Haskell's "unboxed" vectors do this on a library level so `Vector (a, b)` is stored as `V_2 {-# UNPACK #-} !Int !(Vector a) !(Vector b)`. But to implement it fo

Haskell's "unboxed" vectors do this on a library level so `Vector (a, b)` is stored as `V_2 {-# UNPACK #-} !Int !(Vector a) !(Vector b)`. But to implement it for new types you need some boilerplate. Alternatively you could use template haskell or cpp to generate the boilerplate, though. The default instances use cpp https://github.com/haskell/vector/blob/master/internal/unbox-tuple-instances https://github.com/haskell/vector/blob/master/internal/unbox...