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Exercise 4.3
Stream appending and stream merging


We have seen a number of useful stream funtions, as counterparts to well-known list functions.

First, consider if/how to program a stream-append function, as a counterpart to append. Here is a possible recursive append function for lists:

  (define (my-append lst1 lst2)
    (cond ((null? lst1) lst2)
          (else (cons (car lst1) (my-append (cdr lst1) lst2)))))

How will your stream-append function work on infinite streams, such as nat-nums or fibs?

Now program a similar function, stream-merge, that merges two streams by simple alternation.

How will stream-merge function work on initinite streams? And on finite streams?

Can you use stream-merge to produce as stream of all integers (in some arbitrary order), based on two other streams with fixed starting points?

Here is all you need to get started:

(define-syntax cons-stream
  (syntax-rules ()
    ((cons-stream x y)
     (cons x (delay y)))))

(define head car)

(define (tail stream) (force (cdr stream)))

(define empty-stream? null?)

(define the-empty-stream '())

(define (stream-section n stream)
  (cond ((= n 0) '())
        (else (cons (head stream)
              (stream-section 
                (- n 1)
               (tail stream))))))


Solution