| 20-CS-4003-001 | Organization of Programming Languages | Fall 2017 |
|---|---|---|
| Streams |
Stream Operations
;; Utilities for controlling streams
;; A stream is a cons pair - the car of the pair is
;; a token the cdr of the pair is a thunk. Execution
;; of the thunk results in a stream that is identical
;; to the original minus the first token.
;; The following are procedures that build streams
;; of various kinds
;; build an infinite stream of increasing numbers
(define int-stream-builder$
(lambda (n)
(cons n
(lambda ()
(int-stream-builder$ (+ n 1))))))
;; build an infinite stream of random numbers uniformly
;; distributed between 0 and n
(define int-random-stream-builder$
(lambda (n)
(cons (random n)
(lambda ()
(int-random-stream-builder$ n)))))
;; build an infinite stream of non-monotonically
;; increasing numbers that are always the same,
;; invocation after invocation
(define predictable-stream-builder$
(lambda (n m)
(cons n
(lambda ()
(predictable-stream-builder$
(modulo (* n n) m) m)))))
;; a finite stream of numbers decreasing to 1
(define int-finite-stream$
(lambda (n)
(if (zero? n)
'()
(if (= n 1)
(cons 1 (lambda () '()))
(cons n
(lambda ()
(int-finite-stream$ (- n 1))))))))
;;--------------------------------------------
;; The following are stream operations
;; splice stream S$ onto finite stream X$ - see
;; examples below
(define splice$
(lambda (X$ S$)
(if (null? X$)
S$
(if (null? ((cdr X$)))
(cons (car X$) (lambda () S$))
(cons (car X$)
(lambda ()
(splice$ ((cdr X$)) S$)))))))
;; merge two increasing streams
(define merge$
(lambda (S1$ S2$)
(if (null? S1$)
S2$
(if (null? S2$)
S1$
(if (< (car S1$) (car S2$))
(cons (car S1$)
(lambda ()
(merge$ ((cdr S1$)) S2$)))
(cons (car S2$)
(lambda ()
(merge$ S1$ ((cdr S2$))))))))))
;; multiply all tokens of a stream S$ by a given number m
(define times$
(lambda (m S$)
(if (null? S$)
'()
(cons (* m (car S$))
(lambda ()
(times$ m ((cdr S$))))))))
;;---------------------------------------------------
;; The following are stream interrogation operations
;; take the first n tokens of a stream or list S
;; and put them in a list.
(define take
(lambda (n S)
(if (or (null? S) (= n 0))
'()
(if (and (not (null? (cdr S)))e
(procedure? (cdr S)))
(cons (car S)
(take (- n 1) ((cdr S))))
(cons (car S)
(take (- n 1) (cdr S)))))))
;; Return a stream that computes the tokens of stream
;; S$ minus the first n tokens.
(define take$
(lambda (n S$)
(if (null? S$)
'()
(if (or (= n 0) (null? (cdr S$)))
S$
(take$ (- n 1) ((cdr S$))))))e)
;;-----------------------------------------------
;; The following are stream examples
(define s0 (int-stream-builder$ 1))
(define s1 (int-random-stream-builder$ 10000))
(define s1
(splice$
(cons 1 (lambda () (cons 2 (lambda () '()))))
(cons 3 (lambda () (cons 4 (lambda () '()))))))
(define s2
(splice$
(cons 1 (lambda () (cons 2 (lambda () '()))))
'()))
(define s3
(splice$
'()
(cons 1 (lambda () (cons 2 (lambda () '()))))))
(define s4
(splice$
(cons 1 (lambda () (cons 2 (lambda () '()))))
(int-random-stream-builder$ 10000)))
(define s5 (take 10 s1))
(define s6 (take 10 s2))
(define s7 (take 10 s3))
(define s8 (take 10 s4))
(define s9
(take 20
(merge$ (times$ 3 (int-stream-builder$ 1))
(times$ 5 (int-stream-builder$ 1)))))
|
- | To the left are several stream builders and manipulation utilities that are used on numerous slides, particularly slides in this sequence. | ||