racket

In Haskell, like in many other functional languages, the function foldl is defined such that, for example, foldl (-) 0 [1,2,3,4] = -10 . This is OK, because foldl (-) 0 [1, 2,3,4] is, by definition, ((((0 - 1) - 2) - 3) - 4) . But, in Racket, (foldl - 0 '(1 2 3 4)) is 2, because Racket "intelligently" calculates like this: (4 - (3 - (2 - (1 - 0)))) , which indeed is 2. Of course, if we define auxiliary function flip, like this: (define (flip bin-fn) (lambda (x y) (bin-fn y x))) then we could in Racket achieve the same behavior as in Haskell: instead of (foldl - 0 '(1 2 3 4)) we can write:

For the love of the almighty I have yet to understand the purpose of the symbol 'iamasymbol . I understand numbers, booleans, strings... variables. But symbols are just too much for my little imperative-thinking mind to take. What exactly do I use them for? How are they supposed to be used in a program? My grasp of this concept is just fail. In Scheme and Racket, a symbol is like an immutable string that happens to be interned so that symbols can be compared with eq? (fast, essentially pointer comparison). Symbols and strings are separate data types. One use for symbols is lightweight

Racket is a descendant of Scheme. How is Racket different than R6RS? What did it add, or take away, or is just different? I understand that Racket is more than a language, it's a platform for languages. But I'm referring to the main Racket dialect. Zorf Racket is ultimately based on R5RS, and not R6RS and not a strict superset of either. I don't think it can be called 'Scheme' because it's not backwards compatible with any Scheme standard. Most implementations offer extensions, but are otherwise backwards compatible, of course, the compiler that comes with Racket can also run in R5RS or R6RS

so here is the code: (define (time-prime-test n) (newline) (display n) (start-prime-test n (runtime))) (define (start-prime-test n start-time) (if (prime? n) (report-prime (- (runtime) start-time)))) (define (report-prime elapsed-time) (display " *** ") (display elapsed-time)) (define (search-for-primes n m) (if (< n m) ((time-prime-test n) (search-for-primes (+ n 1) m)) (display " calculating stopped. "))) (search-for-primes 100000 100020) and i got this error after "calculating stopped." has been displayed. like below: 100017 100018 100019 * 54 calculating stopped. . . application: not a

I'm using the beginning language with list abbreviations for DrRacket and want to make a powerset recursively but cannot figure out how to do it. I currently have this much (define (powerset aL) (cond [(empty? aL) (list)] any help would be good. What's in a powerset? A set's subsets! An empty set is any set's subset, so powerset of empty set's not empty. Its (only) element it is an empty set: (define (powerset aL) (cond [(empty? aL) (list empty)] [else As for non-empty sets, there is a choice , for each set's element, whether to be or not to be included in subset which is a member of a