expose general .lens(a => b, a => b => a)

[valyagolev]

Fixes #268.

This is badly typed because I couldn't figure out OpticParams machinery just yet (see ignored type-errors in tests). Sending this PR as a WIP in case I received any suggestions or warnings.

[akheron]

Thanks for the PR!

Instead of modifying src/index.ts you should add your changes to scripts/generate-index.ts for them to be correctly applied to all optic types. src/index.ts in generated by running scrips/generate-index.ts. (This limitation doesn't exist for the standalone optics, btw...)

You got pretty close to adding a lens constructor. The correct typings would look like this:

  lens<U>(
    view: (a: A) => U,
    update: (a: A, v: U) => A
  ): Lens<S, NextParams<T, Adapt<A, U>>, U>

This is very close to the generic iso. A limiting factor is that it's monomorphic, meaning that the parameter v of update must have the same type which view returns (U in this case). If you check what Adapt<A, U> HKT does, you see that it make sure that the type that is written must equal U, and in this case it produces the type A.

Building a polymorphic lens (or iso) would require more type juggling, and using it would also require the user to provide a HKT that tells what happens to the types upon a polymorphic write.

[valyagolev]

@akheron Thank you for the help, I updated the PR. I'm confused with the .iso case though. Somehow the last tests, around .iso, do not type check. Perhaps I'm missing something

[akheron]

You should remove all the polymorphic test cases since this lens is not polymorphic. You must pass the same type as the source when you set or modify. It would become more clear if the source type was just string, not string | number.

[valyagolev]

@akheron thank you for the explanation. Ready to merge?

[akheron]

Merged, thanks!