Hi, fusion can only happen when the appropriate rewrite rules have been defined in the library. For Int, that is the case: http://hackage.haskell.org/package/base-4.10.0.0/docs/src/GHC.Enum.html#line... But the enumeration for general numeric types, defined here: http://hackage.haskell.org/package/base-4.10.0.0/docs/src/GHC.Real.html#nume... does not have such a setup. Presumably it could be added, but enumerating doubles is bad style anyways; see https://ghc.haskell.org/trac/ghc/ticket/8012 Greetings, Joachim Am Mittwoch, den 23.08.2017, 14:00 +0100 schrieb Mateusz Kowalczyk:
Hi,
I had few minutes timeout waiting for CI today and I stumbled upon [1]. This is a many years old question and the optimal solution given there was using Vector. I thought that for such a problem it should not be necessary at all.
Indeed I wrote `mean :: [Int] -> Int` and got a much faster solution. However the original signature was `mean :: [Double] -> Int`. After trivial changes (change couple of places to Double and add a single fromIntegral) I expected the same result: after all, code is nearly exactly the same.
However to my disappointment, the code in question was much slower than the vector solution: how could this be? Looking in Core, I saw that GHC was not getting rid of [Double] like it was with [Int]. I was able to convince GHC to go back to fusing the list into the worker with a `map fromIntegral` but I am unhappy: why is this needed? I don't understand why GHC would not decide to fuse the initial attempt. Honestly it seems like a bug to me.
What are your thoughts? For reference here is the core with fromIntegral: as expected, GHC fuses the list and inserts int2Double# as it's generating it:
``` Main.$wgo = \ (w_s5xT :: GHC.Prim.Int#) (ww_s5xX :: GHC.Prim.Int#) (ww1_s5xY :: GHC.Prim.Double#) -> case w_s5xT of wild_Xz { __DEFAULT -> Main.$wgo (GHC.Prim.+# wild_Xz 1#) (GHC.Prim.+# ww_s5xX 1#) (GHC.Prim.+## ww1_s5xY (GHC.Prim.int2Double# wild_Xz)); 30000000# -> (# GHC.Prim.+# ww_s5xX 1#, GHC.Prim.+## ww1_s5xY 3.0e7## #) } ```
Contrast this with version that's commented out in [2]:
``` Main.$wgo = \ (w_s5vE :: [Double]) (ww_s5vI :: GHC.Prim.Int#) (ww1_s5vJ :: GHC.Prim.Double#) -> case w_s5vE of _ [Occ=Dead] { [] -> (# ww_s5vI, ww1_s5vJ #); : y_a3dU ys_a3dV -> case y_a3dU of _ [Occ=Dead] { GHC.Types.D# y1_a3dG -> Main.$wgo ys_a3dV (GHC.Prim.+# ww_s5vI 1#) (GHC.Prim.+## ww1_s5vJ y1_a3dG) } }
…
case Main.$wgo Main.main3 0# 0.0## … Main.main3 = GHC.Real.numericEnumFromTo @ Double GHC.Classes.$fOrdDouble GHC.Float.$fFractionalDouble Main.main5 Main.main4 … Main.main4 = GHC.Types.D# 3.0e7## … Main.main5 = GHC.Types.D# 1.0## ```
Why? There should be nothing stopping it from doing
```
Main.$wgo = \ (w_s5xT :: GHC.Prim.Double#) (ww_s5xX :: GHC.Prim.Int#) (ww1_s5xY :: GHC.Prim.Double#) -> case w_s5xT of wild_Xz { __DEFAULT -> Main.$wgo (GHC.Prim.+# wild_Xz 1#) (GHC.Prim.+# ww_s5xX 1#) (GHC.Prim.+## ww1_s5xY wild_Xz); 3.0e7## -> (# GHC.Prim.+# ww_s5xX 1#, GHC.Prim.+## ww1_s5xY 3.0e7## #) ```
Perhaps it's afraid to make the pattern match on a floating point number?
Insights welcome.
[1]: https://stackoverflow.com/questions/3300995/computing-the-mean-of-a-list-eff... [2]: https://stackoverflow.com/a/45840148/1432740 -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/
-- Joachim “nomeata” Breitner mail@joachim-breitner.de https://www.joachim-breitner.de/