I read too quickly. I might need multi-methods after all, just like I used
in Common Lisp. I want different types of persisted objects to have the
same interface, if possible.
Actually, I'm just experimenting with protocols, so I'm probably better off
looking for a different use case.
On Mon, Aug 29, 2016 at 5:11 PM <jmckitr...@gmail.com> wrote:
> Makes perfect sense. Thanks!
>
> On Mon, Aug 29, 2016 at 5:09 PM Jonathan Fischer <jonat...@mohiji.org>
> wrote:
>
>> The most straightforward one: just make your protocols a thing that your
>> storage medium implements, instead of your records. E.g.:
>>
>> (defprotocol ThingStore
>> (load-item [store item-id])
>> (load-items [store item-ids])
>> (find-item [store item-name])
>> (save-item [store item]))
>>
>> (defrecord DatabaseStore
>> ThingStore
>> (load-item [store item-id] ...))
>>
>> Otherwise, Clojure does have multimethods, but the dispatch mechanism
>> doesn't work the same was as Common Lisp's. You could probably combine
>> multimethods with a marker protocol to do what you're trying.
>>
>> ;; First, an empty marker protocol
>> (defprotocol Loadable)
>>
>> ;; And something that you want to load
>> (defrecord Thing [data]
>> Loadable ;; mark that it implements that empty protocol
>> )
>>
>> ;; Define a multimethod dispatch function that will figure out the right
>> implementation to call
>> (defn loadable-dispatch
>> [item]
>> (cond (satisfies? Loadable item) ::load-one-item
>> (and (sequential? item) (every? #(satisfies? Loadable %) item))
>> ::load-many-items
>> :else nil))
>>
>> ;; The multimethod itself
>> (defmulti load-item loadable-dispatch)
>>
>> ;; And implementations
>> (defmethod load-item ::load-one-item
>> [item]
>> "Load one item.")
>>
>> (defmethod load-item ::load-many-items
>> [items]
>> "Load many items.")
>>
>> user> (load-item (map->Thing {}))
>> "Load one item."
>>
>> user> (load-item [(map->Thing {}) (map->Thing {})])
>> "Load many items."
>>
>> Overall, you're probably better off with the first option, but you can
>> make multimethods work for you here if you want.
>>
>> On Sunday, August 28, 2016 at 7:29:46 AM UTC-7, Jonathon McKitrick wrote:
>>
>>> I'm beginning a foray into protocols after coming from the Common Lisp
>>> world of multimethods. I'm using them initially to implement a consistent
>>> load/save API over the DB layer.
>>>
>>> It's pretty simple to have a Saveable protocol to save one object,
>>> because the first argument is a Record of the type I am saving.
>>>
>>> But since protocols dispatch on the first argument, how do you define a
>>> Loadable protocol that loads an object or handles a collection of objects?
>>>
>>> For example, if I want to load an object by idx, the first argument is
>>> not a Record, but an integer, or perhaps an arbitrary field to used in a
>>> query.
>>>
>>>
>>> --
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> Jonathon McKitrick
>
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