Add prisms or other functionality that tests a lens's applicability

[kstrafe]

Lenses can sometimes fail:

(lens-view (hash-ref-lens 'b) #hash((a . 0)))

This is especially troublesome for composed lenses that act on deeply nested structures. I'd love there to be a sort of lens-test? or lens-correct? to test a lens on some data.

[jackfirth]

Could you describe the composed lens you were working with and a broad overview of what you were trying to do with your data? There's multiple ways of handling this sort of problem, and the extra information would help.

[kstrafe]

Here are the composed lenses

https://github.com/BourgondAries/elevators/blob/master/source/lenses.rkt

Here is an example usage

https://github.com/BourgondAries/elevators/blob/master/source/utilities.rkt#L160-L162

It is sometimes possible that elevators contains no key (id) that's used in (hash-ref-lens id), and as such, this code crashes.

Ideally it'd be possible to check if the composed lens "works" on the data structure. Another possibility (specifically for hash-ref-lens) is to provide a second function that acts as a fallback/default key-value creator.

[jackfirth]

If the key isn't present, what would you like to do? Set a default key and continue? Or something else?

[kstrafe]

In the above case: Log a message and return the elevators structure unedited.

Something along the lines of

(if (lens-works? this:call elevators)
    ... ; do the transformation on elevators
    elevators)