Constructors for immutable unboxed tuple vectors are not exported

[ruifengx]

See the following export list for Data.Vector.Unboxed:

vector/vector/src/Data/Vector/Unboxed.hs

Lines 61 to 63 in eb60526

	module Data.Vector.Unboxed (
	-- * Unboxed vectors
	Vector(V_UnboxAs, V_UnboxViaPrim), MVector(..), Unbox,

All the constructors for the MVector family are exported, which includes constructors for mutable unboxed tuple vectors (e.g., MV_2). However, for immutable unboxed tuple vectors, the constructors are private.

Currently, to construct an unboxed tuple vector from the two underlying parallel vector without copying, one must go through MVector and unsafeFreeze, which seems a bit weird to me.

import Data.Vector.Unboxed qualified as U
pairUnboxed :: U.Vector a -> U.Vector b -> U.Vector (a, b)
pairUnboxed xs ys = do
    xs' <- unsafeThaw xs
    ys' <- unsafeThaw ys
    let len = U.length xs
    unsafeFreeze (MV_2 len xs' ys')

[toyboot4e]

Related issues: #49, #492. Unboxed vector's constructor seem like exposed from Data.Vector.Unboxed.Base. (I'm not sure about the details).

[ruifengx]

Indeed, this seems like a duplicate of #492, then. I guess the only new point is that not exporting V_2 does not actually prevent the user from achieving the same functionality, just with more boilerplate. Maybe I should close this one in favour of #492?

[Shimuuar]

In fact zip/unzip, zip3/unzip3 etc exported from D.V.Unboxed and will grant access to underlying vectors. This is just poorly documented. Basically this is a documentation problem

[ruifengx]

Thanks! Somehow I did not think this way. I must have thought those functions are just aliases to the ones in D.V.Generic (for fusion, of course), and I did not find their implementations on Hackage. Checking their implementations in the included file unboxed-tuple-instances, it is indeed just what I was looking for:

vector/vector/internal/unbox-tuple-instances

Lines 123 to 130 in eb60526

	-- | /O(1)/ Zip 2 vectors.
	zip :: (Unbox a, Unbox b) => Vector a -> Vector b -> Vector (a, b)
	{-# INLINE_FUSED zip #-}
	zip as bs = V_2 len (unsafeSlice 0 len as) (unsafeSlice 0 len bs)
	where len = length as `delayed_min` length bs
	{-# RULES "stream/zip [Vector.Unboxed]" forall as bs .
	G.stream (zip as bs) = Bundle.zipWith (,) (G.stream as)
	(G.stream bs) #-}

Actually, in their documentation they are clearly marked O(1). Maybe it can be improved in discoverability by mentioning this fact at the very top where representations of unboxed vectors are discussed.

[Shimuuar]

they are clearly marked O(1)

That's simple puzzle but even simple puzzles don't make good documentation

[Shimuuar]

I hope that #505 is good enough so I'm closing it. @ruifengx ping if you disagree