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.