[Haskell-cafe] GHC fails to fuse [1 .. 30000000 :: Double] but fuses fromIntegral <$> [1 :: Int .. 30000000]?

[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-efficiently-in-haskell/45840148
[2]: https://stackoverflow.com/a/45840148/1432740
-- 
Mateusz K.