On Mar 10, 10:05 am, Aaron Cohen <aa...@assonance.org> wrote:
> 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}}}}
Ah, this is nicer than mine. Also see https://github.com/amalloy/hot-potato,
which has a reorder HOF so that you could write a perhaps-clearer
(reduce (reorder f) (reverse coll)).
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