Hi everyone,
Does anyone have any experience in mocking multimethods? I'm working
on a version control framework modeled after Git:
(def
^{:private true}
patches- (ref [])
(defn patches []
(seq @patches-))
(defn do-patch! [fn & args]
(dosync
(apply fn args)
(let [patch {:fn fn
:args (vec args)}]
(alter patches- conj patch)
patch)))
I need to be able to undo recorded patches, which means at undo-time,
translating the recorded do-patch! fn and args into an undo-fn which
can be called. If an undo-fn cannot be found, it throws an
exception. This has to be extensible so that as I add operations, I
can add the counterpart undo-fn. Multimethods to the rescue!
(defmulti undo-fn :fn)
(defn undo-patch [patch]
(let [fn (undo-fn patch)]
(dosync
(fn)
(alter patches- #(remove #(= patch %) %)))))
So in another package, I add the operation create-table! to the
system, which has a counterpart drop-table!:
(defn create-table!- [name columns]
(... create table stuff ...))
(defn drop-table!- [name]
(... drop table stuff ...))
(defn create-table! [name columns]
(make-patch! create-table!- name columns))
(defmethod undo-fn create-table!- [patch]
#(drop-table!- (:name patch)))
So calling undo-patch! with a patch that was the result of create-
table! will call undo-fn, which will dispatch on the :fn member of the
patch, which is create-table!-, which will end up returning an
anonymous function that calls drop-table!- with the :name from
the :args member of the patch.
OK, this all works.
But from a unit testing perspective, how do I cleanly test undo-patch!
without any undo-fn methods declared? When testing, I just want to
know that undo-patch! attempts to resolve undo-fn, will throw an error
if it can't, and will call the anonymous function that undo-fn returns
if it can. In other words, how do I mock undo-fn?
Thanks,
Alyssa
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