#7994: Make foldl into a good consumer -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): The trick with `oneShot` is neat, and it works for `foo x = sum $ [f i | i <- [1 .. x]]` and `foo x = sum $ [f i | i <- [1 .. x] , odd i ]` (note the filter), but does not yield optimal code for nested iterations like `foo x = sum $ concat [[f i | i <- [1 .. n]]| n <- [1..x]]`, where we get: {{{ letrec { go_a1mU [Occ=LoopBreaker] :: GHC.Prim.Int# -> Data.Complex.Complex GHC.Types.Double -> Data.Complex.Complex GHC.Types.Double [LclId, Arity=1, Str=DmdType <L,U>] go_a1mU = \ (x_a1mV :: GHC.Prim.Int#) -> let { n_X1nv :: Data.Complex.Complex GHC.Types.Double -> Data.Complex.Complex GHC.Types.Double [LclId, Str=DmdType] n_X1nv = case GHC.Prim.tagToEnum# @ GHC.Types.Bool (GHC.Prim.==# x_a1mV ww_s1ro) of _ [Occ=Dead] { GHC.Types.False -> go_a1mU (GHC.Prim.+# x_a1mV 1); GHC.Types.True -> GHC.Base.id @ (Data.Complex.Complex GHC.Types.Double) } } in case GHC.Prim.tagToEnum# @ GHC.Types.Bool (GHC.Prim.># 1 x_a1mV) of _ [Occ=Dead] { GHC.Types.False -> letrec { go1_X1nG [Occ=LoopBreaker] :: GHC.Prim.Int# -> Data.Complex.Complex GHC.Types.Double -> Data.Complex.Complex GHC.Types.Double [LclId, Arity=2, Str=DmdType <L,U><L,1*U(U(U),U(U))>] go1_X1nG = \ (x1_X1nI :: GHC.Prim.Int#) (eta_B1 :: Data.Complex.Complex GHC.Types.Double) -> let { a_s1pu [Dmd=<L,U(U(U),U(U))>] :: Data.Complex.Complex GHC.Types.Double [LclId, Str=DmdType] a_s1pu = case eta_B1 of _ [Occ=Dead] { Data.Complex.:+ ww2_a10M ww3_a10N -> case ww2_a10M of _ [Occ=Dead] { GHC.Types.D# ww5_s1ub -> case ww3_a10N of _ [Occ=Dead] { GHC.Types.D# ww7_s1ud -> case Foo.f (GHC.Types.I# x1_X1nI) of _ [Occ=Dead] { Data.Complex.:+ ww9_a10Z ww10_a110 -> case ww9_a10Z of _ [Occ=Dead] { GHC.Types.D# ww12_s1uf -> case ww10_a110 of _ [Occ=Dead] { GHC.Types.D# ww14_s1uh -> Data.Complex.:+ @ GHC.Types.Double (GHC.Types.D# (GHC.Prim.+## ww5_s1ub ww12_s1uf)) (GHC.Types.D# (GHC.Prim.+## ww7_s1ud ww14_s1uh)) } } } } } } } in case GHC.Prim.tagToEnum# @ GHC.Types.Bool (GHC.Prim.==# x1_X1nI x_a1mV) of _ [Occ=Dead] { GHC.Types.False -> go1_X1nG (GHC.Prim.+# x1_X1nI 1) a_s1pu; GHC.Types.True -> n_X1nv a_s1pu }; } in go1_X1nG 1; GHC.Types.True -> n_X1nv }; } in go_a1mU 1 Foo.foo1; }}} I guess we’d want `n_X1nv` to have arity one here, and be eta-expanded, so that it turns into a join-point, do we? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7994#comment:9> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler