issue.hs

main :: IO ()
main = toJSONFilter $ \block -> do
  let blocks = toList . convertBlock $ block
  return blocks

-- Ambiguous type variable ‘m0’ arising from a use of ‘return’
-- prevents the constraint ‘(Monad m0)’ from being solved.

ok1.hs

main :: IO ()
main = toJSONFilter $ \block -> do
  let blocks = toList . convertBlock $ block
  return blocks :: IO [Block]

ok2.hs

main :: IO ()
main = toJSONFilter $ fmap (\blocks -> return blocks :: IO [Block]) $ toList . convertBlock