Hi Everyone,
I am using spec for a while (currently on the version shipped with Clojure
1.10). This post is intended to be a kind-of status report, focusing on a
particular issue I am facing at the moment. To explain it in detail, please
have a look on this following macro I wrote, which is an upgraded version
of *spec/keys*:
(ns myns
(:refer-clojure :exclude [keys])
(:require
[clojure.set :as set]
[clojure.core :as core]
[clojure.spec.alpha :as spec]))
(defmacro keys
"Same as `clojure.spec/keys`, but accepts additional boolean option
:additional-keys. Unless :additional-keys is set true, only the declared
keys are allowed, and any additional keys will be invalid."
[& {:keys [additional-keys] :as args}]
(let [args (dissoc args :additional-keys)]
(if additional-keys
`(spec/keys ~@(mapcat identity args))
(let [allowed-keys #{}
allowed-keys
(reduce
(fn [acc k] (into acc (k args)))
allowed-keys [:req :opt])
allowed-keys
(reduce
(fn [acc k] (into acc (map (comp keyword name) (k args))))
allowed-keys [:req-un :opt-un])]
`(spec/and
(spec/keys ~@(mapcat identity args))
(fn [m#] (set/subset? (core/keys m#) ~allowed-keys)))))))
I would like to explain my use-case for writing this, and the reason I
think there is a need for such feature.
I am writing an import/export library from a serialized XML data format,
into in-memory representation. I need the import and export functions to be
invertible:
* given an original external XML data, if I import it, and then
immediately export it, the re-exported data has to be equal to the original.
* given an original internal in-memory data, if I export it, and then
immediately import it, the re-imported data has to be equal to the original.
The main issue is that silent data-loss is absolutely unacceptable, (e.g.
if a newer version of the XML data format has additional unspecified
fields, these additional fields will fail to be imported to the in-memory
model. Also, if any additional unspecified in-memory data gets added, it
will fail to be exported. All this would all happen silently, without any
error/warning).
The problem I am facing is I think one of the valid use-case, spec should
have support for such scenarios.
Another similar use-case is when due to GDPR
<https://en.wikipedia.org/wiki/General_Data_Protection_Regulation>
regulatory rules, extreme care should be taken what information gets
stored, and any unverified data is potentially violating requirements.
(e.g. a user might use clear-text comment fields to store credit-card
information, N.I. numbers, etc.)
The original keys spec allows for additional data to be present, which is
in line with what Rich described as design goals for spec (i.e. "requiring
less", or "providing more" is not breakage but growth, and should be
welcomed, as per the "Speculation
<https://www.youtube.com/watch?v=oyLBGkS5ICk>" talk).
After I started using the above macro, it became obvious that it isn't
versatile enough solution to the problem, because I would rather like to be
notified of additional-keys, and then decide on a case-by-case basis, if in
the given context that is to be considered an error. Instead the macro only
gives me the option to decide this at the time when defining the spec,
rather at the point of data validation.
If I recall correctly, in the "Maybe Not
<https://www.youtube.com/watch?v=YR5WdGrpoug>" talk Rich was talking about
the concerns, that validating required and optional fields should be
separated from the shape/schema of a spec, to be a'la carte, and decided at
the point of validation, not at the time of defining the specs. It feels to
me this concern is quite similar/related issue to the one I am writing
about now.
I have also noticed that my usage pattern is that I rarely use *spec/valid?*,
Instead prefer to use something like:
(if-let [explanation (explain-data :my/spec data)]
(throw (ex-info "Invalid stuff" explanation)
(do-business-logic-with (spec/conform :my/spec data)))
Due to this usage pattern, I am quite reliant on the fact that *spec/valid?*
returns *true* iff <https://en.wikipedia.org/wiki/If_and_only_if>
*spec/explain-data* returns *nil*.
Nevertheless, I was thinking if *spec/explain-data* would return any
unspecified keys found for a keys spec, under an *:unspecified* key in the
explain-data map that would address the issue much better than my barbaric
butchery with macros.
I would be interested to hear what you guys think about all the above, or
if there are better workarounds/recommendations for my use-cases.
Cheers,
Daniel
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