Variant of safe that preserves the argument that lead to an exception

[stereobutter]

I propose a function/decorator (maybe attempt would be a good name?) similar to safe with the twist of returning Failure(bad_input) from safe(f)(bad_input) instead of Failure(SomeException). An example where this would be useful:

(
    attempt(json.loads)('<hello world>')
    .alt(lambda s: ValueError(f'"{s}" is not a valid json')) 
)
# Failure(ValueError('"<hello world>" is not a valid json'))

[sobolevn]

Oh, I see. Do you want to send a PR?

[stereobutter]

I haven't worked with mypy plugins/decorators in a while so I'm not sure if there is a neat way to implement attempt (in a typesafe way) leveraging the existing implementation of safe? Of course one could just copy past the implementation for safe and its plugin but that is imho not a very nice solution. What do you think?

[sobolevn]

It can be a good be a start.

[stereobutter]

After looking into this for the last half hour I think getting this to typecheck without something like https://www.python.org/dev/peps/pep-0612/ will require its own plugin since one has to get at the types of the arguments of f in attempt(f). :(

[sobolevn]

I think so too, that's the big problem with mypy.

[thepabloaguilar]

I was thinking about the implementation here, one thing we need to define is what to do with functions that has multiple parameters, like, if my function has 3 arguments like:

def my_func(a, b, *, c):
    pass

my_func(1, 2, c=3)

What should be inside the Result? A tuple where the first element is the positional args and then the second the keyword args?

[thepabloaguilar]

Any thoughts @sobolevn??

[sobolevn]

I think that functions with multiple arguments should not be allowed in this case.