Add default_error parameter to converters.maybe_to_result (#1914)

Author: sobolevn

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]"