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Use eager indexing for !?#347

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treeowl wants to merge 1 commit into from
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Use eager indexing for !? #347

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9 changes: 7 additions & 2 deletions Data/Vector/Generic.hs
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Expand Up@@ -241,8 +241,13 @@ infixl 9 !?
-- | O(1) Safe indexing
(!?) :: Vector v a => v a -> Int -> Maybe a
{-# INLINE_FUSED (!?) #-}
v !? i | i < 0 || i >= length v = Nothing
| otherwise = Just $ unsafeIndex v i
-- Lengths are never negative, so we can check 0 <= i < length v
-- using one unsigned comparison.

@phadejphadejJan 13, 2021

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I don't understand this comment and logic. Is the idea that

fromIntegral (minBound :: Int) :: Word
9223372036854775808

is always bigger than any realistic length Vector could be?

I think that should explained more clearly, this is "too clever" otherwise.

@treeowltreeowlJan 13, 2021

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@phadej, right; a vector of length over maxBound :: Int will break assumptions all over the place. I can add a comment to that effect.

@BodigrimBodigrimJan 13, 2021

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Existing definition has a benefit of short-circuiting in compile time when i is a statically known negative number. But I do not think that this is an important scenario.

@ShimuuarShimuuarJan 14, 2021

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It's probably worth adding note that fromIntegral (i :: Int) :: Word > fromIntegral (maxBound :: Int) :: Word for all negative Ints. Such optimization is not entirely obvious

v !? i | (fromIntegral i :: Word) >= fromIntegral (length v)
= Nothing
| otherwise
-- Use basicUnsafeIndexM @Maybe to perform the indexing eagerly.
= basicUnsafeIndexM v i

-- | /O(1)/ First element
head :: Vector v a => v a -> a
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