I believe it's because the `mapper` function is just creating and returning
a map literal. The "mapper" function in the evaled version is something
like this:
user> (def names '[n1 n2 n3 n4])
#'user/names
user> (def headers '[h1 h2 h3 h4])
#'user/headers
user> `(fn [[~@names]] ~(zipmap headers names))
(clojure.core/fn [[n1 n2 n3 n4]] {h4 n4, h3 n3, h2 n2, h1 n1}) ;; just a
map literal, whose keys are already known.
Whereas in the first version, zipmap has to be called, iterating over
headers and names each time.
On Fri, Oct 10, 2014 at 1:04 PM, Sean Corfield <s...@corfield.org> wrote:
> It may be more to do with the difference between `for` and `map`. How do
> these versions compare in your benchmark:
>
> (defn read-to-maps-partial [rows]
> (let [headers (->>
> rows
> first
> (take-while (complement #{""}))
> (map keyword))]
> (map (partial zipmap headers) (rest rows))))
>
> (defn read-to-maps-fn [rows]
> (let [headers (->>
> rows
> first
> (take-while (complement #{""}))
> (map keyword))
> mapper (fn [row] (zipmap headers row))]
> (map mapper (rest rows))))
>
> Sean
>
> On Oct 10, 2014, at 11:42 AM, Michael Blume <blume.m...@gmail.com> wrote:
> > So I'm reading a bunch of rows from a huge csv file and marshalling
> those rows into maps using the first row as keys. I wrote the function two
> ways: https://gist.github.com/MichaelBlume/c67d22df0ff9c225d956 and the
> version with eval is twice as fast and I'm kind of curious about why.
> Presumably the eval'd function still implicitly contains a list of keys,
> it's still implicitly treating each row as a seq and walking it, so I'm
> wondering what the seq-destructuring and the map literal are doing under
> the hood that's faster.
>
>
--
Ben Wolfson
"Human kind has used its intelligence to vary the flavour of drinks, which
may be sweet, aromatic, fermented or spirit-based. ... Family and social
life also offer numerous other occasions to consume drinks for pleasure."
[Larousse, "Drink" entry]
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