[racket] for/hash and for/list: adding nothing, adding more than one thing

From: Tim Nelson (tbnelson at gmail.com)
Date: Thu Mar 21 18:55:35 EDT 2013

Dear All,

I've recently started hacking Racket again, and am loving the for/*
Awesome stuff. However, I have two questions about for/hash.

(1) How can I *not* add a hash entry for a given iteration?

When I use for/list, can I abuse the nature of empty and append* (I cringe
as I write this):

(append* (for/list ([i '(1 2 3 4 5)])
             (cond [(even? i) (list (* i 100))]
                   [else empty])))

Surely there is a better way to do this for for/list, and a way in general
for for/hash?

(2) How can I add multiple entries (that is, >1 key-value pair) in a given

Again, using for/list I can cheat via append*:

(append* (for/list ([i '(1 2 3 4 5)])
           (list (* i 100) (* i 200))))

What's the right way to do this in for/hash?

- Tim
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