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Today I was working in transforming some code that used Data.Vector to a more "memory efficient" Data.Vector.Unboxed when I suddenly realized a missing instance.

If I do this in ghci:

import qualified Data.Vector as V
V.singleton V.empty :: V.Vector (V.Vector Int)
[[]]

That is correct, Vector was printed correctly. However, if I try the same with Vector.Unboxed:

import qualified Data.Vector.Unboxed as VU
:set -XFlexibleContexts
VU.singleton VU.empty :: VU.Vector (VU.Vector Int)

I get the error:

<interactive>:10:1: error:
 • No instance for (VU.Unbox (VU.Vector Int))
 arising from a use of ‘VU.singleton’
 • In the expression:
 VU.singleton VU.empty :: VU.Vector (VU.Vector Int)
 In an equation for ‘it’:
 it = VU.singleton VU.empty :: VU.Vector (VU.Vector Int)

Which drives me to the question, how do I correctly implement an unboxed vector of vectors?

Side note:

The objective I was trying to reach was a generic splitChunks function, with the basic premise of splitting a vector into sub vectors of fixed size, a very primitive implementation using common vectors was as follows:

-- With an arbitrary vector and a positive integer of known size it will create a vector that has splitted the original into sections. Example:
-- vec = [1,2,3,4,5,6,7,8,9,10]
-- size = 4
-- result = [[1,2,3,4],[5,6,7,8],[9,10]]
splitChunks :: V.Vector p -> Int -> V.Vector (V.Vector p)
splitChunks vec size
 | size <= 0 = V.empty
 | otherwise = fst $ until condition opr (V.empty, vec)
 where condition = V.null . snd
 opr (x, y) = let (l, r) = V.splitAt size y in (V.snoc x l, r)

Then, when I tried to move from Vector to Vector.Unboxed, I got the same problem as described before.