sicp

This is concerning chapter 3.5 from SICP, in which streams are being discussed. The idea is that: (cons-stream 1 (display 'hey)) Should not evaluate the second part of the cons-stream, so it should not print “hey”. This does happen, I get the following output: hey(1 . #< promise >) So my conclusion is that delay is not implemented as a special form? Or am I doing something wrong? I use the following implementation: (define (cons-stream a b) (cons a (delay b))) With delay being the default R5RS implementation. Is this a fault in the implementation, or am I not doing or understanding it right?

According to SICP section 1.2.6 , exercise 1.22: Most Lisp implementations include a primitive called runtime that returns an integer that specifies the amount of time the system has been running (measured, for example, in microseconds). I'm using DrScheme , where runtime doesn't seem to be available, so I'm looking for a good substitute. I found in the PLT-Scheme Reference that there is a current-milliseconds primitive. Does anyone know if there's a timer in Scheme with better resolution? current-milliseconds is a function that returns the current millisecond count from the system, but it

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: This is concerning chapter 3.5 from SICP, in which streams are being discussed. The idea is that: (cons-stream 1 (display 'hey)) Should not evaluate the second part of the cons-stream, so it should not print “hey”. This does happen, I get the following output: hey(1 . #< promise >) So my conclusion is that delay is not implemented as a special form? Or am I doing something wrong? I use the following implementation: (define (cons-stream a b) (cons a (delay b))) With delay being the default R5RS implementation. Is this a fault in the

As far as I know, eager evaluation/applicative order evaluates all arguments to a function before applying it, on the other hand, lazy evaluation/normal order evaluates the arguments only when needed. So, what are the differences between the pair of terms eager evaluation and applicative order , and lazy evaluation and normal order ? Thanks. Lazy evaluation evaluates a term at most once, while normal order would evaluate it as often as it appears. So for example if you have f(x) = x+x and you call it as f(g(42)) then g(42) is called once under lazy evaluation or applicative order, but twice