Matcher Combinators Save

Library for creating matcher combinator to compare nested data structures

Project README


Library for making assertions about nested data structures.

current version:

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docs: Found on cljdoc

Clojure version compatibility: 1.8 and up


matcher-combinators is maintained by:

For questions and more info, please use the Clojurians channel #matcher-combinators.


Clojure's built-in data structures get you a long way when trying to codify and solve difficult problems. A solid selection of core functions allow you to easily create and access core data structures. Unfortunately, this flexibility does not extend to testing: we seem to be missing a comprehensive yet extensible way to assert that the data fits a particular structure.

This library addresses this issue by providing composable matcher combinators that can be used as building blocks to test functions that evaluate to nested data-structures more effectively.


  • Matchers for scalar and structural values
    • Good readability supported by default interpretations of Clojure types as matchers
  • Pretty-printed diffs when the actual result doesn't match the expected matcher
  • Integration with clojure.test and midje



Require the matcher-combinators.test namespace, which will extend clojure.test's is macro to accept the match? and thrown-match? directives.

  • match?: The first argument should be the matcher-combinator represented the expected value, and the second argument should be the expression being checked.
  • thrown-match?: The first argument should be a throwable subclass, the second a matcher-combinators, and the third the expression being checked.

For example:

(require '[clojure.test :refer [deftest is]]
         '[matcher-combinators.test] ;; adds support for `match?` and `thrown-match?` in `is` expressions
         '[matcher-combinators.matchers :as m])

(deftest test-matching-with-explicit-matchers
  (is (match? (m/equals 37) (+ 29 8)))
  (is (match? (m/regex #"fox") "The quick brown fox jumps over the lazy dog")))

(deftest test-matching-scalars
  ;; most scalar values are interpreted as an `equals` matcher
  (is (match? 37 (+ 29 8)))
  (is (match? "this string" (str "this" " " "string")))
  (is (match? :this/keyword (keyword "this" "keyword")))
  ;; regular expressions are handled specially
  (is (match? #"fox" "The quick brown fox jumps over the lazy dog")))

(deftest test-matching-sequences
  ;; A sequence is interpreted as an `equals` matcher, which specifies
  ;; count and order of matching elements. The elements, themselves,
  ;; are matched based on their types.
  (is (match? [1 3] [1 3]))
  (is (match? [1 odd?] [1 3]))
  (is (match? [#"red" #"violet"] ["Roses are red" "Violets are ... violet"]))

  ;; use m/prefix when you only care about the first n items
  (is (match? (m/prefix [odd? 3]) [1 3 5]))

  ;; use m/in-any-order when order doesn't matter
  (is (match? (m/in-any-order [odd? odd? even?]) [1 2 3]))

  ;; NOTE: in-any-order is O(n!) because it compares every expected element
  ;; with every actual element in order to find a best-match for each one,
  ;; removing matched elements from both sequences as it goes.
  ;; Avoid applying this to long sequences.

(deftest test-matching-sets
  ;; A set is also interpreted as an `equals` matcher.
  (is (match? #{1 2 3} #{3 2 1}))
  (is (match? #{odd? even?} #{1 2}))
  ;; use m/set-equals to repeat predicates
  (is (match? (m/set-equals [odd? odd? even?]) #{1 2 3}))

  ;; NOTE: matching sets is an O(n!) operation because it compares every
  ;; expected element with every actual element in order to find a best-match
  ;; for each one, removing matched elements from both sets as it goes.
  ;; Avoid applying this to large sets.

(deftest test-matching-maps
  ;; A map is interpreted as an `embeds` matcher, which ignores
  ;; un-specified keys
  (is (match? {:name/first "Alfredo"}
              {:name/first  "Alfredo"
               :name/last   "da Rocha Viana"
               :name/suffix "Jr."}))))

(deftest test-matching-nested-datastructures
  ;; Maps, sequences, and sets follow the same semantics whether at
  ;; the top level or nested within a structure.
  (is (match? {:band/members [{:name/first "Alfredo"}
                              {:name/first "Benedito"}]}
              {:band/members [{:name/first  "Alfredo"
                               :name/last   "da Rocha Viana"
                               :name/suffix "Jr."}
                              {:name/first "Benedito"
                               :name/last  "Lacerda"}]
               :band/recordings []})))

(deftest test-matching-transformed-value-via-via
  ;; via applies read-string to the actual value "{:foo :bar}" before
  ;; matching against the expected value {:foo :bar}
  (is (match? {:payloads [(m/via read-string {:foo :bar})]}
              {:payloads [\"{:foo :bar}\"]})))

(deftest exception-matching
  (is (thrown-match? clojure.lang.ExceptionInfo
                     {:foo 1}
                     (throw (ex-info "Boom!" {:foo 1 :bar 2})))))


We've deprecated support for Midje in matcher-combinators. We continue to ship with the matcher-combinators.midje namespace to avoid breaking changes, but we no longer include midje as a transitive dependency.


The matcher-combinators.standalone namespace provides an API for using matcher-combinators outside the context of a test framework.


Default matchers

When an expected value isn't wrapped in a specific matcher the default interpretation is:

  • all scalar and collection types except regex and maps: equals
  • regex: regex
  • map: embeds

You can use the matcher-for function to discover which matcher would be used for a specific value, e.g.

