I'm reading "Programming Clojure" and came across this snippet of
code:
(defn indexed [coll] (map vector (iterate inc 0) coll))
(defn index-filter [pred coll]
(when pred
(for [[idx elt] (indexed coll) :when (pred elt)] idx)))
I'm having a hard time understanding this. What exactly is 'when pred'
checking? I read from the Clojure website that 'when' has the form
'(when test & body)'. Going from the sample call '(index-filter #{\a
\b} "abcdbbb")', I just don't understand what condition is being
checked with 'when pred'. Are we simply checking that pred has values?
Secondly, what is happening inside the for structure? [idx elt] are
being bound to vector returned by indexed, but that assignment only
happens when pred == elt. Finally, we return the vector idx. Is this
correct?
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