Human-optimized error messages for clojure.spec
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Add warning about spec  “0.2.176”
Latest commit 04d13aa Sep 8, 2018


Starting with version “0.2.176”, clojure.spec.alpha changes how specs errors are reported during macroexpansion. If you use Expound to print macro-expansion spec errors (e.g. syntax errors when using let, ns and defn), do not upgrade to “0.2.176” until your REPL has been upgraded to support the new error format.

Expound formats clojure.spec error messages in a way that is optimized for humans to read.

For example, Expound will replace a Spec error message like:

val: {} fails spec: :example/place predicate: (contains? % :city)
val: {} fails spec: :example/place predicate: (contains? % :state)
:clojure.spec.alpha/spec  :example/place
:clojure.spec.alpha/value  {}


-- Spec failed --------------------


should contain keys: `:city`,`:state`

Expound is in alpha while clojure.spec is in alpha.

Expound depends on projects supported by Clojurists Together. If you find this project useful, please consider making a monthly donation to Clojurists Together (or ask your employer to do so).


Clojars Project

Quick start


[expound "0.7.1"]


expound {:mvn/version "0.7.1"}


npm install @bbrinck/expound


API Docs

> brew install clojure
> clj -Sdeps '{:deps {friendly {:git/url "" :sha "bb5806bd655d743f3b48b36ce83c0085a8d7c54a"}}}' -m friendly
user=> (require '[expound.alpha :as expound])
user=> (expound/expound string? 1)
-- Spec failed --------------------


should satisfy


Detected 1 error


Replace calls to clojure.spec.alpha/explain with expound.alpha/expound and to clojure.spec.alpha/explain-str with expound.alpha/expound-str.

(require '[clojure.spec.alpha :as s])
;; for clojurescript:
;; (require '[cljs.spec.alpha :as s])
(require '[expound.alpha :as expound])

(s/def string?)
(s/def string?)
(s/def :example/place (s/keys :req-un []))
(expound/expound :example/place {:city "Denver", :state :CO})
;; -- Spec failed --------------------

;;   {:city ..., :state :CO}
;;                      ^^^

;; should satisfy

;;   string?

;; -- Relevant specs -------

;;   clojure.core/string?
;; :example/place:
;;   (clojure.spec.alpha/keys
;;    :req-un
;;    [])

;; -------------------------
;; Detected 1 error


To use other Spec functions, set clojure.spec.alpha/*explain-out* (or cljs.spec.alpha/*explain-out* for ClojureScript) to expound/printer.

(Setting *explain-out* does not work correctly in ClojureScript versions prior to 1.9.562 due to differences in explain-data)

(require '[clojure.spec.alpha :as s])
;; for clojurescript:
;; (require '[cljs.spec.alpha :as s])
(require '[expound.alpha :as expound])

(s/def string?)
(s/def string?)

;;  Use `assert`
(s/check-asserts true) ; enable asserts

;; Set var in the scope of 'binding'
(binding [s/*explain-out* expound/printer]
  (s/assert 1))

;; Or set it for the current thread.
(set! s/*explain-out* expound/printer)
(s/assert 1)

;; Use `instrument`
(require '[clojure.spec.test.alpha :as st])

(s/fdef pr-loc :args (s/cat :city
(defn pr-loc [city state]
  (str city ", " state))

(st/instrument `pr-loc)
(pr-loc "denver" :CO)

;; You can use `explain` without converting to expound
(s/explain 123)

If you are enabling Expound in a non-REPL environment, remember that set! will only change s/*explain-out* in the current thread. If your program spawns additional threads (e.g. a web server), you can set s/*explain-out* for all threads with (alter-var-root #'s/*explain-out* (constantly expound/printer)). This won't work (and is not necessary) in CLJS.

Using set! will also not work within an uberjar.

Printing results for check

Re-binding s/*explain-out* has no effect on the results of cljs.spec.test.alpha/summarize-results, but Expound provides the function expound/explain-results to print the results from clojure.spec.test.alpha/check.

(require '[expound.alpha :as expound]
         '[clojure.spec.test.alpha :as st]
         '[clojure.spec.alpha :as s]

(s/fdef ranged-rand
  :args (s/and (s/cat :start int? :end int?)
               #(< (:start %) (:end %)))
  :ret int?
  :fn (s/and #(>= (:ret %) (-> % :args :start))
             #(< (:ret %) (-> % :args :end))))
(defn ranged-rand
  "Returns random int in range start <= rand < end"
  [start end]
  (+ start (long (rand (- start end)))))

(set! s/*explain-out* expound/printer)
(expound/explain-results (st/check `ranged-rand))
;;== Checked user/ranged-rand =================
;;-- Function spec failed -----------
;;  (user/ranged-rand -3 0)
;;failed spec. Function arguments and return value
;;  {:args {:start -3, :end 0}, :ret -5}
;;should satisfy
;;  (fn
;;   [%]
;;   (>= (:ret %) (-> % :args :start)))

Error messages for predicates

Adding error messages

If a value fails to satisfy a predicate, Expound will print the name of the function (or <anonymous function> if the function has no name). To improve the error message, you can use expound.alpha/def to add a human-readable error message to the spec.

