GoCrest - Hamcrest-like matchers for Go



A hamcrest-like assertion library for Go. GoCrest matchers are composable, self-describing and can be strung together in a more readable form to create flexible assertions.

Inspired by Hamcrest.

Build status Go Report Card GoDoc BCH compliance Coverage Status

Package import

import (


then.AssertThat(testing, "hi", is.EqualTo("bye").Reason("we are going"))


we are going
Expected: value equal to <bye>
     but: <hi>

Composed with AllOf:

then.AssertThat(t, "abcdef", is.AllOf(is.ValueContaining("abc"), is.LessThan("ghi")))

Matchers so far..

  • is.EqualTo(x)
  • is.EqualToIgnoringWhitespace(string) - compares two strings without comparing their whitespace characters.
  • is.Nil() - value must be nil
  • is.ValueContaining(expected) -- acts like containsAll
  • is.Not(m *Matcher) -- logical not of matcher's result
  • is.MatchForPattern(regex string) -- a string regex expression
  • has.FunctionNamed(string x) - checks if an interface has a function (method)
  • has.FieldNamed(string x) - checks if a struct has a field named x
  • is.AllOf(... *Matcher) - returns true if all matchers match
  • is.AnyOf(... *Matcher) - return true if any matcher matches
  • is.GreaterThan(expected) - checks if actual > expected
  • is.LessThan(expected)
  • is.Empty() - matches if the actual is "", nil or len(actual)==0
  • is.LessThan(x)
  • is.LessThanOrEqualTo(x)
  • is.GreaterThan(x)
  • is.GreaterThanOrEqualTo(x)
  • has.Length(x) - matcher if given value (int or matcher) matches the len of the given
  • has.Prefix(x) - string starts with x
  • has.Suffix(x) - string ends with x
  • has.Key(x) - map has key x
  • has.AllKeys(T x, T y) (or has.AllKeys([]T{x,y})) - finds key of type T in map
  • has.EveryElement(x1...xn) - checks if actual[i] matches corresponding expectation (x[i])
  • has.StructWithValues(map[string]*gocrest.Matcher) - checks if actual[key] matches corresponding expectation (x[key])

For more comprehensive documentation see godoc.

  • Matchers: has.ElementsWith and has.StructWithValues

    Matchers: has.ElementsWith and has.StructWithValues

    I implemented two new matchers:

    has.ElementsWith: Checks wether each element of an array/slice matches all expectations has.StructWithValues: Allows to check single struct fields

    Can be combined in a way that now we can check if f.e.:

        "id": "someId1",
        "someOmittedField": true
        "id": "someId2",
        "someOmittedField": false

    has structs that have an id field with the prefix someId

    opened by jpolack 6
  • Implement the gomock.Matcher interface

    Implement the gomock.Matcher interface

    I was using gomock and wanted to use our matchers. Sadly it didn't work out because our matchers don't implement the gomock.Matcher interface:

    type Matcher interface {
      // Matches returns whether x is a match.
      Matches(x interface{}) bool
      // String describes what the matcher matches.
      String() string

    To implement the gomock.Matcher interface we would have to:

    1. implement a .Matches method as a struct method, not only as a field
    2. implement the Stringer interface

    The Stringer interface should be fairly easy. But the Matches method would cause trouble, because Matcher already has a field called Matches of type func(actual interface{}) bool.

    At this point I see two options:

    1. add a mapping method from gocrest.Matcher to gomock.Matcher:

    A quick win, but kind of dirty because it doesn't really take advantage of go's interface system. Additionally it kind of breaks gomocks syntax:


    2. rename the Matches method on the Matcher struct:

    Seems like the cleaner way to me, but would cause a breaking change in how matchers are written. Would result in the following gomock syntax:


    I would love to discuss this and find a way that suites us best!

    opened by jpolack 2
  • does a NoError() matcher make sense?

    does a NoError() matcher make sense?


    since foo, err := bar() is a typical go style, I wonder, if a NoError() matcher would make sense to add?


    foo, err := bar()
    then.assertThat(t, err, is.NoError())

    What do you think? I would be open to create a PR ... but would like to get your view first :)

    opened by nitram509 1
  • Implement Testify custom matchers via mock.MatchedBy

    Implement Testify custom matchers via mock.MatchedBy

    Hi, I saw the recent gomock discussion and would love documentation on how to Implement Testify's custom matchers via mock.MatchedBy using this library.

    // Anything and AnythingOfType matchers
        On("Do", mock.Anything, mock.AnythingOfType("string"))
    // Custom matcher
        On("Do", 1, mock.MatchedBy(func(x string) bool {
            return strings.HasPrefix(x, "abc")
    opened by StevenACoffman 2
  • Describe only failed matchers in AnyOf

    Describe only failed matchers in AnyOf

    AnyOf reports all matcher's descriptions when it fails.

    Given a number of matchers, when less than all of them fail, we should only report the description of the failing matchers.

    opened by corbym 1
  • v1.0.5(Dec 21, 2020)

    Two more matchers added:

    • has.EveryElement(x1...xn) - checks if actual[i] matches corresponding expectation (x[i])
    • has.StructWithValues(map[string]*gocrest.Matcher) - checks if actual[key] matches corresponding expectation (x[key])

    Bugs fixed:


    Source code(tar.gz)
    Source code(zip)
  • v1.0.4(Nov 26, 2020)

  • v1.0.3(Feb 1, 2018)

    Added matchers:

    • has.TypeName - checks if an instance has the struct name x where x is a string or a matcher
    • shortcut matchers is.True and is.False
    Source code(tar.gz)
    Source code(zip)
  • v1.0.2(Jan 21, 2018)


    • has length panicked with nil actual Matt Corby 21/01/2018 20:48
    • godoc for AppendActual Matt Corby 21/01/2018 16:40
    • anyof and all of now carry actuals over Matt Corby 21/01/2018 16:36
    • Length matcher actual was only useful if you know the length of actual
    • not now carries the actual over.
    Source code(tar.gz)
    Source code(zip)
  • v1.0.1(Jan 17, 2018)

Java Dev, 20 years industry experience. Full stack developer (Team Lead) at eBay
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