On Mon, 1 Apr 2019 at 09:22, Simon Peyton Jones simonpj@microsoft.com http://mailto:simonpj@microsoft.com wrote: That does look odd. Can you explain how to reproduce this with HEAD? That is, are we getting a newtype constructor in the argument position of an StgApp in HEAD too? Newtype “data constructors” are always inlined, which is why I don’t understand what’s happening. OK, that’s good to know. I must be missing a Core-to-Core ot Core-to-STG transform step. I’ll try to make a single-file repro case against HEAD and see what happens. Maybe in doing so I’ll uncover what caused this output for me. On Mon, 1 Apr 2019 at 09:49, Ömer Sinan Ağacan omeragacan@gmail.com http://mailto:omeragacan@gmail.com wrote: I’m a bit confused about your findings because I’m unable to reproduce them. I wasn’t aware that we’re generating terms for newtype constructors, and when I try I can see that this is really the case. For example, when I compile a module with just this line: newtype MyInt = MyInt Int in the Core or STG dumps I don’t see any terms for the MyInt constructor. I also checked the Core for System.Timeout.timeout module, and I don’t see any Timeout constructor applications anywhere. I compiled with -O0 -ddump-simpl. Here’s what I get for the fmap Timeout application in that module: Right, I see in yours that the newtype is properly removed and mine isn’t (via -ddump-simpl): -- RHS size: {terms: 201, types: 218, coercions: 11, joins: 0/1}timeout :: forall a. Int -> IO a -> IO (Maybe a) [GblId, Arity=2]timeout = \ (@ a_a27I4) (n_a27GX :: Int) (f_a27GY :: IO a_a27I4) -> case < @ Int GHC.Classes.$fOrdInt n_a27GX (GHC.Types.I# 0#) of { False -> case == @ Int GHC.Classes.$fEqInt n_a27GX (GHC.Types.I# 0#) of { False -> case rtsSupportsBoundThreads of { False -> >>= @ IO GHC.Base.$fMonadIO @ ThreadId @ (Maybe a_a27I4) myThreadId (\ (pid_a27Hb :: ThreadId) -> >>= @ IO GHC.Base.$fMonadIO @ Timeout @ (Maybe a_a27I4) (fmap @ IO GHC.Base.$fFunctorIO @ Unique @ Timeout System.Timeout.Timeout <- *NEWTYPE HERE* newUnique) So I must be missing a pass somewhere. I’ll try to reproduce on HEAD and get back to you. Cheers! Chris