possible tuple elimination optimization?

[Dan Doel]

By the way, in case this helps your mental model, if you modify sumr to be:

    sumr n = snd $ as' 1 0
      where
      as' i s
        | i >= n = (i, s)
        | otherwise = ...

Then it has the same problem as sumh. Your original as' for sumr is
strict in s, but this modified one isn't.

This shows another way to fix sumh, too. Create a version of until
that separates out the part of the state that is only for testing.
Then the until loop will be strict in the result part of the state,
and the desired optimizations will happen (in this case):

    until' p step = go
     where
     go t r
       | p t = r
       | otherwise = uncurry go $ step (t, r)

-- Dan

On Sat, Mar 26, 2016 at 1:50 PM, George Colpitts
<george.colpitts at gmail.com> wrote:
> The following higher order function, sumh, seems to be 3 to 14 times slower
> than the equivalent recursive function, sumr:
>
> sumh :: Double -> Double
> sumh n =
>     snd $ until ((>= n) . fst) as' (1, 0)
>     where
>       as' (i,s) =
>           (i + 2, s + (-1) / i + 1 / (i + 1))
>
> sumr :: Double -> Double
> sumr n =
>     as' 1 0
>     where
>       as' i  s
>           | i >= n    = s
>           | otherwise = as' (i + 2) (s + (-1) / i + 1 / (i + 1))
>
> This is true in 7.10.3 as well as 8.0.1 so this is not a regression. From
> the size usage my guess is that this is due to the allocation of tuples in
> sumh. Maybe there is a straightforward way to optimize sumh but I couldn't
> find it. Adding a Strict pragma didn't help nor did use of
> -funbox-strict-fields -flate-dmd-anal. Have I missed something or should I
> file a bug?
>
> Timings from 8.0.1 rc2:
>
> ghc --version
> The Glorious Glasgow Haskell Compilation System, version 8.0.0.20160204
> bash-3.2$ ghc -O2 -dynamic sum.hs
> ghc -O2 -dynamic sum.hs
> [1 of 1] Compiling Main             ( sum.hs, sum.o )
> Linking sum ...
> bash-3.2$ ghci
> Prelude> :load sum
> Ok, modules loaded: Main.
> (0.05 secs,)
> Prelude Main> sumh (10^6)
> -0.6931466805602525
> it :: Double
> (0.14 secs, 40,708,016 bytes)
> Prelude Main> sumr (10^6)
> -0.6931466805602525
> it :: Double
> (0.01 secs, 92,000 bytes)
>
> Thanks
> George
>
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