【SICP练习】142 练习3.77

练习3-77

原文

Exercise 3.77. The integral procedure used above was analogous to the “implicit” definition of the infinite stream of integers in section 3.5.2. Alternatively, we can give a definition of integral that is more like integers-starting-from (also in section 3.5.2):

(define (integral integrand initial-value dt)
   (cons-stream initial-value            
                (if (stream-null? integrand)         
                    the-empty-stream           
                    (integral (stream-cdr integrand)                             
                              (+ (* dt (stream-car integrand))                                
                                 initial-value)                             
                              dt))))

When used in systems with loops, this procedure has the same problem as does our original version of integral. Modify the procedure so that it expects the integrand as a delayed argument and hence can be used in the solve procedure shown above.

代码

 (define (integral delayed-integrand initial-value dt) 
         (cons-stream initial-value 
                 (let ((integrand (force delayed-integrand))) 
                         (if (stream-null? integrand) 
                                 the-empty-stream 
                                 (integral (delay (stream-cdr integrand)) 
                                           (+ (* dt (stream-car integrand)) 
                                                      initial-value) 
                                                dt))))) 

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