Add default_error parameter to converters.maybe_to_result (#1914)

CHANGELOG.md

@@ -14,6 +14,8 @@ See [0Ver](https://0ver.org/).
 - Improve inference of `ResultLike` objects when exception catching
  decorator is applied with explicit exception types
 - Add picky exceptions to `impure_safe` decorator like `safe` has. Issue #1543
+- Adds `default_error` parameter to `returns.converters.maybe_to_result`,
+ which provides a default error value for `Failure`
 
 ### Misc
 

docs/pages/converters.rst

@@ -22,8 +22,8 @@ That's how they work:
 .. code:: python
 
  >>> from returns.converters import maybe_to_result, result_to_maybe
- >>> from returns.maybe import Maybe, Some
- >>> from returns.result import Result, Success
+ >>> from returns.maybe import Maybe, Some, Nothing
+ >>> from returns.result import Failure, Result, Success
 
  >>> result: Result[int, Exception] = Success(1)
  >>> maybe: Maybe[int] = result_to_maybe(result)
@@ -32,6 +32,9 @@ That's how they work:
  >>> new_result: Result[int, None] = maybe_to_result(maybe)
  >>> assert new_result == Success(1)
 
+ >>> failure_with_default: Result[int, str] = maybe_to_result(Nothing, 'abc')
+ >>> assert failure_with_default == Failure('abc')
+
 Take a note, that type changes.
 Also, take a note that ``Success(None)`` will be converted to ``Nothing``.
 

returns/converters.py

@@ -1,4 +1,4 @@
-from typing import TypeVar
+from typing import TypeVar, Union, overload
 
 from returns.functions import identity
 from returns.interfaces.bindable import BindableN
@@ -71,12 +71,30 @@ def result_to_maybe(
  return Nothing
 
 
+@overload
 def maybe_to_result(
  maybe_container: Maybe[_FirstType],
 ) -> Result[_FirstType, None]:
+ """No default case."""
+
+
+@overload
+def maybe_to_result(
+ maybe_container: Maybe[_FirstType],
+ default_error: _SecondType,
+) -> Result[_FirstType, _SecondType]:
+ """Default value case."""
+
+
+def maybe_to_result(
+ maybe_container: Maybe[_FirstType],
+ default_error: Union[_SecondType, None] = None,
+) -> Result[_FirstType, Union[_SecondType, None]]:
  """
  Converts ``Maybe`` container to ``Result`` container.
 
+ With optional ``default_error`` to be used for ``Failure``'s error value.
+
  .. code:: python
 
  >>> from returns.maybe import Some, Nothing
@@ -86,7 +104,9 @@ def maybe_to_result(
  >>> assert maybe_to_result(Some(None)) == Success(None)
  >>> assert maybe_to_result(Nothing) == Failure(None)
 
+ >>> assert maybe_to_result(Nothing, 'error') == Failure('error')
+
  """
  if is_successful(maybe_container):
  return Success(maybe_container.unwrap())
- return Failure(None)
+ return Failure(default_error)

typesafety/test_converters/test_maybe_to_result.yml

@@ -5,3 +5,12 @@
  from returns.maybe import Maybe
 
  reveal_type(maybe_to_result(Maybe.from_value(1))) # N: Revealed type is "returns.result.Result[builtins.int, None]"
+
+
+- case: maybe_to_result_default_error
+ disable_cache: false
+ main: |
+ from returns.converters import maybe_to_result
+ from returns.maybe import Maybe
+
+ reveal_type(maybe_to_result(Maybe.from_value(1), 'a')) # N: Revealed type is "returns.result.Result[builtins.int, builtins.str]"