Library for creating matcher combinator to compare nested data structures
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Library for creating matcher combinator to compare nested data structures

current version:

Current Version

docs: Found on cljdoc


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: a comprehensive yet extensible way to assert that the data fits a particular structure seems to be lacking.

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


  • Pretty-printed diffs when the actual result doesn't match the expected matcher
  • Integrates with clojure.test and midje
  • Good readability by providing default interpretations of core clojure data-structures as matcher combinators
Midje checkers Matcher combinators
midje checkers matcher combinators check
Midje checkers failure output Matcher combinators failure output
midje checker failure output matcher combinators failure output



The matcher-combinators.midje namespace defines the match midje-style checker. This checker is used to wrap matcher-combinators to be used on the right-side of the fact check arrows

For example:

(require '[midje.sweet :refer :all]
         '[matcher-combinators.matchers :as m]
         '[matcher-combinators.midje :refer [match]])
(fact "matching a map exactly"
  {:a {:bb 1 :cc 2} :d 3} => (match (m/equals {:a (m/embeds {:bb 1}) :d 3}))
  ;; but when a map isn't immediately wrapped, it is interpreted as an `embeds` matcher
  ;; so you can write the previous check as:
  {:a {:bb 1 :cc 2} :d 3} => (match (m/equals {:a {:bb 1} :d 3})))

Note that you can also use the match checker to match arguments within midje's provided construct:

(unfinished f)
(fact "using matchers in provided statements"
  (f [1 2 3]) => 1
    (f (match [odd? even? odd?])) => 1))


Require the matcher-combinators.test namespace, which will extend clojure.test's is macro to accept the match? directive. The first argument to match? should be the matcher-combinator represented the expected value, and the second argument should be the actual value being checked.

For example:

(require '[clojure.test :refer :all]
         '[matcher-combinators.test] ;; needed for defining `match?`
         '[matcher-combinators.matchers :as m])
(deftest basic-sequence-matching
  ;; by default a sequentials are interpreted as a `equals` matcher
  (is (match? [1 odd?] [1 3]))
  (is (match? (m/prefix [1 odd?]) [1 1 2 3])))


default matchers

If a data-structure isn't wrapped in a specific matcher-combinator the default interpretation is:

  • map: embeds
  • sequential: equals
  • set: equals
  • number, date, and other base data-structure: equals
  • regex: regex

built-in matchers

  • equals operates over base values, maps, sequences, and sets

    • base values (string, int, function, etc.): 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
    • 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 arround this one should use (set-equals [odd? odd]).

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

building new matchers

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

An example of this in the wild can be seen in the abracad library here.

Running tests

The project contains both midje and clojure.test tests.

Midje is capable of running both types of tests:

lein midje