(require '[matcher-combinators.matchers :as matchers])

(matchers/matcher-for {:this :map})
;; => #function[matcher-combinators.matchers/embeds]

built-in matchers

  • equals operates over any scalar value or collection

    • scalars: matches when the given value is exactly the same as the expected.
    • map: matches when
      1. the keys of the expected map are equal to the given map's keys
      2. the value matchers of expected map matches the given map's values
        • Note: Given that the default matcher for maps is embeds, nested maps continue being matched with embeds (instead of also being matched with equals). Check out 'Overriding default matchers' below for instructions on how to match nested maps with equals too.
    • sequence: matches when the expected sequences's matchers match the given sequence. Similar to midje's (just expected)
    • set: matches when all the elements in the given set can be matched with a matcher in expected set and each matcher is used exactly once.
  • embeds operates over maps, sequences, and sets

    • map: matches when the map contains some of the same key/values as the expected map.
    • sequence: order-agnostic matcher that will match when provided a subset of the expected sequence. Similar to midje's (contains expected :in-any-order :gaps-ok)
    • set: matches when all the matchers in the expected set can be matched with an element in the provided set. There may be more elements in the provided set than there are matchers.
  • prefix operates over sequences

    matches when provided a (ordered) prefix of the expected sequence. Similar to midje's (contains expected)

  • in-any-order operates over sequences

    matches when the given a sequence that is the same as the expected sequence but with elements in a different order. Similar to midje's (just expected :in-any-order)

  • set-equals/set-embeds similar behavior to equals/embeds for sets, but allows one to specify the matchers using a sequence so that duplicate matchers are not removed. For example, (equals #{odd? odd?}) becomes (equals #{odd}), so to get around this one should use (set-equals [odd? odd]).

  • seq-of takes an expected matcher and creates a new matcher over a sequence, where each element matches the provided expected matcher. Analogous to clojure.core/every?, although seq-of expects a non-empty sequence.

  • any-of given any number of matchers, successfully matches if at least one of them matches.

  • all-of given any number of matchers, successfully matches if all of them match.

  • regex: matches the actual value when provided an expected-regex using (re-find expected-regex actual)

  • match-with: overrides default matchers for expected (scalar or arbitrarily deep structure) (see Overriding default matchers, below)

  • within-delta: matches numeric values that are within expected +/- delta (inclusive)

via matcher: transform the actual before matching

In some cases one might want to match a serialized string against a parsed data-structure.

Without help this might look like the following, which becomes tedious for deeply nested structures:

(let [result {:payloads ["{:foo :bar :baz :qux}"]}]
 (is (match? {:payloads [{:foo :bar}]}
      (update result :payloads (partial map read-string)))))

The via matcher can help us out with this:

(let [result {:payloads ["{:foo :bar :baz :qux}"]}]
  (is (match? {:payloads [(m/via read-string {:foo :bar})]}
              {:payloads result})))

via, when paired with match-with, can be used to apply actual pre-processing before applying an underlying matcher:

(testing "using `match-with` + `via` we can sort the actual result before matching"
  (is (match? (m/match-with
               [vector? (fn [expected] (m/via sort expected))]
               {:payloads [1 2 3]})
              {:payloads (shuffle [3 2 1])}))))

In this example we decorate vector?'s matcher to first sort the actual and then do matching. When operating over sort-able values this can be a stand-in for the computationally slower in-any-order.

negative matchers

Negative matchers, that is, those asserting the absence of something, are generally discouraged due to the adverse effect they can have on code readability.

  • mismatch: negation matcher that takes in an expected matcher and passes when it doesn't match the actual. For example, to assert the absence of an entry in a list (is (match? (mismatch (embeds [odd?])) actual)). Considering the mental burden of reasoning about negation, please use sparingly.
  • absent: for use in the context of maps. Matches when the actual map is missing the key pointing to the absent matcher. For example (is (match? {:a absent :b 1} {:b 1})) matches but (is (match? {:a absent :b 1} {:a 0 :b 1})) won't. absent should only be used when the absence of a key is behaviourly important.
readability concerns with negation matchers
(deftest avoid-negative-matchers
  (testing "normal assertion that `:a` is present"
    (match? {:a any?}
  (testing "double negation version"
    (match? (matcher-combinators.matchers/mismatch {:a matcher-combinators.matchers/absent})

building new matchers

You can extend your data-types to work with matcher-combinators by implemented the Matcher protocol.

In the Matcher protocol -name and -matcher-for are largely boilerplate while the important implementation is -match, who should return a map adhering to the result spec.

Overriding default matchers

Inside the context of match? (clojure.test) / match (midje), data-structures are assigned default matchers, which eliminates the need to wrap data-structures with matcher-combinators when your desired matching behavior matches the defaults.

But what if your desired matching behavior deviates from the defaults?

For example, if you want to do exact map matching you need to use a log of m/equals:

(deftest exact-map-matching-by-hand
  (is (match? (m/equals {:a (m/equals {:b (m/equals {:c odd?})})})
              {:a {:b {:c 1}}}))
  ;; without m/equals, the system defaults to m/embeds for maps,
  ;; which has looser matching properties
  (is (match? {:a {:b {:c odd?}}}
              {:a {:b {:c 1 :extra-c 0} :extra-b 0} :extra-a 0})))

This verbosity can be avoided by redefining the matcher data-type defaults using the match-with matcher:

(deftest exact-map-matching-with-match-with
  (is (match? (m/match-with [map? m/equals] {:a {:b {:c odd?}}}))
              {:a {:b {:c 1}}}))


Start nREPL

bb dev

(requires babashka to run bb commands)

Running tests

The project contains midje, clojure.test, and cljs.test tests.

bb test:clj   # run only Clojure tests
bb test:midje # run only Midje tests
bb test:node  # run only ClojureScript tests
bb test:browser # run ClojureScript tests in browser at `http://localhost:9158/`

Linting and formatting

Check formatting and linting:

bb lint

Auto-fix formatting and linting:

bb lint:fix
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