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The language and speech, especially the jargon, slang, or argot, of a particular field, group, or individual

Trying to make spec explain message more usable with small, composable additions.

The goal is to provide the spec users with more data from errors and and give means to render helpful error messages.

It differs from other similar libraries in that the focus is more on (explain-)data first, then leveraging that for potential rendering. You can use lingo without its error rendering facilities in order to just infer more information about your errors.

Internally building this data is done via transducers that are run against the explain-data problems. It's quite easy to pull this apart and build your own if want/need to.


lingo is available on Clojars.

Add this to your dependencies:

Clojars Project


Adds 3 functions similar to clojure.spec.alpha/explain-*

  • exoscale.lingo/explain-data: returns clojure.spec.alpha/explain-data for spec/value with extra fields

  • exoscale.lingo/explain: uses exoscale.lingo/explain-data but prints a nicely formatted message)

  • exoscale.lingo/explain-str: same as exoscale.lingo/explain but returns a string instead of printing

exoscale.lingo/explain-data is the most important one really

It will take a spec, a value and potentially options and return clojure.spec explain-data for it with extra fields added.

Some very high level:

  • :exoscale.lingo.explain/message inferred error message you might want to display
  • :exoscale.lingo.explain/path humanised path
  • :exoscale.lingo.explain/highlight potential highlight output (shows error value in context with blanked surroundings)

Keys related to a potential custom error message found at spec level:

  • :exoscale.lingo.explain.spec/spec which spec had a message registered for it
  • :exoscale.lingo.explain.spec/message the message in question

Keys related to a potential error inferred from pred data

  • :exoscale.lingo.explain.pred/spec the spec used to match the predicate (via conform)
  • :exoscale.lingo.explain.pred/vals the values destructured via conformed
  • :exoscale.lingo.explain.pred/message the message generated from the values extracted + potential formater registered for this predicate key

So that's quite a lot of information, in most cases you will just care about the first three, but the rest is available to you via explain-data if you need/want to build your own outputs. If you look at the way lingo/explain-data is implemented you will also see that all this is very composable you can easily enrich/lighten this information.

Ok, now examples:

First out of the box, without any custom message added to a spec:

(s/def :foo/t-shirts (s/coll-of :foo/t-shirt))
(s/def :foo/t-shirt (s/keys :req-un [:foo/size :foo/color]))
(s/def :foo/size (s/int-in 1 3))
(s/def :foo/color #{:red :blue :green})

(exoscale.lingo/explain :foo/t-shirts [{:size 5 :color :pink}])

Let's see how it's done under the hood

(exoscale.lingo/explain-data :foo/t-shirts [{:size 5 :color :pink}])

                     ({:path [:size],
                       :via [:foo/t-shirts :foo/t-shirt :foo/size],
                       :val 5,
                       :in [0 :size]
                       :pred (clojure.core/fn [%] (clojure.spec.alpha/int-in-range? 1 3 %)),
                       :exoscale.lingo.explain/highlight "[{:size 5, :color _}]\n        ^ should be an Integer between 1 3",
                       :exoscale.lingo.explain.pred/spec :exoscale.lingo.pred/int-in-range,
                       :exoscale.lingo.explain.pred/message "should be an Integer between 1 3",
                       :exoscale.lingo.explain.pred/vals [:_ {:_ %, :min 1, :max 3}],
                       :exoscale.lingo.explain/message "should be an Integer between 1 3",
                       :exoscale.lingo.explain/path "[0].size"}
                      {:path [:color],
                       :pred #{:green :red :blue},
                       :via [:foo/t-shirts :foo/t-shirt :foo/color],
                       :val :pink,
                       :in [0 :color]
                       :exoscale.lingo.explain.pred/spec :exoscale.lingo.pred/set,
                       :exoscale.lingo.explain.pred/message "should be one of :blue, :green, :red",
                       :exoscale.lingo.explain.pred/vals #{:green :red :blue},
                       :exoscale.lingo.explain/highlight "[{:size _, :color :pink}]\n                  ^^^^^ should be one of :blue, :green, :red",
                       :exoscale.lingo.explain/message "should be one of :blue, :green, :red",
                       :exoscale.lingo.explain/path "[0].color"}),
                     :spec :foo/t-shirts,
                     :value [{:size 5, :color :pink}]}

As you can see there's a lot of more information available than what spec returns alone. There seem to be repetitions in the data, but that will make more sense after it's explained.

