Use vars to enable terser lens declarations

[marcprux]

Is there any reason vars shouldn't be used in data model structs? The struct will still be immutable in the sense that there is no shared mutable state, and it would permit lenses to be expressed without having the call the struct's initializer for each individual lens.

[CodaFi]

I like the idea of using benign effects to clean up our interface but this permits arbitrary mutation and (mutating) effects to be executed against the lens subject. One of the key points in the README is that our lenses are to be used to subsume the use of mutatable structures. This might make more sense as a motivating example with classes but our goal with structs is to be as immutable as possible both locally and globally.

[marcprux]

Mutability could be restricted to the module level with internal(set). Might that allay concerns about arbitrary mutation?

[CodaFi]

Mm, could be. If you wanted to include this (for lack of a better word) sugar, break it into its own file and create some documentation showing a motivating use case please. For now, I'd like to leave the existing tests alone but feel free to add your own.

[marcprux]

OK, I'll play around with it a bit. Thanks for the feedback.

[CodaFi]

Sounds good. Thanks for your efforts here, I hope we can get something merged. This is a great idea! ✨