Why does ghc generate so many useless-looking nested case expressions?

Question

I have some Haskell code

indLoc index way = do
    Store p <- ask
    return $ plusPtr p localAddr
    where localAddr = (stHeader + stIndices + index + stIndices * way) * blockSize
--snip...
    (Just way, _) -> do
        liftIO $ pokeElemOff loc way a
        indLoc index way

Which is generating some rather strange-looking core:

  case GHC.Prim.writeInt64OffAddr#
         @ GHC.Prim.RealWorld a4_s4UV i_s4UW x_s4UX new_s_s4UY
  of s2_s4V0 { __DEFAULT ->
  let {
    sat_s528 :: GHC.Ptr.Ptr b_a1UV
    [LclId]
    sat_s528 =
      case r_s4Us `cast` ... of _ { GHC.Ptr.Ptr addr_s4Vb ->
      case index_s4Um of _ { GHC.Types.I# y_s4Vc ->
      case ds1_s4Ub of _ { (blk1_s529, offs_s4V8) ->
      case offs_s4V8 of _ { GHC.Types.I# d_s4Vk ->
      case GHC.Prim.*# 2 i_s4UW of sat_s4Vf { __DEFAULT ->
      case GHC.Prim.+# 3 y_s4Vc of sat_s527 { __DEFAULT ->
      case GHC.Prim.+# sat_s527 sat_s4Vf of sat_s526 { __DEFAULT ->
      case GHC.Prim.*# sat_s526 4096 of sat_s4Vi { __DEFAULT ->
      case GHC.Prim.plusAddr# addr_s4Vb sat_s4Vi
      of sat_s525 { __DEFAULT ->
      case GHC.Prim.plusAddr# sat_s525 d_s4Vk of sat_s524 { __DEFAULT ->
      GHC.Ptr.Ptr @ b_a1UV sat_s524
      }
      }
      }
      }
      }
      }
      }
      }
      }
      } } in
  (# s2_s4V0, sat_s528 #)
  }

The case expressions seem like no-ops. What are they there for? Strictness?

Answer 1

The case expression forces evaluation. Using let will build closures instead. Note, GHC Core differs from regular Haskell in the semantics of case.