20-CS-4003-001 |
Organization of Programming Languages |
Fall 2017 |
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Streams |

**Topological Sort**

;; See extensive notes on newObject to the right (define newObject (lambda (idx id deps) (lambda () (letrec ((visitall (lambda (s d) (if (null? d) (begin (vector-set! vec idx '()) (cons id (lambda () s)) ) (if (null? (vector-ref vec (car d))) (visitall s (cdr d)) (let* ((p (vector-ref vec (car d))) (x ((cdr p)))) (visitall (splice$ x s) (cdr d)))))))) (visitall '() deps))))) ;; splice stream X$ onto stream S$ (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$)) S$))))))) ;; take the first n tokens of a stream or list S (define take (lambda (n S) (if (or (null? S) (= n 0)) '() (if (and (not (null? (cdr S))) (procedure? (cdr S))) (cons (car S) (take (- n 1) ((cdr S)))) (cons (car S) (take (- n 1) (cdr S))))))) ;; An example: ;; format: '((<identity> (list-of-dependencies)) ...) (define input '((Cincinnati (1 5 7 9)) (Cleveland (4 5 8)) (Columbus (0 1 6 10 12 18)) (Chicago (5 9 12 13 14)) (Calumet (9 11 12)) (Corman ()) (Denver (3 9 10 11)) (Dallas (4 9 12 13)) (Durango (10 11 20 21)) (Durea (25)) (Detroit (25)) (Edwards ()) (Echemonte (8 9 10 11)) (Eagle_Creek (20 21 22)) (Erasmus (15 19 21)) (Fullman (10 11 19 24)) (Fortnight (19 20)) (Fallow (15 20 21)) (Fables (19 20)) (Finese (22)) (Gordon ()) (Gallop (10 20)) (Gormon (11 21 25)) (Harmon (12 22 24)) (Halpern (22)) (Hornwich ()))) ;; this vector stores the newObject procedures ;; for each object (define vec (make-vector (length input) '())) ;; create a vector of newObject procedures (define populate-vector (lambda (inp i) (if (null? inp) '() (let* ((id (caar inp)) (deps (cadar inp)) (no (newObject i id deps))) (vector-set! vec i (cons id no)) (populate-vector (cdr inp) (+ i 1)))))) ;; populate the vector with given input (populate-vector input 0) ;; See notes to the right (define solve (lambda (i len) (if (= i 0) '() (if (null? (vector-ref vec (- len i))) (solve (- i 1) len) (let* ((p (vector-ref vec (- len i))) (lst (take len ((cdr p))))) (append (solve (- i 1) len) lst)))))) ;; this makes it easier to run solve (define solveit (lambda (inp) (solve (length inp) (length inp)))) |
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The code to the left implements a solution to the problem of
topologically sorting a partial order. This solution uses streams.
The following notes pertain to the procedures defined in the code.
Procedure (cons id (lambda () s))To prevent recomputation of the stream, in case object idx is a
dependency of more than one object, the stream is closed to further hits by
setting the idx position of the vector to '(). This is
done by the line
(vector-set! vec idx '())If the vector had been maintained as a list this side effect would not be necessary but then the vector would have to be disassembled to replace a list member and this would be expensive, at least theoretically.
visitall parameters: s is the stream of all dependency outputs, d is a list vector indices of dependenciesObserve, even though vec is not yet defined in this file, loading proceeds normally because all references to vec are behind a (lamba
()..., also called a thunk. This is important because the definition
of the vector cannot be known until an input is given.
Procedure |
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