Int mean/median wrap at 2^63

[skymanbp]

median' (VU.fromList [maxBound - 1, maxBound :: Int]) gives -1.5 - the
sum wraps in Int. mean has the same problem.

Widening per element fixes it but adds a conversion in the hot loop
(#214 review), so it needs db-benchmark numbers first. Will send a PR
with benchmarks.

[mchav]

int2Double might be a better solution than the hand-rolled/specialized realToFrac. Something like:

sumDoubles :: Vector a -> Double
sumDoubles xs = case testEquality (typeRep @a) (typeRep @Double) of
    Just Refl -> VG.sum xs
    Nothing -> case testEquality (typeRep @a) (typeRep @Int) of
        Just Refl -> V.foldl' (\acc x -> acc + (int2Double x)) 0 xs
        Nothing -> -- other cases here

This means we branch once per container as opposed to branching and converting on every element of the loop.

[skymanbp]

int2Double might be a better solution than the hand-rolled/specialized realToFrac. Something like:

sumDoubles :: Vector a -> Double
sumDoubles xs = case testEquality (typeRep @A) (typeRep @double) of
Just Refl -> VG.sum xs
Nothing -> case testEquality (typeRep @A) (typeRep @int) of
Just Refl -> V.foldl' (\acc x -> acc + (int2Double x)) 0 xs
Nothing -> -- other cases here
This means we branch once per container as opposed to branching and converting on every element of the loop.

Yeah, thatd work. Ig calling it directly doesn't depend on the rules firing. I'll try it out.