I use the following idiom:

(m/rewrite [1 2 3]
   [(m/and !a1 !a2) ...]
   {:a1 [!a1 ...] :a2 [!a2 ...]})

How can I bind nested values without failing if a parent is absent?

(m/match {:a 1}
  {:b {:c ?c}}
  ?c)
Execution error (ExceptionInfo) at clubhouse.resolvers.datalayer/eval641426 (REPL:203).
non exhaustive pattern match

You can always add an extra case like _ nil or ?x ?x

Or use m/or at the variable

in my real use case, I still want to bind a bunch of other things at other levels, something like

(m/match {:a 1 :c {:foo 3}}
  {:a ?a
   :b ?b
   :c {:foo ?c}
   :d {:bar ?d}}
  [?a ?b ?c ?d])

I thought I could do :d {:bar (m/or ?d (m/let [?d 1]))}, but that still gives me non-exhaustive pattern match. Is that not the right level for the m/or?

ah, if I move m/or up a level it works

Yeah at that level there would have to be a map at :d. The key couldn't be missing. Moving it up one level allows that.

When matching a map, how can I distinguish between a key’s absence and its having the value nil ?

This is one of those areas where I don't have a great answer. When I went to put together the map pattern matching semantics I based them on the observations that 1) find use is uncommon, and 2) most folks are concerned with observable equivalence and not semantic equivalence when it comes to get . The subtle difference between the following two expressions do not weight heavily on most. Its a pragmatic trade off I suppose.

(get {} :m)
(get {:m nil} :m)

If it is cheap, you could preprocess the map to have "semantic nils" i.e. ::blank / ::undefined and then pattern match on those. So

user=> (def blank (fnil identity ::blank))
#'user/blank
user=> (blank (get {:m nil} :m ::undefined))
:user/blank
user=> (blank (get {} :m ::undefined))
:user/undefined
user=> (blank (get {:m 10} :m ::undefined))
10

Interesting, that makes sense. It’s good to know I’m not missing an obvious way to do this.

Perhaps regrettably, we’ve got an API contract for updates where {:foo nil} means “unset the :foo property,” while the absence of :foo means “leave the :foo property as it is.”

Ha! Yep. We have a situation like that with one of our APIs too. {:foo nil} is fine but if :foo is missing the API yells at you. 😐