An atom that can be watched for changes under particular paths. Useful for simple front-end application state stores.

(require '[clj-arsenal.madam :as madam])

(def !store (madam/madam {:foo {:bar 2}}))

(madam/watch !store ::my-watch [:foo :bar]
 (fn [old-val new-val _changed-paths]
   (prn :old-val old-val :new-val new-val)))

(madam/patch! !store
  {:path [:foo :bar]
   :change [:call inc]})
; >> :old-val 2 :new-val 3

(madam/unwatch !store ::my-watch)

Patching a 'madam' keeps track of which paths the applied operator could have changed. The default operators are :assoc, :merge, :clear (dissoc, disj, remove index from vector), :conj, :into, :call, :value (replace value).

An :fnil entry can be provided on the patch, which is substituted in for the current value if the current value is nil or doesn't exist.

A custom set of operators can be given as a map under the :operators option of the madam constructor. These operators replace the defaults, so make sure to merge in default-operators to keep them. The keys of the operator map are operator names, and the values are functions of the form (f basis & args) -> [new-value #{& changed-subpaths}]. Where basis is given as either the current value at the patch path, or the :fnil value if the current value is nil or non-existent. args is given as the remaining items in the :change vector, after the operator. The new-value is assoc-ed at the path path, and the changed-subpaths help determine which subpaths (under the patch path) could have been changed by the operation.

Patches can be given as either a single patch map, or a collection of patch maps.

Instead of a :change, a patch map can specify a :subpatch, which is another patch that'll be applied at the patch path.