This may be really basic. So much so that it's hard to find on the
internet, but is there something like a copy constructor in clojure,
where I can copy everything in a structure except one or two keys?
Maybe something like struct-map, but fills in the other variables not
supplied by another data structure.
There's no need for copying with Clojure's native data structures.
Clojure data structures are persistent: for example, if you dissoc a
key from a map, you get a new map with the same contents as the
original minus the dissociated key. The original map is unchanged. The
new map safely shares structure with the original.
This is both efficient and safe.
E.g.,
user=> (def aaa {:foo 1 :bar 2 :baz 3})
#'user/aaa
user=> (dissoc aaa :foo :baz)
{:bar 2}
user=> aaa
{:foo 1, :bar 2, :baz 3}
If you want to draw values from two maps, use merge:
user=> (merge {:foo 0 :bar 0 :baz 0} (dissoc aaa :foo :baz))
{:foo 0, :bar 2, :baz 0}
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