Have you tried adding another (dummy) method to the class? GHC used to have problems with optimising single-method classes in the past. Roman On 18 Jan 2011, at 10:33, José Pedro Magalhães wrote:

Hello all,

I fail to understand the behavior of the inliner in the following example:

module M1 where

class MyEnum a where myEnum :: [a]

instance MyEnum () where myEnum = [()]

module M2 where

import M1

f1 = map (\() -> 'p') [()] f2 = map (\() -> 'q') myEnum

The generated core code for M2 with ghc-7.0.1 -O is:

M2.f22 :: GHC.Types.Char [GblId, Caf=NoCafRefs, Str=DmdType m, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 2}] M2.f22 = GHC.Types.C# 'q'

M2.f11 :: GHC.Types.Char [GblId, Caf=NoCafRefs, Str=DmdType m, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 2}] M2.f11 = GHC.Types.C# 'p'

M2.f21 :: () -> GHC.Types.Char [GblId, Arity=1, Caf=NoCafRefs, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=1, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=True)}] M2.f21 = \ (ds_dch :: ()) -> case ds_dch of _ { () -> M2.f22 }

M2.f2 :: [GHC.Types.Char] [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=False, ConLike=False, Cheap=False, Expandable=False, Guidance=IF_ARGS [] 3 0}] M2.f2 = GHC.Base.map @ () @ GHC.Types.Char M2.f21 M1.$fMyEnum()_$cmyEnum

M2.f1 :: [GHC.Types.Char] [GblId, Caf=NoCafRefs, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 3}] M2.f1 = GHC.Types.: @ GHC.Types.Char M2.f11 (GHC.Types.[] @ GHC.Types.Char)

So, why does the inliner fail to get rid of the map in f2, while correctly ditching it in f1? Note that using two modules is essential here: if the instance is in M2 (and thus becoming orphan), the inliner works "correctly". Adding INLINE/INLINABLE pragmas to myEnum doesn't improve things either. Is this a bug, or is there a reason for this behavior?

Thanks, Pedro _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Yes, that's wrong. Thank you. Now fixed; see http://hackage.haskell.org/trac/ghc/ticket/4918 Simon From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of José Pedro Magalhães Sent: 18 January 2011 10:33 To: GHC users Subject: Behavior of the inliner on imported class methods Hello all, I fail to understand the behavior of the inliner in the following example: module M1 where class MyEnum a where myEnum :: [a] instance MyEnum () where myEnum = [()] module M2 where import M1 f1 = map (\() -> 'p') [()] f2 = map (\() -> 'q') myEnum The generated core code for M2 with ghc-7.0.1 -O is: M2.f22 :: GHC.Types.Char [GblId, Caf=NoCafRefs, Str=DmdType m, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 2}] M2.f22 = GHC.Types.C# 'q' M2.f11 :: GHC.Types.Char [GblId, Caf=NoCafRefs, Str=DmdType m, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 2}] M2.f11 = GHC.Types.C# 'p' M2.f21 :: () -> GHC.Types.Char [GblId, Arity=1, Caf=NoCafRefs, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=1, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=ALWAYS_IF(unsat_ok=True,boring_ok=True)}] M2.f21 = \ (ds_dch :: ()) -> case ds_dch of _ { () -> M2.f22 } M2.f2 :: [GHC.Types.Char] [GblId, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=False, ConLike=False, Cheap=False, Expandable=False, Guidance=IF_ARGS [] 3 0}] M2.f2 = GHC.Base.map @ () @ GHC.Types.Char M2.f21 M1.$fMyEnum()_$cmyEnum M2.f1 :: [GHC.Types.Char] [GblId, Caf=NoCafRefs, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, Cheap=True, Expandable=True, Guidance=IF_ARGS [] 1 3}] M2.f1 = GHC.Types.: @ GHC.Types.Char M2.f11 (GHC.Types.[] @ GHC.Types.Char) So, why does the inliner fail to get rid of the map in f2, while correctly ditching it in f1? Note that using two modules is essential here: if the instance is in M2 (and thus becoming orphan), the inliner works "correctly". Adding INLINE/INLINABLE pragmas to myEnum doesn't improve things either. Is this a bug, or is there a reason for this behavior? Thanks, Pedro