On Thu, Mar 10, 2011 at 12:46 PM, Damien Lepage <damienlep...@gmail.com> wrote:
> Hi
>
> I wrote a function to transform a variable number of arguments into embedded
> maps.
> Here is what it does:
>> (enmap 1 2)
> {1 2}
>> (enmap 1 2 3)
> {1 {2 3}}
>> (enmap 1 2 3 4)
> {1 {2 {3 4}}}
>> (enmap 1 2 3 4 {5 6 7 8})
> {1 {2 {3 {4 {5 6, 7 8}}}}}
> Here is my implementation:
> (defn enmap [arg & args]
> (if-let [more (butlast args)]
> (let [k (last more), v (last args)]
> (if-let [even-more (butlast more)]
> (apply enmap arg (concat even-more (list (hash-map k v))))
> (enmap arg (hash-map k v))))
> (apply hash-map arg args)))
> Two things bother me:
>
> Is there a way to make this function less complicated? without recursion
> maybe?
> Is there something simpler than (concat even-more (list (hash-map k v)) to
> append an element at the end of a sequence?
>
When I see a pattern like this of repeatedly doing something to a
collection, I immediately think of "reduce".
For fun, let's see what happens when I try it:
user=> (reduce hash-map [1 2 3 4 5])
{{{{1 2} 3} 4} 5}
Close, but backwards.
I know reduce is equivalent to a left-fold, so to get the opposite I
want a right-fold. Clojure doesn't have that built-in, but one way to
define it:
user=>(defn foldr [f coll]
(reduce #(f %2 %1) (reverse coll)))
Now, we can:
user=>(foldr hash-map [1 2 3 4 5])
{1 {2 {3 {4 5}}}}
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