#expect(throws:) on enum with associated value?

Using Swift
1

I'd like to write a test that expects this error:

enum Errors : Error, Equatable {
	case invalidToken(String, String.Index)
}

like so:

#expect(throws: Lexer.Errors.invalidToken)
{
	try lexer.tokenize(s)
}

I also tried appending .self, as shown in the docs. But I get Type '@Sendable (String, String.Index) -> Lexer.Errors' cannot conform to 'Equatable' (and Error).

I don't think there's any way to express this in Swift, is there?

What is the example doing in this section? It doesn’t show the definition of PizzaToppings.InvalidToppingError.

2

The self is used to refer to the type, not the associated case or its constructor. So #expect(throws: Lexer.Errors.self) should work.

You can also match a specific value with #expect(throws: Lexer.Errors.invalidToken(s, s.startIndex)), but there currently isn't a way to use wildcards _ like in a switch statement.

4 Likes
3

In Swift 6.1 it looks like #expect(throws:) returns the error caught. Combined with something like swift-case-paths, you can get something like:

let error = #expect(throws: Lexer.Errrors.self) {
  try lexer.tokenize(s)
}

#expect(error?.is(\.invalidToken) == true)
3 Likes
4

You don't actually ever need _. This is still horrible but I don't think it's worse than case paths:

// 1 "line"
#expect({
  if case .invalidToken = (#expect(throws: Lexer.Errors.self) { try lexer.tokenize(s) })
  { true } else { false }
}())

// 2 "lines"
let error = #expect(throws: Lexer.Errors.self) { try lexer.tokenize(s) }
#expect({
  if case .invalidToken = error { true } else { false }
}())
5

As it turns out, this is what I need to do anyway (I was taking the resulting values and testing them separately, but this is much tidier). E.g:

s = "‑3⁄8"
var error = #expect(throws: Lexer.InvalidTokenError.self)
{
    tokens = try lexer.tokenize(s)
}
#expect(error?.source == "‑3⁄8")
#expect(error?.location == error?.source.startIndex)

vs

s = "‑3⁄8"
#expect(throws: Lexer.Errors.invalidToken(source: s, range: s.startIndex ..< s.index(after: s.startIndex)))
{
    tokens = try lexer.tokenize(s)
}