It does seem to depend highly on the context in which the array is used. Strictly defined algorithm with an off-by-one: bug. User-defined indexing as you say is

It does seem to depend highly on the context in which the array is used. Strictly defined algorithm with an off-by-one: bug. User-defined indexing as you say is a counter. I suppose it is hinging on arrays being used in less dynamic contexts where you're likely to know the length, so to save you from handling all the Nones. I think I'd prefer a parallel api that returns Option, so you don't need to duplicate the bounds checking.