//I first tried :

fun Application.main() {    
    routing {
        route("/api"){
            this@routing.fooApi()
            this@routing.bazApi()
        }
    }
}

fun Routing.fooApi() {
    route("/foo"){
        route("/bar"){
            get{ DoMagic() }
        }
    }
}

fun Routing.bazApi() {
    route("/baz"){
        get{ DoMagic() }
    }
}

//Which doesn't work but this does :

fun Application.main() {    
    routing {
        fooApi()
        bazApi()
    }
}

fun Routing.fooApi() {
    route("/api"){
        route("/foo"){
            route("/bar"){
                get{ DoMagic() }
            }
        }
    }
}

fun Routing.bazApi() {
    route("/api"){
        route("/baz"){
            get{ DoMagic() }
        }
    }
}