(def email-regex #"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,63}$")

(expound/def :example/email (s/and string? #(re-matches email-regex %)) "should be a valid email address")

(expound/expound :example/email "sally@")

;; -- Spec failed --------------------
;;   "sally@"
;; should be a valid email address

If you prefer to use clojure.spec.alpha/def, you can still add a message using expound.alpha/defmsg:

(s/def :ex/name string?)
(expound/defmsg :ex/name "should be a string")
(expound/expound :ex/name :bob)
;; -- Spec failed --------------------
;; :bob
;; should be a string

Built-in predicates with error messages

Expound provides a default set of type-like predicates with error messages. For example:

(expound/expound :expound.specs/pos-int -1)
;; -- Spec failed --------------------
;; -1
;; should be a positive integer

You can see the full list of available specs with expound.specs/public-specs.

Printer options

Create a custom printer by changing the following options e.g.

(set! s/*explain-out* (expound/custom-printer {:show-valid-values? true :print-specs? false :theme :figwheel-theme}))
name spec default description
:show-valid-values? boolean? false If false, replaces valid values with ... (example below)
:value-str-fn ifn? provided function Function to print bad values (example below)
:print-specs? boolean? true If true, display "Relevant specs" section. Otherwise, omit that section.
:theme #{:figwheel-theme :none} :none Enables color theme.


By default, printer will omit valid values and replace them with ...

(set! s/*explain-out* expound/printer)
(s/explain :example/place {:city "Denver" :state :CO :country "USA"})

;; -- Spec failed --------------------
;;   {:city ..., :state :CO, :country ...}
;;                      ^^^
;; should satisfy
;;   string?

You can configure Expound to show valid values:

(set! s/*explain-out* (expound/custom-printer {:show-valid-values? true}))
(s/explain :example/place {:city "Denver" :state :CO :country "USA"})

;; -- Spec failed --------------------
;; {:city "Denver", :state :CO, :country "USA"}
;;                         ^^^
;; should satisfy
;;   string?

You can provide your own function to display the invalid value.

;; Your implementation should meet the following spec:
(s/fdef my-value-str
        :args (s/cat
               :spec-name (s/nilable #{:args :fn :ret})
               :form any?
               :path :expound/path
               :value any?)
        :ret string?)
(defn my-value-str [_spec-name form path value]
  (str "In context: " (pr-str form) "\n"
       "Invalid value: " (pr-str value)))

(set! s/*explain-out* (expound/custom-printer {:value-str-fn my-value-str}))
(s/explain :example/place {:city "Denver" :state :CO :country "USA"})

;; -- Spec failed --------------------
;;   In context: {:city "Denver", :state :CO, :country "USA"}
;;   Invalid value: :CO
;; should satisfy
;;   string?

Manual clojure.test/report override

Clojure test allows you to declare a custom multi-method for its clojure.test/report function. This is particularly useful in ClojureScript, where a test runner can take care of the boilerplate code:

(ns pkg.test-runner
  (:require [cljs.spec.alpha :as s]
            [cljs.test :as test :refer-macros [run-tests]]
            [expound.alpha :as expound]
            ;; require your namespaces here


(set! s/*explain-out* expound/printer)

;; We try to preserve the clojure.test output format
(defmethod test/report [:cljs.test/default :error] [m]
  (test/inc-report-counter! :error)
  (println "\nERROR in" (test/testing-vars-str m))
  (when (seq (:testing-contexts (test/get-current-env)))
    (println (test/testing-contexts-str)))
  (when-let [message (:message m)] (println message))
  (let [actual (:actual m)
        ex-data (ex-data actual)]
    (if (:cljs.spec.alpha/failure ex-data)
      (do (println "expected:" (pr-str (:expected m)))
          (print "  actual:\n")
          (print (.-message actual)))
      (test/print-comparison m))))

;; run tests, (stest/instrument) either here or in the individual test files.
(run-tests 'pkg.namespace-test)

Using Orchestra

Use Orchestra with Expound to get human-optimized error messages when checking your :ret and :fn specs.

(require '[orchestra.spec.test :as st])

(s/fdef location
        :args (s/cat :city
        :ret string?)
(defn location [city state]
  ;; incorrect implementation

(set! s/*explain-out* expound/printer)
(location "Seattle" "WA")

;;ExceptionInfo Call to #'user/location did not conform to spec:
;; form-init3240528896421126128.clj:1
;; -- Spec failed --------------------
;; Return value
;; nil
;; should satisfy
;; string?
;; -------------------------
;; Detected 1 error


Expound will not give helpful errors (and in some cases, will throw an exception) if you use conformers to transform values. Although using conformers in this way is fairly common, my understanding is that this is not an intended use case.

If you want to use Expound with conformers, you'll need to write a custom printer. See "Printer options" above.

Related work

  • Inspectable - Tools to explore specs and spec failures at the REPL
  • Pretty-Spec - Pretty printer for specs
  • Phrase - Use specs to create error messages for users
  • Pinpointer - spec error reporter based on a precise error analysis

Prior Art


Pull requests are welcome, although please open an issue first to discuss the proposed change. I also answer questions on the #expound channel on clojurians Slack.

If you are working on the code, please read the Development Guide


Copyright © 2017-2018 Ben Brinckerhoff

Distributed under the Eclipse Public License version 1.0, just like Clojure.