Hi Chris,
Thanks a lot for your clear explanations! Now all the pieces suddenly
make sense to me.
Stefan
On Nov 26, 9:30 pm, Chris Perkins <chrisperkin...@gmail.com> wrote:
> On Nov 26, 7:42 am, Stefan Rohlfing <stefan.rohlf...@gmail.com> wrote:
>
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> > Dear Clojure Group,
>
> > Today I took a closer look at the 'merge-with' function of Clojure
> > Core and changed some parts to better understand its implementation.
>
> > Now I still have two questions regarding the following code:
>
> > (defn my-merge-with [f & maps]
> > (when (some identity
> > maps) ;; question 1
> > (reduce (fn [acc m]
> > (let [key (key (first m)) val (val (first
> > m))] ;; question 2
> > (assoc acc key (if-let [old-val (get acc key)]
> > (f old-val val)
> > val))))
> > {} maps)))
>
> > (my-merge-with + {"t" 1} {"a" 1} {"c" 1} {"g" 1} {"g" 1} {"g" 1} {"a"
> > 1} {"c" 1} )
> > ;; {"g" 3, "c" 2, "a" 2, "t" 1}
>
> > Question 1:
> > (when (some identity maps)
>
> > This expression from the original implementation checks if the
> > provided coll is empty.
> > However, why not just use (when (empty? maps) or (when (seq maps)
> > instead?
>
> It has the effect that calling (merge-with f nil), or (merge-with f
> nil nil nil), returns nil rather than an empty map. I guess that's the
> reason for it.
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> > Question 2:
> > In order to return the key of a map consisting of one key/value pair I
> > have to use the 'first' function, otherwise an exception will be
> > thrown:
>
> > (key {:a "a"})
> > ;; clojure.lang.PersistentArrayMap cannot be cast to java.util.Map
> > $Entry
> > ;; [Thrown class java.lang.ClassCastException]
>
> > (key (first {:a "a"}))
> > ;; :a
>
> > I understand that 'key' expects a java.util.Map$Entry object, but how
> > exactly does using 'first' accomplish this?
>
> Calling seq on a map returns a sequence of Map$Entry objects (which
> look a lot like two-element vectors, but are not quite the same).
> first calls seq.
>
> - Chris
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