Consider adding validators package

[sobolevn]

Currently a lot of python tools just throw exceptions at us or even uses: .is_valid() and .errors properties. It is not type-safe at all. Sometimes it is hard to explain to people how to really use type-driven design and make invalid states unreachable.

My idea is that we need to write thin wrappers around popular typed validation packages: like pydantic, cerebrus, schema, etc to return really typed things.

We also need to think about API to be expressive and recognisable enough.

Related: https://gist.github.com/maksbotan/14b4bebda2acab98cdd158f85a970855
CC @maksbotan

[sobolevn]

Shame on me! Not validators but parsers!

See https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/

[stereobutter]

there is also https://validators.readthedocs.io/en/latest/ which imho fortunately returns errors instead of raising them

[sobolevn]

1. https://hackage.haskell.org/package/monad-validate-1.2.0.0/docs/Control-Monad-Validate.html
2. https://github.com/debasishg/hask/blob/master/hfrdomain/README.md

[sobolevn]

Or we can add validate function to the pipeline.py.
Probably in the next release.

[sobolevn]

https://dry-rb.org/gems/dry-monads/1.3/validated/

[sobolevn]

https://typelevel.org/cats/datatypes/validated.html

[sobolevn]

So, it looks like Validated should be defined like so:

class Validated(Generic[_ValueType, _ErrorType]):
    _inner_value: Iterable[
        Callable[[_ValueType], Result[_ValueType, _ErrorType]],
    ]

It should work with all types: Maybe, FutureResult, IOResult, and Result, somehow. Union?

[sobolevn]

https://typelevel.org/cats/datatypes/validated.html#validated-vs-either

[maksbotan]

Take a look at http://hackage.haskell.org/package/these as well.

http://hackage.haskell.org/package/these-1.1/docs/Data-These.html#t:These
http://hackage.haskell.org/package/monad-chronicle-1/docs/Control-Monad-Chronicle.html

It's Monad instance has the same problems as Validated's, of course.

[sobolevn]

I am using These in fp-ts, it is quite useful when working with components in vue.
But, I don't know how one can use it in Python? Can you please share some usecases?

[astynax]

Here is another nice lib for validation: https://github.com/typeable/validationt
This one collects warnings but stops on errors - a pretty convenient behavior IMHO. Also you can get more than one warning for each field - it's nice too.