That's impossible, but that's not what's happening here. There is no "-> b" at the end of the type signature upthread. The example in the thread is more like

That's impossible, but that's not what's happening here. There is no "-> b" at the end of the type signature upthread. The example in the thread is more like foo :: Either (b -> Int) b which is much easier: foo = Left (const 1) :: Either (b -> Int) b