42 lines
1.1 KiB
Scheme
42 lines
1.1 KiB
Scheme
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;;; scons.ss
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;;; a stream-construction facility
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;;; The scons special form performs a cons, suspending the cdr field
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;;; by enclosing it in a procedure of no arguments. scdr tests to see
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;;; if the cdr is a procedure, and if so, invokes it. scar is provided
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;;; for symmetry; it is just car.
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;;; The function stream-ref is simply list-ref defined in terms of
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;;; scdr and scar.
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;;; factlist and fiblist are two infinite streams.
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;;; Try (stream-ref factlist 10) or (stream-ref fiblist 20).
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;;; scons could easily suspend the car field as well. This would
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;;; implement the lazy cons of Friedman & Wise.
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(define-syntax scons
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(syntax-rules ()
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((_ car cdr) (cons car (lambda () cdr)))))
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(define scar car)
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(define scdr
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(lambda (x)
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(when (procedure? (cdr x)) (set-cdr! x ((cdr x))))
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(cdr x)))
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(define stream-ref
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(lambda (x n)
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(if (zero? n)
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(scar x)
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(stream-ref (scdr x) (1- n)))))
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(define factlist
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(let fact ([a 1] [n 1])
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(scons a (fact (* a n) (1+ n)))))
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(define fiblist
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(let fib ([fib-2 0] [fib-1 1])
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(scons fib-1 (fib fib-1 (+ fib-2 fib-1)))))
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