Is there a matcher that simultaneously asserts the type of an object and casts it?

[amomchilov]

• I have read CONTRIBUTING and have done my best to follow them.

I'm trying to make an assertion about the type of an error, and of a field on it. This requires me to cast.

This was my work-around:

expect(try somethingThatThrows()).to(throwError { error in
    expect(error).to(beAnInstanceOf(MyCustomError))
    expect((error as! MyCustomError).cause) == POSIXError(.ENOMEM)
}

Notice how I needed to force cast from Error to MyCustomError in order to gain access to the cause property. This force-cast shouldn't ever fail (because of the typecheck on the previous line), but it's still a little crufty.

Is there a build in matcher that can help with this?

Here are some other alternatives I considered:

expect(try somethingThatThrows()).to(throwError { error in
    if let error = error as? MyCustomError {
         expect(error.cause) == POSIXError(.ENOMEM)
    } else {
        fail()
    }

expect(try somethingThatThrows()).to(throwError { error in
    expect((error as? MyCustomError).cause) == POSIXError(.ENOMEM)
}

Environment

List the software versions you're using:

• Quick: 4.0.0
• Nimble: 9.2.0
• Xcode Version: 13.0 (13A5201i)
• Swift Version: 5.5

Please also mention which package manager you used and its version. Delete the
other package managers in this list:

• Swift Package Manager 5.5.0

[ikesyo]

• Nimble/Sources/Nimble/Matchers/ThrowError.swift

  Lines 138 to 141 in 2c44aee

  	public func throwError<T: Error, Out>(
  	errorType: T.Type,
  	closure: ((T) -> Void)? = nil
  	) -> Predicate<Out> {

• Nimble/Tests/NimbleTests/Matchers/ThrowErrorTest.swift

  Lines 55 to 59 in 2c44aee

  	// Typed closure over errorType argument
  	expect { throw EquatableError.parameterized(x: 42) }.to(throwError(errorType: EquatableError.self) { error in
  	guard case .parameterized(let x) = error else { fail(); return }
  	expect(x) >= 1
  	})

I think this overload is what you want.

expect(try somethingThatThrows()).to(throwError(errorType: MyCustomError.self) { error in
    expect(error.cause) == POSIXError(.ENOMEM)
}