Structural pattern matching not matching stored failure value

[ogabrielsantos]

Bug report

What's wrong

I'm trying to match some exceptions using structural pattern matching after reading the documentation, which says that I can match by result stored inside Failure (here):

from returns.result import Failure, Success, safe

@safe
def div(first_number: int, second_number: int) -> int:
    return first_number // second_number

match div(1, 0):
    # [...] omitted

    # Matches if the result stored inside `Failure` is `ZeroDivisionError`
    case Failure(ZeroDivisionError):
        print('"ZeroDivisionError" was raised')

    # [...] omitted

So, I've tried to implement my own code and looks like the first defined failure will be called, independent of it's value. Lets try it:

from returns.result import Failure, Success, Result

def foo(x) -> Result[float, str]:
    return Failure("Some dumb error")

match foo(10):
    case Failure(ZeroDivisionError):
        print("matched ZeroDivisionError failure")
    case Failure(str):
        print("matched string failure")
    case _:
        print("no failure matched")

# prints: matched ZeroDivisionError failure

How is that should be

Assuming above code, it should've printed matched string failure

System information

• python version: 3.10.5 (main, Jun 23 2022, 17:14:57) [Clang 13.1.6 (clang-1316.0.21.2.5)]
• returns version: 0.19.0
• mypy version: 0.961
• hypothesis version (if any): absent
• pytest version (if any): 6.2.5

[thepabloaguilar]

I'll take a look to see why this is happening!!

[thepabloaguilar]

Ok, I just found the problem and it's in our documentation 😆

To match correctly use parenthesis, Failure(ZeroDivisionError())! Could you test again using parenthesis @ogabrielsantos, please? I knew that I've tested a case with exceptions inside a Failure container: https://github.com/dry-python/returns/blob/master/tests/test_pattern_matching.py

cc @sobolevn

[ogabrielsantos]

@thepabloaguilar I've solved this issue yesterday reading the above mentioned test but had no time to get here and report it because I was out of office. Thanks!

For the documentation issue, how about a more non-generic example, showing the advantage/usage of Result for multiple Failure types?

[thepabloaguilar]

What do you have in mind? Feel free to submit a PR to us, help is always welcome!!