github.com/onsi/gomega@v1.32.0/matchers/with_transform_test.go

package matchers_test

import (
	"errors"

	. "github.com/onsi/ginkgo/v2"
	. "github.com/onsi/gomega"
	. "github.com/onsi/gomega/matchers"
)

var _ = Describe("WithTransformMatcher", func() {

	var plus1 = func(i int) int { return i + 1 }

	Context("Panic if transform function invalid", func() {
		panicsWithTransformer := func(transform interface{}) {
			Expect(func() { WithTransform(transform, nil) }).WithOffset(1).To(Panic())
		}
		It("nil", func() {
			panicsWithTransformer(nil)
		})
		Context("Invalid number of args, but correct return value count", func() {
			It("zero", func() {
				panicsWithTransformer(func() int { return 5 })
			})
			It("two", func() {
				panicsWithTransformer(func(i, j int) int { return 5 })
			})
		})
		Context("Invalid number of return values, but correct number of arguments", func() {
			It("zero", func() {
				panicsWithTransformer(func(i int) {})
			})
			It("two", func() {
				panicsWithTransformer(func(i int) (int, int) { return 5, 6 })
			})
		})
		Context("Invalid number of return values, but correct number of arguments", func() {
			It("Two return values, but second return value not an error", func() {
				panicsWithTransformer(func(interface{}) (int, int) { return 5, 6 })
			})
		})
	})

	When("the actual value is incompatible", func() {
		It("fails to pass int to func(string)", func() {
			actual, transform := int(0), func(string) int { return 0 }
			success, err := WithTransform(transform, Equal(0)).Match(actual)
			Expect(success).To(BeFalse())
			Expect(err).To(HaveOccurred())
			Expect(err.Error()).To(ContainSubstring("function expects 'string'"))
			Expect(err.Error()).To(ContainSubstring("have 'int'"))
		})

		It("fails to pass string to func(interface)", func() {
			actual, transform := "bang", func(error) int { return 0 }
			success, err := WithTransform(transform, Equal(0)).Match(actual)
			Expect(success).To(BeFalse())
			Expect(err).To(HaveOccurred())
			Expect(err.Error()).To(ContainSubstring("function expects 'error'"))
			Expect(err.Error()).To(ContainSubstring("have 'string'"))
		})

		It("fails to pass nil interface to func(int)", func() {
			actual, transform := error(nil), func(int) int { return 0 }
			success, err := WithTransform(transform, Equal(0)).Match(actual)
			Expect(success).To(BeFalse())
			Expect(err).To(HaveOccurred())
			Expect(err.Error()).To(ContainSubstring("function expects 'int'"))
			Expect(err.Error()).To(ContainSubstring("have '<nil>'"))
		})

		It("fails to pass nil interface to func(pointer)", func() {
			actual, transform := error(nil), func(*string) int { return 0 }
			success, err := WithTransform(transform, Equal(0)).Match(actual)
			Expect(success).To(BeFalse())
			Expect(err).To(HaveOccurred())
			Expect(err.Error()).To(ContainSubstring("function expects '*string'"))
			Expect(err.Error()).To(ContainSubstring("have '<nil>'"))
		})
	})

	It("works with positive cases", func() {
		Expect(1).To(WithTransform(plus1, Equal(2)))
		Expect(1).To(WithTransform(plus1, WithTransform(plus1, Equal(3))))
		Expect(1).To(WithTransform(plus1, And(Equal(2), BeNumerically(">", 1))))

		// transform expects custom type
		type S struct {
			A int
			B string
		}
		transformer := func(s S) string { return s.B }
		Expect(S{1, "hi"}).To(WithTransform(transformer, Equal("hi")))

		// transform expects interface
		errString := func(e error) string {
			if e == nil {
				return "safe"
			}
			return e.Error()
		}
		Expect(nil).To(WithTransform(errString, Equal("safe")), "handles nil actual values")
		Expect(errors.New("abc")).To(WithTransform(errString, Equal("abc")))
	})

	It("works with negative cases", func() {
		Expect(1).ToNot(WithTransform(plus1, Equal(3)))
		Expect(1).ToNot(WithTransform(plus1, WithTransform(plus1, Equal(2))))
	})

	Context("failure messages", func() {
		When("match fails", func() {
			It("gives a descriptive message", func() {
				m := WithTransform(plus1, Equal(3))
				Expect(m.Match(1)).To(BeFalse())
				Expect(m.FailureMessage(1)).To(Equal("Expected\n    <int>: 2\nto equal\n    <int>: 3"))
			})
		})

		When("match succeeds, but expected it to fail", func() {
			It("gives a descriptive message", func() {
				m := Not(WithTransform(plus1, Equal(3)))
				Expect(m.Match(2)).To(BeFalse())
				Expect(m.FailureMessage(2)).To(Equal("Expected\n    <int>: 3\nnot to equal\n    <int>: 3"))
			})
		})

		When("transform fails", func() {
			It("reports the transformation error", func() {
				actual, trafo := "foo", func(string) (string, error) { return "", errors.New("that does not transform") }
				success, err := WithTransform(trafo, Equal(actual)).Match(actual)
				Expect(success).To(BeFalse())
				Expect(err).To(HaveOccurred())
				Expect(err.Error()).To(MatchRegexp(": that does not transform$"))
			})
		})

		Context("actual value is incompatible with transform function's argument type", func() {
			It("gracefully fails if transform cannot be performed", func() {
				m := WithTransform(plus1, Equal(3))
				result, err := m.Match("hi") // give it a string but transform expects int; doesn't panic
				Expect(result).To(BeFalse())
				Expect(err).To(MatchError("Transform function expects 'int' but we have 'string'"))
			})
		})
	})

	Context("MatchMayChangeInTheFuture()", func() {
		It("Propagates value from wrapped matcher on the transformed value", func() {
			m := WithTransform(plus1, Or()) // empty Or() always returns false, and indicates it cannot change
			Expect(m.Match(1)).To(BeFalse())
			Expect(m.(*WithTransformMatcher).MatchMayChangeInTheFuture(1)).To(BeFalse()) // empty Or() indicates cannot change
		})
		It("Defaults to true", func() {
			m := WithTransform(plus1, Equal(2)) // Equal does not have this method
			Expect(m.Match(1)).To(BeTrue())
			Expect(m.(*WithTransformMatcher).MatchMayChangeInTheFuture(1)).To(BeTrue()) // defaults to true
		})
	})
})