Discussion: High-level functions that can convert between vector types

[nh2]

Edit: Issue description of the underlying problem https://github.com/haskell/vector/issues/373#issuecomment-808821411

Below is the original question that led to it:

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I often work with indices that I'd like to store in Unboxed.Vector Int vectors, and Generic.Vector vector a vectors into which those indices point.

At some point I have to resolve the indices back into the original generic vector type. I would like to do:

import qualified Data.Vector.Generic as VG
import qualified Data.Vector.Unboxed as VU

myIndexVector :: VU.Vector Int

atIndices :: (VG.Vector vector a)  => vector a -> VU.Vector Int -> vector a
atIndices vals indices = VG.map (vals VG.!) indices

(This is basically the same as myVector[indicesVector] does in Python's numpy.)

However in the above, VG.map (xs VG.!) indices does not typecheck, beause map requires that the input vector type be the same as the output vector type.

Thus it would be awesome to have a convertingMap function that combines map with convert, of type:

convertingMap :: (Vector v a, Vector w b) => (a -> b) -> v a -> w b

Edit: Solution for this specific function due to @lehins in https://github.com/haskell/vector/issues/373#issuecomment-808820750.

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Alternatives considered:

• convertingMap vals f = VG.map f (VG.convert vals) or VG.convert (VG.map f vals) isn't great, because it requires that v and w can store the same elements, which sometimes isn't the case, e.g. I can have a data D1 that only works with Unboxed and data D2 that only works with Storable.
• I currently use:

  convertingVectorMap :: (VG.Vector vectorA a, VG.Vector vectorB b) => (a -> b) -> vectorA a -> vectorB b
  convertingVectorMap f va =
    VG.generate (VG.length va) $ \i -> f (va VG.! i)
  

  but I don't know if that's optimal stream-fusion-wise.