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When I compile

test :: (a -> a -> a) -> Int -> a -> HashMap Int a -> HashMap Int a
test f k a m = insertModifying a (blink (f a)) k m

blink :: (a -> b) -> a -> (# b #)
-- Or blink g = \a -> (# g a #) ; it makes no difference.
blink g a = (# g a #)

I get

test
  = \ (@ a_a9o3)
      (f_a7iP :: a_a9o3 -> a_a9o3 -> a_a9o3)
      (k_a7iQ :: Int)
      (a1_a7iR :: a_a9o3)
      (m_a7iS :: HashMap Int a_a9o3) ->
      case k_a7iQ of { GHC.Types.I# ww1_sa1Z ->
      RULES.$w$sinsertModifying
        @ a_a9o3
        a1_a7iR
        (let {
           g_s9E1 [Dmd=<L,C(U)>] :: a_a9o3 -> a_a9o3
           [LclId]
           g_s9E1 = f_a7iP a1_a7iR } in
         \ (a2_a7iU :: a_a9o3) -> (# g_s9E1 a2_a7iU #))
        ww1_sa1Z
        m_a7iS
      }

We build g_s9E1 = f_a7iP a1_a7iR for no apparent reason. Trouble persists into STG:

RULES.test
  :: forall a.
     (a -> a -> a)
     -> GHC.Types.Int
     -> a
     -> Data.HashMap.Base.HashMap GHC.Types.Int a
     -> Data.HashMap.Base.HashMap GHC.Types.Int a
[GblId,
 Arity=4,
 Str=<L,1*C1(C(U))><S(S),1*U(U)><L,U><S,1*U>,
 Unf=OtherCon []] =
    [] \r [f_saiX k_saiY a1_saiZ m_saj0]
        case k_saiY of {
          GHC.Types.I# ww1_saj2 [Occ=Once] ->
              let {
                g_saj3 [Occ=OnceL!, Dmd=<L,C(U)>] :: a_a9o3 -> a_a9o3
                [LclId] =
                    [f_saiX a1_saiZ] \u [] f_saiX a1_saiZ; } in
              let {
                sat_saj6 [Occ=Once] :: a_a9o3 -> (# a_a9o3 #)
                [LclId] =
                    [g_saj3] \r [a2_saj4]
                        let {
                          sat_saj5 [Occ=Once] :: a_a9o3
                          [LclId] =
                              [g_saj3 a2_saj4] \u [] g_saj3 a2_saj4;
                        } in  Unit# [sat_saj5];
              } in  RULES.$w$sinsertModifying a1_saiZ sat_saj6 ww1_saj2 m_saj0;
        };

insertModifying uses its function argument at most once, so there is no possible benefit to this partial application.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	nomeata
Operating system
Architecture