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Hello, I just stumbled upon this blog (got linked to it by a friend who’s studying functional programming using Racket). I’m enjoying the blog so far. ^^ Just thought I’d mention that while “zip” isn’t completely built-in to Racket, the built-in map function can perform zip easily enough since it can operate on multiple lists at once. (map (lambda (a b) (list a b)) (list 1 2 3 4 5) (list ‘a ‘b ‘c ‘d ‘e) ) - ( (1 ‘a) (2 ‘b) (3 ‘c) (4 ‘d) (5 ‘e) ) The function takes n arguments, where n in the number of lists.

All lists must be the same length, because the first element from each list is used as the arguments to f. (Similarly, foldl can perform the same way, taking a function that takes n+1 arguments for n lists.) Anyway, I thought I’d toss that out there, just in case it comes in handy. You’re very right! Racket’s version of map can do a lot of great things. It turns out that with a clever use of the apply function, we can implement zip for an arbitrary number of lists: (define (zip list.

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Lists) (apply map (cons (lambda (x. Xs) (cons x xs)) (cons list lists)))) (zip '(1 2 3) '(4 5 6)) '((1 4) (2 5) (3 6)) (zip '(1 2 3) '(3 4 5) '(4 5 6)) '((1 3 4) (2 4 5) (3 5 6)) The apply function takes as its first argument the apply-function which you want to apply, and as its second argument it takes a list of arguments to the apply-function. For instance: (apply + ‘(1 2 3 4)) is transformed into (+ 1 2 3 4). The new notation “list. Lists” is the way to support variable-length argument lists. Here we force there to be at least one argument (the “list”) and 0 or more remaining arguments (the “lists”) which are stored internally to the function as a list.

Then when we apply it to map, we bundle up the mapping function into the list, and ensure the mapping function can take a variable number of arguments as well. Pretty neat, huh?

Here the λ character has special binding in the Dr. Racket programming. 17 thoughts on “ A Taste of Racket. I was just providing examples of programs. Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family. Feedback is appreciated! You can reach me at @th3rac25 or th3rac25 [at.

Features

J2kun, you could simplify your “zip” definition to simply: (define (zip. Args) (apply map list args)) -we can just package all arguments into a list, instead of treating the first argument specially -apply takes as many arguments as you like – (apply fn a b cs) is equivalent to (apply fn (cons a (cons b cs))) Even better, we would simply write: (define zip (curry map list)) -((curry fn arg1 arg2) arg3 arg4) is equivalent to (fn arg1 arg2 arg3 arg4), so (zip a b c) = ((curry map list) a b c) = (map list a b c), which is exactly what we want! I love Racket!

An excellent book that goes well with HtDP is Discrete Mathematics and Functional Programming which basically teaches you SML through math. Afterwards the reader can slide right into the lecture notes for 15-150 at Carnegie Mellon U and their parallel/functional algorithms course 15-210 (which includes a free book too). Another good sequel to HtDP is PAPL by the same authors using a ‘functional’ Python dialect called Pyret they wrote using Racket. I love how you can finish both HtDP/PAPL and jump right into writing large programs with no fear.