I'm a big fan of trees and promises as more complete representatives of monads. Trees give us free monads, where `join` grafts the child tree contained in a lea

I'm a big fan of trees and promises as more complete representatives of monads. Trees give us free monads, where `join` grafts the child tree contained in a leaf onto the parent tree; and promises are a bit more viscerally monadic, as you really can't get at the value in a promise, since it probably isn't even there yet.