Hi everybody, I'm playing around with concatMap in stream fusion (the vector package to be exact). concatMapM :: Monad m => (a->m (Stream m b)) -> Stream m a -> Stream m b concatMapM f (Stream ...) = ... I can get my concatMap to behave nicely and erase all Stream and Step constructors but due to the existential nature of the Stream seeds, they are re-boxed for the inner stream (which is kind-of annoying given that the seed is immediately unboxed again ;-). seq doesn't help here. Otherwise, fusion happens for streams and vectors, so that is ok. But boxing kills performance, criterion says. Do we have s.th. in place that could help here? Currently I could use the vector-concatMap which creates intermediate arrays, my version which has boxed seeds, or hermit but that is too inconvenient for non-ghc savy users. Viele Gruesse, Christian Fusing concatMapM: concatMapM f (SM.Stream ostep t _) = SM.Stream step (Left t) Unknown where step (Left t) = do r <- ostep t case r of SM.Done -> return $ SM.Done SM.Skip t' -> return $ SM.Skip (Left t') SM.Yield a t' -> do s <- f a return $ SM.Skip (Right (s,t')) step (Right (SM.Stream istep s _,t)) = do r <- istep s case r of SM.Done -> return $ SM.Skip (Left t) SM.Skip s' -> return $ SM.Skip (Right (SM.Stream istep s' Unknown,t)) SM.Yield x s' -> return $ SM.Yield x (Right (SM.Stream istep s' Unknown,t)) {-# INLINE [0] step #-} {-# INLINE [1] concatMapM #-} testConcatMapM :: Int -> Int testConcatMapM k = seq k $ U.unId . SM.foldl' (+) 0 . concatMap (\i -> SM.enumFromTo 5 k) $ SM.enumFromTo 3 k {-# NOINLINE testConcatMapM #-} CORE: testConcatMapM testConcatMapM = \ k_aCA -> let! { I# ipv_s1xv ~ _ <- k_aCA } in ### inner loop letrec { $s$wfoldlM'_loop_s29q $s$wfoldlM'_loop_s29q = \ sc_s29i sc1_s29j sc2_s29k -> ### unboxing let! { I# x_a1LA ~ _ <- sc1_s29j } in case tagToEnum# (<=# x_a1LA ipv_s1xv) of _ { False -> $s$wfoldlM'_loop1_s29c sc_s29i sc2_s29k; True -> $s$wfoldlM'_loop_s29q ### reboxing (+# sc_s29i x_a1LA) (I# (+# x_a1LA 1)) sc2_s29k }; ### outer loop $s$wfoldlM'_loop1_s29c $s$wfoldlM'_loop1_s29c = \ sc_s29a sc1_s29b -> case tagToEnum# (<=# sc1_s29b ipv_s1xv) of _ { False -> sc_s29a; True -> case tagToEnum# (<=# 5 ipv_s1xv) of _ { False -> $s$wfoldlM'_loop1_s29c sc_s29a (+# sc1_s29b 1); ### boxed seed (I# 6) True -> $s$wfoldlM'_loop_s29q (+# sc_s29a 5) (I# 6) (+# sc1_s29b 1) } }; } in let! { __DEFAULT ~ ww_s20G <- $s$wfoldlM'_loop1_s29c 0 3 } in I# ww_s20G