[racket] Problem Stream with alternating values

On 19/05/2013 21:22, Jos Koot wrote:
> Your function H1 is not lazy enough.
> 
> Closely following the instructions of the exercise I immediately come with:
> 
> (define (dan-then-dog)
>  (cons "dan.jpg"
>   (lambda () ???)))
> 
> Can you fill in the question-marks?
> 
> Can easily be done with stream-cons too:

Right. I was stuck in the 'C' paradigm of passing everything in the
parameter list, wheras, I think, the stream should only ever be referred
to as the return value from a function call.

I like my original approach, using b0 as a state variable, because it
can be extended to produce a stream of any repeating set of values. The
more concise double stream-cons is not so general.  Fixing the syntax of
my original code gives me:-

(define (dan-then-dog)
  (define (H1 b0)
    (cond
      [(= b0 1) (stream-cons dan (H1 0))]
      [else     (stream-cons dog (H1 1))]
      )
    )
  (H1 1))


> 
> (define dan-then-dog-str (stream-cons "dan.jpg" ...))
> 
> but this is not according to the exercise, I think. In the exersize a stream
> is defined as a thunk.

I saw that. The requirement for a stream of alternating values was
clearly stated, but then it confusingly talked about the internal
structure of a stream.  Surely if the stream is constructed by stream
primitives then it will all be thunked.  Or do you read the problem as
requiring that the stream itself be separately thunked?

Patrick

> 
> Jos
> 
>> -----Original Message-----
>> From: users-bounces at racket-lang.org 
>> [mailto:users-bounces at racket-lang.org] On Behalf Of Patrick Sweetman
>> Sent: jueves, 16 de mayo de 2013 14:31
>> To: users at racket-lang.org
>> Subject: [racket] Problem Stream with alternating values
>>
>> I'm doing this problem:
>>
>> 6. Write a stream dan-then-dog, where the elements of the stream
>> alternate between the strings "dan.jpg" and "dog.jpg" (starting with
>> "dan.jpg"). More specically, dan-then-dog should be a thunk that when
>> called produces a pair of "dan.jpg" and a thunk that when called
>> produces a pair of "dog.jpg" and a thunk that when called... 
>> etc. Sample
>> solution: 4 lines.
>>
>> and I cannot for the life of me understand why the following code does
>> not produce a properly thunked stream. It evaluates the 
>> entries until it
>> runs out of memory
>>
>> (define dan "dan.jpg")
>> (define dog "dog.jpg")
>>
>> (define (dan-then-dog)
>>   (define (H1 st0 b0)
>>     (cond
>>       [(= b0 1) (H1 (stream-cons dan st0) 0)]
>>       [else     (H1 (stream-cons dog st0) 1)]
>>       )
>>     )
>>   (H1 empty-stream 1))
>>
>> Can somebody please explain why my understanding of Racket streams is
>> faulty?
>>
>> Thanks,
>> Patrick Sweetman
>> ____________________
>>   Racket Users list:
>>   http://lists.racket-lang.org/users
> 
>