On Wed, Jun 24, 2009 at 6:42 AM, Rich Hickey <richhic...@gmail.com> wrote:
>
> On Tue, Jun 23, 2009 at 8:02 PM, Stephen C. Gilardi<squee...@mac.com>
> wrote:
> >
> > On Jun 23, 2009, at 6:22 PM, pupsik wrote:
> >
> >> The following recursive function does not
> >> terminate if I exexute it in my REPL.
> >> What is wrong?
> >> (This example is from the official Clojure-website).
> >>
> >> (defn my-zipmap [keys vals]
> >> (loop [my-map {}
> >> my-keys (seq keys)
> >> my-vals (seq vals)]
> >> (if (and my-keys my-vals)
> >> (recur (assoc my-map (first my-keys) (first my-vals))
> >> (rest my-keys)
> >> (rest my-vals))
> >> my-map)))
> >>
> >> (my-zipmap [:a :b :c] [1 2 3])
> >
> > The example needs to be updated for the lazier seqs that were implemented
> > before the Clojure 1.0 release. As part of that change, we lost "nil
> > punning". "rest" no longer returns nil if there are no more items. A new
> > function "next" does though:
> >
> > user=> (source zipmap)
> > (defn zipmap
> > "Returns a map with the keys mapped to the corresponding vals."
> > [keys vals]
> > (loop [map {}
> > ks (seq keys)
> > vs (seq vals)]
> > (if (and ks vs)
> > (recur (assoc map (first ks) (first vs))
> > (next ks)
> > (next vs))
> > map)))
> > nil
>
> Yes, this (Steve's) version, using next and testing directly with (and
> ks vs) is the idiomatic way when you are not in turn producing a lazy
> seq.
>
> (if-not (or (empty? ks) (empty? vs))
>
> is to be avoided. (No offense Cosmin :)
>
None taken. As I was writing that code I was wondering why it had to look so
ugly. I completely forgot about next.
--
Cosmin Stejerean
http://offbytwo.com
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