add Vector (a,b) -> (Vector a, Vector b) et al to Unboxed Vector and MVector

[cartazio]

I'd like this for Unboxed Vectors and MVector

roughly

Unbox (a,b) =>Vector (a,b) -> (Vector a,Vector b)
Unbox (a,b) => (Vector a,Vector b) -> Vector (a,b)
Unbox (a,b) => (MVector s a,MVector s b) -> MVector s (a,b)
Unbox (a,b) =>MVector (a,b) -> (MVector a,MVector b)

should do this for (,)...(,,,) and Complex at least

[cartazio]

https://github.com/wellposed/numerical/blob/d133ff5d982c5e83e711e1f364678a5455fe9b2b/src/Numerical/Array/Shape.hs#L481-L527
is some code i wrote that uses this, and I actually crucially need these sorts of transformations in another module of my code

or just export the constructors, because its totally type safe

[ttuegel]

If there is any concern about exporting the constructors, there should at least be zero-copy versions of these functions.

[cartazio]

Looking at how the unboxed tuples instances are defined, I think these have to be zero copy helper functions

eg

data instance Vector (a, b)    
      = V_2 {-# UNPACK #-} !Int  !(Vector a)  !(Vector b)

[cartazio]

and I guess in the tuple of (Vector a, Vector b) -> Vector (a,b) direction, they need to validate that the lenghts are the same

[neongreen]

unzip* and zip* are already provided for unboxed vectors and they are O(1): https://github.com/haskell/vector/blob/master/internal/unbox-tuple-instances

This issue should probably be closed.

[ttuegel]

We also need the same for Complex.

[yongqli]

It would be great if there was a way to expose the underlying "structure of arrays". It might be too much to ask for though. See ekmett/linear#61

[cartazio]

@yongqli i have something like that in my numerical repo
https://github.com/wellposed/numerical/blob/master/src/Numerical/Data/Vector/Pair.hs

[ekmett]

I personally would have no problem with exporting the constructors from somewhere.

[lehins]

Constructors for unboxed vectors have been exported from Data.Vector.Unboxed.Base from the beginning of time. It is an internal module as it very much should be, but it does not prevent others to shoot themselves in the foot.