Stream Operations

The query system uses a few stream operations that were not presented in chapter 3.

Stream-append-delayed and interleave-delayed are just like streamappend and interleave (section ), except that they take a delayed argument (like the integral procedure in section ). This postpones looping in some cases (see exercise ).

(define (stream-append-delayed s1 delayed-s2)
  (if (stream-null? s1)
      (force delayed-s2)
      (cons-stream
       (stream-car s1)
       (stream-append-delayed (stream-cdr s1) delayed-s2))))

(define (interleave-delayed s1 delayed-s2)
  (if (stream-null? s1)
      (force delayed-s2)
      (cons-stream
       (stream-car s1)
       (interleave-delayed (force delayed-s2)
                           (delay (stream-cdr s1))))))

Stream-flatmap, which is used throughout the query evaluator to map a procedure over a stream of frames and combine the resulting streams of frames, is the stream analog of the flatmap procedure introduced for ordinary lists in section . Unlike ordinary flatmap, however, we accumulate the streams with an interleaving process, rather than simply appending them (see exercises  and  ).

(define (stream-flatmap proc s)
  (flatten-stream (stream-map proc s)))

(define (flatten-stream stream)
  (if (stream-null? stream)
      the-empty-stream
      (interleave-delayed
       (stream-car stream)
       (delay (flatten-stream (stream-cdr stream))))))

The evaluator also uses the following simple procedure to generate a stream consisting of a single element:

(define (singleton-stream x)
  (cons-stream x the-empty-stream))