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

For example, Expound will replace a clojure.spec error message like:

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


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


should contain keys: :city, :state

| key    | spec    |
| :city  | string? |
| :state | string? |

Comparison with clojure.spec error messages

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

Expound is 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).


If you are using recent versions of ClojureScript, please check the compatibility guide


[expound "0.9.0"]


expound/expound {:mvn/version "0.9.0"}


npm install @bbrinck/expound


API docs

Quick start via clj

> brew install clojure/tools/clojure
> clojure -Sdeps '{:deps {friendly/friendly {:git/url "" :sha "d532662414376900c13bed9c920181651e1efeff"}}}' -X friendly/run
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])
(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} {:print-specs? false})
;; -- Spec failed --------------------

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

;; should satisfy

;;   string?

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


To use other Spec functions, set clojure.spec.alpha/*explain-out* to expound/printer.

(require '[clojure.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))

(set! s/*explain-out* expound/printer)
;; (or alter-var-root - see doc/
(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)

ClojureScript considerations

Due to the way that macros are expanded in ClojureScript, you'll need to configure Expound in Clojure to use Expound during macro-expansion. This does not apply to self-hosted ClojureScript. Note the -e arg when starting ClojureScript:

clj -Srepro -Sdeps '{:deps {expound {:mvn/version "0.9.0"} org.clojure/test.check {:mvn/version "0.9.0"} org.clojure/clojurescript {:mvn/version "1.10.520"}}}' -e "(require '[expound.alpha :as expound]) (set! clojure.spec.alpha/*explain-out* expound.alpha/printer)" -m cljs.main -re node

As of this commit, ClojureScript instrumentation errors only contain data and leave formatting/printing errors to the edge of the system e.g. the REPL. To format errors in the browser, you must set up some global handler to catch errors and call repl/error->str. For instance, here is a custom formatter for Chrome devtools that uses Expound:

(require '[cljs.repl :as repl])
(require '[clojure.spec.alpha :as s])
(require '[expound.alpha :as expound])
(set! s/*explain-out* expound/printer)

(def devtools-error-formatter
  "Uses cljs.repl utilities to format ExceptionInfo objects in Chrome devtools console."
      (fn [object _config]
        (when (instance? ExceptionInfo object)
          (let [message (some->> (repl/error->str object)
                                 (re-find #"[^\n]+"))]
            #js["span" message])))
      :hasBody (constantly true)
      :body (fn [object _config]
              #js["div" (repl/error->str object)])})
(defonce _
         (some-> js/window.devtoolsFormatters
                 (.unshift devtools-error-formatter)))

See this ticket for other solutions in the browser.

Printing results for check

Re-binding s/*explain-out* has no effect on the results of clojure.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)
;; (or alter-var-root - see doc/
(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 specs

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/defmsg to add a human-readable error message to the spec.

(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

You can also reuse messages by defining specs in terms of other specs:

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

;; 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

expound and expound-str can be configured with options:

(expound/expound :example/place {:city "Denver", :state :CO} {:print-specs? false :theme :figwheel-theme})

or, to configure the global printer:

(set! s/*explain-out* (expound/custom-printer {:show-valid-values? true :print-specs? false :theme :figwheel-theme}))
;; (or alter-var-root - see doc/
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)
;; (or alter-var-root - see doc/
(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}))
;; (or alter-var-root - see doc/
(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}))
;; (or alter-var-root - see doc/
(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 [clojure.spec.alpha :as s]
            [clojure.test :as test :refer-macros [run-tests]]
            [expound.alpha :as expound]
            ;; require your namespaces here


(set! s/*explain-out* expound/printer)
;; (or alter-var-root - see doc/

;; 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 (:clojure.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 Expound as printer for 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)
;; (or alter-var-root - see doc/
(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 © 2017-2022 Ben Brinckerhoff

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