lisp

This is trivial implement of course, but I feel there is certainly something built in to Racket that does this. Am I correct in that intuition, and if so, what is function? Strangely, there isn't a built-in procedure in Racket for finding the 0-based index of an element in a list (the opposite procedure does exist, it's called list-ref ). However, it's not hard to implement efficiently: (define (index-of lst ele) (let loop ((lst lst) (idx 0)) (cond ((empty? lst) #f) ((equal? (first lst) ele) idx) (else (loop (rest lst) (add1 idx)))))) But there is a similar procedure in srfi/1 , it's called

Why do we have to use funcall to call higher order functions in Common Lisp? For example, why do we have to use: (defun foo (test-func args) (funcall test-func args)) instead of the simpler: (defun bar (test-func args) (test-func args)) Coming from a procedural background, I'm a bit surprised by that since the languages I'm more used to (e.g. Python, C#) don't need the distinction. In particular, on the source level at least, the C# compiler transforms it to something like func.invoke() . The only problem I see is that this would mean we couldn't call a global function test-func anymore

I'm bewildered by all the built-in Mathematica functions that purport to prevent evaluation in some way: Unevaluated , Defer , Hold , and over half a dozen of the form Hold* . The Mathematica documentation just explains each function in isolation without explaining why you would choose one or the other. Can anyone offer a coherent explanation of all these functions? The whole thing seems like a convoluted mess to me. Relating it all to Lisp macros might be a good place to start. Most of the Mathematica language is amazingly well-designed but it seems like Wolfram really painted himself into a

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

I've been working on solving Project Euler problems in Clojure to get better, and I've already run into prime number generation a couple of times. My problem is that it is just taking way too long. I was hoping someone could help me find an efficient way to do this in a Clojure-y way. When I fist did this, I brute-forced it. That was easy to do. But calculating 10001 prime numbers took 2 minutes this way on a Xeon 2.33GHz, too long for the rules, and too long in general. Here was the algorithm: (defn next-prime-slow "Find the next prime number, checking against our already existing list" (