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