I noticed a reoccurring help function across several applications that I've written, and I think it would be nice to have these in vector itself:

traverse_ ::
     (a -> m b)
  -> MVector (PrimState m) a
  -> m ()

foldM :: -- Should probably be named foldlM instead as in Data.Foldable, but vector uses foldM elsewhere
     (b -> a -> m b)
  -> b
  -> MVector (PrimState m) a
  -> m b

The implementations are both straightforward, and I'm happy to implement this if others find it worthwhile. There is the possibility of a traverse that either (a) mutates the vector in-place or (b) creates a new mutable vector. There's more room to disagree there, so if anyone is interested in that, I think that discussing it on a separate issue would be better.