There are 2 ways to specify custom messages, depending on what is the source you start from:

If you are working from spec identifiers or static forms you can use set-spec-error!, it will dispatch on the problem spec (value we get from explain-data :via), potentially resolving aliases too, up to the pred failing at the end. spec errors will result in addition of :exoscale.lingo.explain.spec/* keys to the problem data.

(set-spec-error! `string? "should be a String")
(set-spec-error! ::thing "should be a Thing")
(set-spec-error! (s/coll-of ::thing) "should be a collection of Things")

What I meant by "resolving aliases" is that for something like this

(s/def ::foo ::bar)
(s/def ::bar ::baz)

If you have a custom message on ::baz or ::bar and your value blows up at ::foo level, lingo will pick up the first message in the alias chain (so checks ::foo, then ::bar and then ::baz). Alias information is not data available from raw explain-data, lingo has to infer by itself.

If you want to have more precise error handling based on the problem pred only (usually it's the best thing to do) you can use set-pred-error!. This will result in the addition of the :exoscale.lingo.explain.pred/* keys to the problem.

There is this libary called clojure.spec that's pretty good to parse forms and extract information from them, we happen to use it to do just that:

(s/def ::int-in-range (s/cat :_ #{'clojure.spec.alpha/int-in-range?}
                        :min number?
                        :max number?
                        :_ #{'%}))

(set-pred-error! ::int-in-range
                 (fn [{:keys [min max]} _opts]
                   (format "should be an Integer between %d %d" min max)))

This will use the first argument to perform conforming against the pred in the explain-data problems. From there it will add the destructured values to the explain-data and also the key of the conformer that matched (here ::int-in-range). From this conformer key and extracted values you/lingo are able to generate an appropriate message.

Internally these are really 2 distinct operations, we first destructure via the spec then render in another step.

so for an error like this:

                     [{:path [],
                        (clojure.spec.alpha/int-in-range? 0 3 %)),
                       :val 4,
                       :via [],
                       :in []}],
                     :value 4}

It will pass the abbreviated (clojure.spec.alpha/int-in-range? 0 3 %) to the conforming spec we defined.

(s/cat :_ #{'clojure.spec.alpha/int-in-range?}
                        :min number?
                        :max number?
                        :_ #{'%})

Which will destructure it to {:min 0 :max 3} and call the following function on it.

(fn [{:keys [min max]} _opts]
   (format "should be an Integer between %d %d" min max))

This is a trivial example, but if you take a s/coll-of (or any of s/every variants), which can return a myriad of preds depending on how failure happened, this will return very fine grained error message that pin-points exactly how the value failed. This is also very handy for more "custom" other uses cases, such as exoscale.specs/string-of and other parameterised specs we might have.

By default a lot of common predicates are supported out of the box , most common math operators and compositions of these with count for instance, Set/Map membership (or lack thereof), and most of clojure.spec custom predicates.

set-pred-error! calls internally set-pred-conformer! and set-pred-message!, the operations are decoupled. If you only care about the destructuring just set a pred conformer for your use and leave the formater out.

You can in theory have a custom error message at both spec level and pred level and mix them to create something very detailed, but by default lingo sets :exoscale.lingo.explain/message to the value that makes more sense for you (all the rest is still available in the explain-data), it will try to get something from the spec explain we generate first, and fallback to the pred message.

The tests demonstrate some of these :


  • :registry defaults to lingo's internal registry, but you can have your own, could be handy if you need to support multiple languages for instance
  • :highlight? defaults to true, whether we should try to provide an highlight of the error value
  • :highlight-inline-message? defaults to true, whether we should show the explain message inline with the error marker in the highlight
  • :colors? defaults to false, whether to use terminal colors for the highlight
  • :path? defaults to true, whether to include a prettified path (js like path vs get-in/associative path)
  • :message? defaults to true, whether to include string messages derived from the pred/spec data extracted
  • :header? defaults to true, whether to show a header with problems count in explain message
  • :focus? defaults to true, whether to blank out the values that are not relevant in the highlight value.
  • :group-missing-keys? defaults to true, whether to group all "missing keys" problems for the same map into a single problem.
  • :group-or-problems? defaults to true, whether to group all problems concerning the same value into one: typically that's used for nilable, since by default a nilable failure will cause 2 problems and also for s/or clauses (to be able to express "should be a string OR an int" instead of having 2 problems).

replacing clojure spec printer

You can use (exoscale.lingo/set-explain-printer! ?opts) to set a default printer for spec.


(s/explain string? 1)

;; 1 is invalid - should be a String


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