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