Hi,
I would like to suggest an enhancement to the clojure.core/group-by
function. The idea came from using Enumerable.GroupBy
<http://msdn.microsoft.com/en-us/library/bb534304.aspx>extension method in
.NET quite much. It is really handy to have an optional value-mapper
function which transforms the elements before adding them to the collection
under the key. It is backward compatible, because calling the overload with
2 parameters can call the 3 parameter one with clojure.corj/identity as
value-mapper function.
The implementation is easy-peasy (almost the same as the original):
(defn group-by
([f g coll]
(persistent!
(reduce
(fn [ret x]
(let [k (f x)]
(assoc! ret k (conj (get ret k []) (g x)))))
(transient {}) coll)))
([f coll]
(group-by f identity coll)))
Without the value-mapper argument it is very awkward to achieve the same
structure after the group-by call. Also, doing the transformation before
the group-by is often impossible, because the key function depends on some
property of the source element, which would be removed after the
transformation.
To demonstrate the usage, check out the below calls:
(def animals [{:name "Betsy" :type :cow}
{:name "Murmur" :type :cat}
{:name "Lessie" :type :dog}
{:name "Dingo" :type :dog}
{:name "Rosie" :type :cat}
{:name "Rex" :type :dog}
{:name "Alf" :type :cat}])
(group-by :type animals) ; old usage
> ... ugly stuff
(group-by :type :name animals) ; new usage
> {:cow ["Betsy"], :cat ["Murmur" "Rosie" "Alf"], :dog ["Lessie" "Dingo"
"Rex"]}
(group-by :type #(.toUpperCase (:name %)) animals) ; hell yeah!
> {:cow ["BETSY"], :cat ["MURMUR" "ROSIE" "ALF"], :dog ["LESSIE" "DINGO"
"REX"]}
It would be so cool to have this in the core. What do you guys think?
Regards,
Daniel Dinnyes
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