How to match error with boolean function?

[kaoet]

I want to test whether my err is "not exist", I found os.IsNotExist() in the standard library. How can I use it in my tests? I'm wondering something like

Expect(err).To(MatchError(os.IsNotExist))

But that doesn't work.

[blgm]

Expect(err).NotTo(HaveOccurred())

Does this help?

[onsi]

or if you want to assert the exact error you can:

Expect(err).To(MatchError(os.ErrNotExist))

that's the actual error. os.IsNotExist() is a function - if you wanted to use it you would:

Expect(os.IsNotExist(err)).To(BeTrue())

[kaoet]

@onsi Thanks for helping.
Actually, I over simplified my use case. I was just taking os.IsNotExist() as an example.

My original use case is that, I have a library, which returns library specific error and provides IsXXXError() functions just like os.IsNotExist(). But that library doesn't provide a convient way to construct an error of specific type like os.ErrNotExist.

What I want to test is wheter my error is nil or is some error matching IsXXXError(). I want to write something like:

Expect(err).To(Or(BeNil(), MatchError(IsXXXError)))

As you can see, since I want to use it inside Or(), I must convert IsXXXError into a matcher.

I think those IsXXXError() functions, which have signature func(error)bool is a common pattern in Go. Can we support those functions as parameter of MatchError()?

Moreover, can we have a quick way to convert arbitrary func(...)bool to matchers rather than having to write an implementation of GomegaMatcher interface?

---

My current workaround is to use an if statement like:

if err := DoSomething(); err != nil {
  Expect(IsXXXError(err)).To(BeTrue())
}

But it's long and doesn't give me useful error message when it fails. Gomega just says "false is not true".

[onsi]

hey @kaoet I think your best bet here is to make one custom matcher that takes a func(...) bool as an argument and handles your use case.

Something like: BeNilOrSatisfy(f func(...) bool{}). You could then write tests like:

Expect(err).To(BeNilOrSatisfy(os.IsNotExist))

I imagine such a matcher would not be too hard to write - but let me know if you'd like some help with it.

[blgm]

A helper function is another approach:

err := DoSomething()
checkError(err)

func checkError(err error) {
  switch err.(type) {
    case nil, OtherAcceptableErrorType:
      return
    default:
      ExpectWithOffset(1, err).NotTo(HaveOccurred(), "extra message can be added here")
    }
  }
}

[blgm]

Hi @kaoet. Have the responses above been helpful?

[kaoet]

@blgm No, finally we implemented a MatchErrorFn() matcher in our own project, because it's so commonly used.

I'm always thinking MatchError() function in Gomega should support pass in a func (error) bool function as parameter in addition to string and error.