[bedobi]

The sane thing to do is to let lower layer functions return

    Either<Error, Foo>

then, in the HTTP layer, your endpoint code just looks like

    return fooService
                .getFoo(
                    fooId
                ) //Either<Error, Foo>
                .fold({ //left (error) case
                    when (it) {
                        is GetFooErrors.NotAuthenticated -> { Response.status(401).build() }
                        is GetFooErrors.InalidFooId -> { Response.status(400).build() }
                        is GetFooErrors.Other -> { Response.status(500).build() }
                    }
                }, { //right case
                   Response.ok(it)
                })

the benefits of this are hard to overstate.

* Errors are clearly enumerated in a single place. * Errors are clearly separated from but connected to HTTP, in the appropriate layer. (=the HTTP layer) Developers can tell from a glance at the resource method what the endpoint will return for each outcome, in context. * Errors are guaranteed to be exhaustively mapped because Kotlin enforces that sealed classes are exhaustively mapped at compile time. So a 500 resulting from forgetting to catch a ReusedPasswordException is impossible, and if new errors are added without being mapped to HTTP responses, the compiler will let us know.

It beats exceptions, it beats Go's shitty error handling.