Unit method like `IOResultE.from_value` must return `IOResult[_V, Exception]` type

[sobolevn]

Currently, they do return incorrect types:

reveal_type(IOResultE.from_value(int))
# => IOResult[int, Any]

# Should be:
# => IOResult[int, Exception]

To achieve what we want we might tweak the way we create an alias in some way:

1. We can use inheritance instead of aliasing
2. We can use some magic
3. mypy plugin?

[sobolevn]

The same applies for all aliases with E suffix: ResultE, IOResultE, FutureResultE, ReaderResultE, etc

[thepabloaguilar]

Inheritance does not work:

class IOResultE(IOResult[_ValueType, Exception]):
    pass

reveal_type(IOResultE.from_value(42))  # N: Revealed type is 'returns.io.IOResult[builtins.int*, builtins.Exception*]'

Output:

main:6: note: Revealed type is 'returns.io.IOResult[builtins.int*, Any]'

[thepabloaguilar]

I got it, the actual signature is:

class IOResult:
    def from_value(cls, inner_value: _NewValueType) -> 'IOResult[_NewValueType, Any]':
        ...

I've just adjusted the return type to:

class IOResult:
    def from_value(cls, inner_value: _NewValueType) -> 'IOResult[_NewValueType, _ErrorType]':
        ...

Test case:

- case: test_io_result_e_from_value
  disable_cache: true
  main: |
    from returns.io import IOResultE

    reveal_type(IOResultE.from_value(10))  # N: Revealed type is 'returns.io.IOResult[builtins.int*, builtins.Exception]'

[sobolevn]

@thepabloaguilar can you please test that these tests still work: https://github.com/dry-python/returns/blob/master/typesafety/test_io/test_ioresult_container/test_ioresult_typecast.yml ?

[thepabloaguilar]

Just one test was broken:

- case: ioresult_from_value
  disable_cache: true
  main: |
    from returns.io import IOResult

    reveal_type(IOResult.from_value(1))  # N: Revealed type is 'returns.io.IOResult[builtins.int*, Any]'

Output:

E   pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output: 
E   Expected:
E     main:3: note: Revealed type is 'returns.io.IOResult[builtins.int*, Any]' (diff)
E   Actual:
E     main:3: note: Revealed type is 'returns.io.IOResult[builtins.int*, <nothing>]' (diff)
E   
E   Alignment of first line difference:
E     E: ...io.IOResult[builtins.int*, Any]'
E     A: ...io.IOResult[builtins.int*, <nothing>]'

[sobolevn]

I remember that we had problems with <nothing> somewhere. It does not compose with something.

Possible somewhere in:

• Typing issue with chained Result #124
• Question on the algebra of the Result type #196
• python3.7.1 has incorrect NoReturn handling #134
• Make Success and Failure proper types/classes #140

[sobolevn]

Ok, I found it: #124

[sobolevn]

I am still not sure, why are we using Any?

[thepabloaguilar]

In fact we have this problem (Any) with IOSuccess and IOFailure too!

>>> reveal_type(IOSuccess(1))
# note: Revealed type is 'returns.io.IOResult[builtins.int*, Any]'

Unless we annotate the type explicit:

>>> io: IOResult[int, str] = IOSuccess(10)
>>> reveal_type(io)
# note: Revealed type is 'returns.io.IOResult[builtins.int*, builtins.str*]'