Array lens created through Prism sets array element to `undefined`

[tuxslayer]

When a Lens created through a Lens.index(...) prism with default value V it sets associated array element into undefined in being set with that value V.
E.g.:

const states = Atom.create([])  // Array of strings
states.set(['hidden', 'hidden'])
const lens = states.lens(Lens.index(0).compose(Lens.withDefault('hidden')))
// ...
lens.set('visible') // works just fine
lens.set('hidden') // sets states[0] to `undefined`

Two points here:

• I assume expected behavior is to return default value if array[i] is undefined by set whatever is passed to lens.set()
• Returning an optional from states.lens(Lens.index(0)) is not very convenient IMO

[blacktaxi]

Hi, thanks for reporting, I'll look into it – seems like a buggy behavior.

For your second point, returning an optional from an index lens is necessary to be type-safe, as the lens could be used to view the array for which the index will be out of bounds.

[blacktaxi]

Tracking this in #18