There are a variety of possible extensions to what is currently offered by
spec wrt regex derivatives but that's up to Rich.
On Friday, August 5, 2016 at 12:37:15 PM UTC-5, Russell Mull wrote:
>
> Suppose you have a reactive process, something that receives a message,
> processes it, updates its, state, and repeats. For example:
>
> (go-loop [s :initial]
> (case (<! input-chan)
> :a (recur :processing)
> :b (recur :stopping)
> nil [:success]))
>
> In this contrived example, there are some implied valid state transitions:
> :initial -> :processsing
> :processing ->: :processing
> :processing -> :stopping
> :stopping -> (done)
>
> This is pretty easy to write as a regular expression, using clojure.spec:
> (s/def ::state-seq
> (s/cat :s1 #{:initial}
> :s2 (s/+ #{:processing})
> :s3 #{:stopping}))
>
> But with the clojure.spec api, we'd have to store the whole sequence of
> states and validate it once at the end. To be useful for specifying a
> sequence of values that are spread out over time, I'd really like to be
> able to partially evaluate the schema with a single value when it is at
> hand. The API doesn't expose this, but the implementation appears to
> support it. It would require the ability to track the intermediate states
> of the regex derivative, and the ability to do an explicit complete
> (nullable) check. It could look something like this:
> (go-loop [st :initial, sch (s/get-spec ::state-seq)]
> (let [[valid sch'] (s/partial-valid? sch st)]
> (when-not (valid)
> [:error "everything is terrible"]
> (throw "everything is terrible"))
> (case (<! input-chan)
> :a (recur :processing sch')
> :b (recur :stopping sch')
> nil (if (s/complete? sch')
> [:success]
> [:error "input channel closed, but we weren't in a done state"
> ]))))
>
>
> This could also be useful for specifying something about the order of
> values expected on a channel:
> (s/def ::channel-message-seq
> (s/cat :s1 #{:a}
> :s2 (s/+ #{:b})))
>
> (go-loop [sch (s/get-spec ::channel-message-seq)]
> (let [[sch' msg] (s/partial-conform sch (<! input-chan))
> (case msg
> :a (recur sch')
> :b (recur sch')
> ::s/invalid [:error "got unexpected message"])))
>
> There are of course many convenient ways this could be packaged.
>
> Is it feasible or desirable to add this to clojure.spec?
>
> - Russell Mull
>
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