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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