Support a "left" constructor, for example in EitherAsync

[kbirger]

Currently, it looks like Left values can be returned only by doing throwE. If I'm pursuing a functional approach, where errors are truly for exceptional cases, I would rather avoid going through the intermediate step of raising an error just to map to a Left value. Additionally, there are cases where a Left is not the result of an error thrown. For example, validation.

class-validator, and many other validation packages will already return something like a result object. Currently, I would have to do something like this:

      const validationResult = await dto.validate();
      if (validationResult.length > 0) {
        throwE(validationResult);
      }

I would rather simply have a helper left that I can return. It seems like this would also make code flow more consistent, since throwE throws, thereby interrupting other code, which decreases readability.

In JS, there is a performance penalty for throwing anything. This is especially true for Error types, because of stack generation, but is still true for other throwables.

[gigobyte]

I assume your example code is ran inside the EitherAsync constructor. If you're using async/await the code is supposed to look procedural on purpose. I think you would find using EitherAsync without syntax sugar more elegant (i.e. with map, chain and all of the other utility methods).

Regarding the performance aspect of things, I agree and I would not recommend using purify for performance-critical applications, especially for async code, as the abstractions provided by the library are quite costly. That's not to discourage people from using the library, I mean it in the sense that if you really care about performance I wouldn't recommend neither purify nor JavaScript as a language.