ability Emit a where
  emit : a ->{Emit a} ()

type Stream a = Stream ('{Emit a} ())

use Stream Stream
use Optional None Some

namespace Stream where

  unfold : s -> (s -> Optional (a, s)) -> Stream a
  unfold s f = Stream 'let
    -- step : (s -> Optional (a,s)) -> s ->{Emit a} ()
    step f s = match f s with
      None        -> ()
      Some (a, s) -> emit a
                     step f s
    step f s

---

I found a value of type a where I expected to find one of type a:

   11 |   unfold : s -> (s -> Optional (a, s)) -> Stream a
   12 |   unfold s f = Stream 'let
   13 |     -- step : (s -> Optional (a,s)) -> s ->{Emit a} ()
   14 |     step f s = match f s with
   15 |       None        -> ()
   16 |       Some (a, s) -> emit a
   17 |                      step f s
   18 |     step f s

  from right here:

    4 | type Stream a = Stream ('{Emit a} ())