On Apr 4, 6:50 am, David Jagoe <davidja...@gmail.com> wrote:
> Particularly I very often find myself doing
>
> (apply hash-map (flatten (for [[k v] some-map] ...)))
:( :( :( flatten is vile, never use it[1]. What if the value in the
map is a list, or the key is a vector? Now you've broken it. Instead
use into:
(into {}
(for [[k v] some-map]
[k (f v)])) ;; or whatever k/v pair you want
I'm also very surprised to see (apply merge-with vector ...), because
this seems like it is rarely right. If you have three maps, {:a 1},
{:a 2}, and {:a 3}, do you really want the result to be {:a [[1 2]
3]}? You're probably better off converting to {:a [1]} {:a [2]} {:a
[3]} beforehand and using (apply merge-with into ...).
Your implementation of selected? is also strange - why are you
iterating through the keys, instead of using their fast lookup? I'd
write (selected? [coll] (contains? select coll)). But that brings up
another point: selected? and agg are just partial applications of
contains and get:
(let [selected? (partial contains? select)
agg (partial get select)]
...)
Or, if you prefer never to repeat anything:
(let [[selected? agg] (for [f [contains? get]]
(partial f select))]
...)
[1] technically you want it sometimes, but I strongly suggest staying
away.
>
> Any pointers or advice on improving my style will be much appreciated!
>
> (defn aggregate
> "Aggregate rows using the functions provided in select and after
> filtering the rows by where.
>
> E.g.
>
> (aggregate [{:a 1 :b 2} {:a 3 :b 4} {:a 5 :b 6}] :select {:a +}
> :where (fn [r] (< (:b r) 6))) --> {:a 4}"
>
> [rows & {select :select where :where}]
> (letfn [(selected? [col] (some #{col} (keys select)))
> (agg [col] (get select col))]
> (apply hash-map
> (flatten
> (for [[k vs] (apply merge-with vector (filter where
> rows)) :when (selected? k)]
> [k (apply (agg k) vs)])))))
>
> Thanks,
>
> --
> David Jagoe
>
> davidja...@gmail.com
> +447535268218
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