Consider adding IO monad

[sobolevn]

It might be a good idea to promote https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell idea in IO[] type.

Let's see an example:

from returns.io import IO, impure

@impure
def get_username() -> str:
     return input('Hi, what is your name?')

reveal_type(get_username())  # => IO[str]
get_username().split('@')  # nope! type error!
get_username().map(lambda name: name.split('@'))  # => IO[List[str]]

It is important to restrict any access to the internal state of this object.

[sobolevn]

Let's say we have this code:

def index_view(request, user_id):
     user = User.objects.get(id=user_id)  # io operation
     return render('index.html', { user: user })

In this case we cannot just use unsafe(User.objects.get)(...) since it won't render in the template.
We cannot even .map it or do any other monadic transformations. We will need to have something like this:

# returns/unsafe.py

def unsafe_io_unwrap(io_monad: IO[T]) -> T:
     return io_monad.__internal_state

And it will be possible to forbid this import by tools like https://github.com/seddonym/import-linter
So, you will achieve at least some safety!

All in all:

def index_view(request, user_id):
     user: IO[User] = impure(User.objects.get)(id=user_id)
     return render('index.html', { user: unsafe_io_unwrap(user) })

I know it looks not that good, since it is used in a single function just for the example.
But, using it across your whole code base is great.

[sobolevn]

Major con: you have to manually mark all your IO operations. No one will stop you from doing it wrong.

[sobolevn]

Closed with baf049e