Here is a snippet from a real code that could benefit from non-recursive let. The example is notable because it incrementally constructs not one but two structures (both maps), ast and headers. The maps are constructed in a bit interleaved fashion, and stuffing them into the same State would be ungainly. In my real code -- Add an update record to a buffer file do_update :: String -> Handle -> Parsed -> [Parsed] -> IO () do_update "TAF" h ast asts@(_:_) = do rv <- reflect $ get_trange "TRange" ast headers0 <- return . M.fromList =<< sequence (map (\ (n,fld) -> reflect (fld ast) >>= \v -> return (n,v)) fields_header) let headers = M.insert "_report_ranges" (format_two_tstamps rv) headers0 foldM write_period (rv,headers,(snd rv,snd rv)) asts return () where write_period (rv,headers,mv) ast = do pv@(p_valid_from,p_valid_until) <- reflect $ get_trange "TRange" ast check_inside pv rv let prevailing = M.lookup "PREVAILING" ast (mv,pv) <- case prevailing of Just _ -> return (pv,pv) -- set the major valid period -- Make sure each VAR period occurs later than the prevailing -- period. If exactly at the same time add 1 min Nothing -> case () of _ | fst mv < p_valid_from -> return (mv,pv) _ | fst mv == p_valid_from -> return (mv,(p_valid_from + 60, p_valid_until)) _ -> gthrow . InvalidData . unwords $ [ "VAR period begins before prevailing:", show ast, "; prevailing TRange", show mv] let token = maybe (M.findWithDefault "" "VAR" ast) id prevailing let ast1 = M.insert "_token" token . M.insert "_period_valid" (format_two_tstamps pv) . M.unionWith (\_ x -> x) headers $ ast let title = M.member "Title" ast let headers1 = if title then headers else M.delete "_limit_to " . M.delete "_limit_recd" $ headers write_fields h ast1 fields return (rv,headers1,mv)