On: CHIP-8 in Common Lisp: Graphics

[stevelosh]

If you've never done Common Lisp before, it's better to start by looking at `loop`, which is a built-in macro that provides a bunch of different ways to loop/iterate: http://www.gigamonkeys.com/book/loop-for-black-belts.html

iterate is a Common Lisp library that aims to be a replacement for loop, being cleaner (subjectively) and portably extensible: https://common-lisp.net/project/iterate/

`repeat` is an iterate clause that tells iterate to perform N iterations of the loop:

    (iterate (repeat 3) (print 'beep))
    BEEP
    BEEP
    BEEP

As for this particular `for`, it's an iterate clause that handles numeric iteration (among other things). So:

    (iterate (for x :from 0)
             (for y :from 10 :downto 7)
             (print (list x y)))

Would give you:

    (0 10)
    (1 9)
    (2 8)
    (3 7)

iterate will terminate the loop the first time any one of its clauses runs out of values/iterations. So in the post I had something like

    (iterate (repeat N)
             (for x :from start)
             ...)

You could write this as

    (iterate (for x :from start :below (+ start N))
             ...)

But manually calculating the ending value seems ugly to me. I know I want the loop to happen N times so I just say `(repeat N)` and let the computer figure out when it's time to be done.

[SixSigma → stevelosh]

I cant remember the original or who I'm almost quoting but it goes something like this :

> `loop` is so complex, LISP programmers invented functional style so they didn't have to learn how to use it