x (no arguments)
>>
>> C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 255:4
>>         (if (list? body) (append (free-vars-list (car body)) (free-vars-list
>> (cdr body)))
>>                        (cases expression body
>>                           ;; When we have a var-exp, return the variable
>>                           (var-exp (id) id)
>>                           (primapp-exp (prim rands) (append(free-vars-list
>> (car rands)) (free-vars-list (cdr rands))))
>>                           (if-exp (test-exp true-exp false-exp)
>>                                   (append (free-vars-list test-exp)
>> (free-vars-list true-exp) (free-vars-list false-exp)))
>>                           (begin-exp (exp1 exps) (append (free-vars-list
>> exp1) (free-vars-list exps)))
>>                           (let-exp (ids rands body) (append (free-vars-list
>> body)))
>>                           (proc-exp (ids body) (append (free-vars-list
>> body)))
>>                           (app-exp (rator rands) (append (free-vars-list
>> rator) (free-vars-list rands)))
>>                           (else '())
>>                        )
>>       )
>>     )
>>   )
>> )
>>
>> (define closure
>>   (lambda (ids body env)
>>
>> C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 247:4
>>       (list (free-vars-list body))
>>   )
>> )
>>
>> (define free-vars-list
>>   (lambda (body)
>>
>> C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 276:4
>>       (let ((freevars (difference todd ids)))
>>       (let ((saved-env
>>                    (extend-env
>>                          freevars
>>                          (map (lambda (v) (apply-env env v)) freevars)
>>                          (empty-env))
>>            ))
>>          (lambda (args)
>>                   (eval-expression body (extend-env ids args env)))
>>       )
>>     )
>>   ))
>> )
>>
>> (run str2)
>>
>> C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 138:4
>>       (map (lambda (x) (eval-rand x env)) rands)))
>>
>> C:\Users\Todd\Documents\CS784\Assign2\3-5-mod.scm: 120:8
>>           (let ((args (eval-rands rands env)))
>>           (eval-expression body (extend-env ids args env))))
>>
>> Thanks for all your help!
>> Todd
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