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Add intersperse #20

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Add intersperse #20

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17 changes: 17 additions & 0 deletions src/Data/Vector/NonEmpty.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -180,6 +180,9 @@
, scanl, scanl', scanl1, scanl1', iscanl, iscanl'
, prescanr, prescanr', postscanr, postscanr'
, scanr, scanr', scanr1, scanr1', iscanr, iscanr'

-- * ??
, intersperse
) where


Expand All @@ -191,7 +194,7 @@

import qualified Data.Foldable as Foldable
import Data.Either (Either(..))
import Data.Functor hiding (unzip)

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import Data.Int
import Data.List.NonEmpty (NonEmpty(..))
import qualified Data.List.NonEmpty as NonEmpty
Expand Down Expand Up @@ -2646,3 +2649,17 @@
iscanr' :: (Int -> a -> b -> b) -> b -> NonEmptyVector a -> NonEmptyVector b
iscanr' f b = NonEmptyVector . V.iscanr' f b . _neVec
{-# INLINE iscanr' #-}

-- | \(\mathcal{O}(n)\). The 'intersperse' function takes an element and a NonEmptyVector
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@emilypi emilypi Nov 13, 2023 •
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It looks like you copypasted something like the implementation for intersperse from Data.List.NonEmpty, which isn't a very good implementation for something like vector or nonempty-vector for several reasons:

  1. This function is not O(n): cons for Vector is O(n), and vector consing for n-1-many elements is actually O(n^2). This makes the implementation rather unsatisfactory, and in fact, worse than the just converting to and from a list for very large vectors.

  2. There is an issue out to resolve the suboptimal vector cons-based implementation (posted here: Adding intersperse to the API haskell/vector#421), which discusses the potential streams-based implementation that I would welcome in this package when the time comes for that work to be done in vector. Otherwise, we'd be importing a very suboptimal implementation into this package, breaking from vector's API. Presumably such a streams-based implementation could amortize away some of the array resizing needed under the hood so we don't have to take the quadratic hit so hard.

  3. I think there's benefit in matching the vector API, since it makes this package a near drop-in replacement for vector, minus some fiddling and pattern matching to enforce the nonempty invariants in one's code. We'd be deviating from that until point 2 is implemented.

So i have to reject this on those grounds.

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@emilypi emilypi Nov 13, 2023 •
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At least, in its current incarnation. If you want to help the vector package move the work along for intersperse, I'm happy to keep this open and accept it with a deference to the vector implementation!

-- and \`intersperses\' that element between the elements of the NonEmptyVector.
--
intersperse :: a -> NonEmptyVector a -> NonEmptyVector a
intersperse sep vne =
let (h, t) = uncons vne
in consV h (prependToAll sep t)
{-# INLINE intersperse #-}

prependToAll :: a -> Vector a -> Vector a
prependToAll sep vec = case V.uncons vec of
Nothing -> V.empty
Just (h, t) -> V.cons sep (V.cons h (prependToAll sep t))
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