# Pastebin vhNbTLNJ
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE LambdaCase #-}
{-# OPTIONS_GHC -Wall #-}

import Data.Conduit
import Data.Function (fix)

data A

data B

data C

f :: A -> Either B C
f = undefined

fc :: Monad m => ConduitT A C m (Maybe B)
fc = fix $ \loop -> do
  await >>= \case
    Nothing -> (pure Nothing)
    Just a -> case f a of
      Left b -> pure (Just b)
      Right c -> yield c *> loop

fc' :: Monad m => ConduitT A C m (Maybe B)
fc' = fix $ \loop ->
  await
    >>= maybe
      (pure Nothing)
      (either (pure . Just) ((*> loop) . yield) . f)