Behavior of the inliner on imported class methods

[Simon Peyton-Jones]

Yes, that's wrong. Thank you.  Now fixed; see http://hackage.haskell.org/trac/ghc/ticket/4918

Simon

From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at 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
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