Question

I have a type

type T s r a = StateT [s] (ListT (Reader r)) a

I’d like to create a monadic value that yields each of the current states

states :: T a r a

But I’m feeling stupid today, and I can’t figure out how to transform a value of T a r [a] to T a r a.

states = do
  as <- get
  -- insert magic
  return a

Answer 1

Figured a way out

states = do
  as <- get
  a <- lift . ListT $ return as
  return a

Which can be compacted down to

states = get >>= lift . ListT . return

ok, feel smarter now