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Simpliest pattern matching you've ever seen.

Clojars Project


The main goal is to provide the simpliest DSL to describe pattern of expected value.


Not so easy to write tests with multiple asserts. Code grows fast, a lot of repetitions, hard to read.

(ns hello-world.core-test
  (:require [clojure.test :refer :all]
            [clojure.spec.alpha :as s]
            [matcho.core :refer :all :as m]))

(defn patch-article [patch]
  {:status 200
   :body   (assoc {:article (merge patch
                                   {:text "nice article text"})}
                  :article-id 1
                  :meta {:tags ["nature" "bears"]})})

(def patch {:title       "Article about bears"
            :description "Very good article"})
(def resp  (patch-article patch))

(s/def ::str-coll (s/coll-of string?))

(deftest general-patch-test
  (let [body (:body resp)]
    (is (< (:status resp) 300))
    (is (= (:title patch) (get-in body [:article :title])))
    (is (= (:description patch) (get-in body [:article :description])))
    (is (s/valid? ::str-coll (get-in body [:meta :tags])))))


More declarative and readable approach:

  • Describe part of the value with nodes, which have to be checked
  • Place a predicate, spec or regexp instead of node value or not
  • Assert with (m/assert pattern value)
  • Profit
(deftest matcho-patch-test
  (def pattern
    {:status #(< % 300)
     :body   {:article patch
              :meta    {:tags ::str-coll}}})
  (m/assert pattern resp))

Full example can be found here.



Add following project dependency to deps.edn:

{healthsamurai/matcho {:mvn/version "RELEASE"}}

Understand and pick out needed parts:


(ns hello-world.core
  (:require [clojure.test :refer :all]
            [clojure.spec.alpha :as s]
            [matcho.core :refer :all :as m]))

Spec-like interface

There are few main vars in core ns: valid?, explain-data, assert and dessert. First one is a function, which takes pattern and value returns true if value conforms the pattern and false in other case. The second one is a function, which returns a vector of errors or nil. The third one is a macro, it works the same way as valid?, but additionally asserts with is and provide a vector of errors using expalin-data. dessert is opposit to assert, test fails only if value is valid?.

(m/valid? [int? string?] [1 "test"])
;; => true

(deftest int-str-pair-test
  (m/assert [int? string?] [1 "test"]))

(m/explain-data [int? int? string?] [1 "test"])
;; => [{:expected "#function[clojure.core/int?]", :but "test", :path [1]} {:expected "#function[clojure.core/string?--5132]", :but nil, :path [2]}]

(deftest int-str-pair-fail-test
  (m/assert [int? int? string?] [1 "test"]))

;; [{:expected "#function[clojure.core/int?]", :but "test", :path [1]} {:expected "#function[clojure.core/string?--5132]", :but nil, :path [2]}] [1 "test"] [[#function[clojure.core/int?] #function[clojure.core/int?] #function[clojure.core/string?--5132]]]

(deftest dessert-test
  (m/dessert [int? int?] [1 "test"]))
;; is ok!

How it works?

matcho takes a data structure with "special" leaf nodes (a pattern) and smartly compares them with corresponding nodes in the original value. If nodes looks too different an explanation will be added to list of errors and the process will continue.

The pattern can be much smaller (has less keys, elements in vector and so on) than the original value and it is a common case. The open-world assumption implemented in matcho allows developer to check only "interesting" parts, but if it needed some parts of patterns can be marked as closed-world.

(m/valid? {:status 200} {:status 200 :body "ok"})
;; => true

(m/valid? {:status 200 :body string?} {:status 200})
;; => false

Strict match

In some cases it necessary to check that there are no additional elements in a vector or no additional keys in a map. To make sure that no sensetive data exposed for example. This can be done using metadata inside pattern. Needed node should be prepended with ^{:matcho/strict true} or ^:matcho/strict (alternative shorter form).

(deftest user-sensitive-data-test

  (testing "open-world exposes sensitive data"
      {:username string?
       :age      int?}}
      {:username "bob"
       :age      42
       :password "my-password"}}))

  (testing "closed-world will catch accidentially exposed password"
      {:username string?
       :age      int?}}
      {:username "bob"
       :age      42
       :password "my-password"}})))

(deftest vector-strict-match
  (def vector-123 [1 2 3])
  (m/assert [1 2] [1 2 3])
  (m/dessert ^:matcho/strict [1 2] [1 2 3])
  (m/assert ^:matcho/strict [1 2] [1 2])
  ;; ^:matcho/strict works only for current element of the pattern and
  ;; not inherited by nested nodes
  (m/assert ^:matcho/strict {:a [1 2]} {:a [1 2 3]}))

In examples above the presence of unnecessary will be catched. Strictness not inherited by child nodes of data structure.

Special leaf nodes

There are several options for pattern leaf values. It can be:

  • Any simple value
  • Regular expression
  • Clojure spec (keyword or spec directly)
  • Predicate (boolean-valued function)
(s/def ::pos-coll (s/coll-of pos?))

(deftest readme-test
  (is (m/valid? pos? 1))
  (m/assert 1 1)
  (m/assert {:status #(< % 300)
             :body   #(not (empty? %))}
            {:status 200
             :body   "hello"})
  (m/assert ::pos-coll [1 2 3])
  (m/assert [{:expected #"conforms.*pos-coll"}]
            (m/explain-data ::pos-coll [1 -1 2])))

More advanced examples can be found here.

Why not just use a clojure.spec?

Because matcho, that's why.

(def response {:status 200
               :body   "ok"})

(deftest with-spec-test
  (s/def ::status #(= 200 %))
  (s/def ::body #(not-empty %))
  (s/def ::response (s/keys :req-un [::status ::body]))
  (is (s/valid? ::response response)))

(deftest without-spec-test
  (m/assert {:status 200 :body not-empty} response))


Copyright © 2016 HealthSamurai

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.

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