Daniel Fischer wrote:
Except that with optimisations turned on, GHC ties the knot for you (at least if always isn't exported). Without -fno-state-hack, the knot is tied so tightly that always (return ()) is never descheduled (and there's no leak).
Yes, I was concentrating on -O2, without -fno-state-hack.
With -fno-state-hack, I get
Rec { Main.main_always :: GHC.Types.IO () -> GHC.Types.IO () GblId [Arity 1 NoCafRefs Str: DmdType L] Main.main_always = \ (a_aeO :: GHC.Types.IO ()) -> let { k_sYz :: GHC.Types.IO () LclId [Str: DmdType] k_sYz = Main.main_always a_aeO } in (\ (eta_ann :: GHC.Prim.State# GHC.Prim.RealWorld) -> case (a_aeO `cast` (GHC.Types.NTCo:IO () :: GHC.Types.IO () ~ (GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)))) eta_ann of _ { (# new_s_anz, _ #) -> (k_sYz `cast` (GHC.Types.NTCo:IO () :: GHC.Types.IO () ~ (GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)))) new_s_anz }) `cast` (sym (GHC.Types.NTCo:IO ()) :: (GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)) ~ GHC.Types.IO ()) end Rec }
Which is always = \a_aeO -> let k_sYz = always a_aeO in a_aeO >> k_sYz specialised to IO, and with (>>) inlined. Where is the knot? regards, Bertram