Packed vectors : use these whenever possible. The regular vector type is just a gussied up linked list, but when vector functions are applied to these types, bracketed by pack and unpack, then things unfold into neatly optimized code.

Storable, Num, Fractional, Fold, Map, and ZipWith instances are provided for packed vectors, so some operations do not require pack/unpack. For example, dot does not require pack/unpack because it is defined in terms of zipWith and fold. However transpose, det, gaussElim and most others are recursive, and so you'll still need to use pack/unpack with these. This goes for multmm as well because it uses transpose, and multmv does not need its arguments to be unpacked, but the result will be a polymorphic vector of type (:.), so you will want to pack it again. This is all very experimental and likely to change.