From[1]:
> conj expects another (possibly single entry) map as the item, and returns a
> new map which is the old map plus the entries from the new, which may
> overwrite entries of the old.
=> (conj {:a 5} {:b 6})
{:b 6, :a 5}
This doesn't make much sense to me; it seems that the values one can add to a
map should be limited to map entries / vector pairs, if for no other reason
than to correspond with the the "granularity" of values in sequences obtained
from maps:
=> (first (seq {:a 5})) ;; a vector / map entry
[:a 5]
I can't remember ever seeing conj used to add all the entries of a map (or,
maps) to another map in the wild; that's what `merge` is for, after all.
I wonder if this is vestigial. FWIW, APersistentMap.cons(Object)[2] checks for
the value being consed as a map entry and as a vector before getting around to
the case where it can be a seqable of map entries. Also, this behaviour of
conj w.r.t. maps seems to make `merge` redundant:
=> (merge {:a 5} {:b 5} {:c 5} {:d 7})
{:d 7, :c 5, :b 5, :a 5}
=> (conj {:a 5} {:b 5} {:c 5} {:d 7})
{:d 7, :c 5, :b 5, :a 5}
I feel like I may have known of the rationale for this being as it is at some
point, but perhaps I've forgotten now. :-) Can anyone remind me?
Thanks,
- Chas
[1]
http://clojure.org/data_structures#Data%20Structures-Maps%20%28IPersistentMap%29
[2]
https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/APersistentMap.java#L23
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