Coalesce Optional Query Param to Nil

[heyltsjay]

In the following example I would expect some way to coalesce Field("password") to nil without Optionally, but it seems like there is no way to distinguish nil as being the default value itself, rather than the absence of one. I was surprised the compiler was happy with the pattern but that the tests were failing to parse app://[email protected]

public extension AppRoute {
    
    enum Auth: Equatable {
        case signIn(email: String, password: String?)
    }

    static let authRouter = OneOf {
        Route(.case(Auth.signIn)) {
            Path { "signIn" }
            Query {
                Field("email")
                Optionally {
                    Field("password")
                }
            }
        }
    }

}

[stephencelis]

@heyltsjay I think this is working as intended. All parser optionality is handled explicitly via Optionally, and the Field("name") initializer is a shorthand for Field("name", .string).

It might be possible that another overload could be introduced, but we're not sure result builder type inference and/or type ambiguity would interfere. For example, if it weren't for your Route mapping the String? to an enum's associated value, what kind of parser would this be in isolation?

let p = Field("email") // Parser<Substring, ???>

Some magic use of @_disfavoredOverload could work, and if you explore it by defining an overload of Field, please do let us know what you find, and we'd definitely consider a PR if such an overload doesn't cause other problems.

[stephencelis]

I'm also going to move this to a discussion since it's more about discussing a potential feature than a bug in the library.

[heyltsjay]

This all sounds fair enough, albeit breaking my mental model somewhat of how other optional query params are treated.

[heyltsjay]

The nuts and bolts of keeping the compiler happy is probably too academic for me, but I am a little surprised that it is happy to match against the optional whereas I don't think there is an outcome that would write nil without throwing an error.