Typed exception

Author: sobolevn

dry_monads/do_notation.py

@@ -1,24 +1,18 @@
 # -*- coding: utf-8 -*-
 
 from functools import wraps
-from typing import TYPE_CHECKING, Callable, TypeVar, Union
+from typing import Callable
 
-from dry_monads.either import Either
 from dry_monads.primitives.exceptions import UnwrapFailedError
-
-if TYPE_CHECKING:  # pragma: no cover
-    from dry_monads.primitives.monad import Monad  # noqa: Z435, F401
-
-# We need to have this ugly type because there is no other way around it:
-_MonadType = TypeVar('_MonadType', bound=Union['Monad', Either])
+from dry_monads.primitives.types import MonadType
 
 
 def do_notation(
-    function: Callable[..., _MonadType],
-) -> Callable[..., _MonadType]:
+    function: Callable[..., MonadType],
+) -> Callable[..., MonadType]:
     """Decorator to enable 'do-notation' context."""
     @wraps(function)
-    def decorator(*args, **kwargs) -> _MonadType:
+    def decorator(*args, **kwargs) -> MonadType:
         try:
             return function(*args, **kwargs)
         except UnwrapFailedError as exc:

dry_monads/primitives/exceptions.py

@@ -1,10 +1,20 @@
 # -*- coding: utf-8 -*-
 
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:  # pragma: no cover
+    from dry_monads.primitives.types import MonadType  # noqa: Z435, F401
+
 
 class UnwrapFailedError(Exception):
     """Raised when a monad can not be unwrapped into a meaningful value."""
 
-    def __init__(self, monad) -> None:
-        """Saves halted monad in the inner state."""
+    def __init__(self, monad: 'MonadType') -> None:
+        """
+        Saves halted monad in the inner state.
+
+        So, this monad can later be unpacked from this exception
+        and used as a regular value.
+        """
         super().__init__()
         self.halted_monad = monad

dry_monads/primitives/types.py

@@ -0,0 +1,10 @@
+# -*- coding: utf-8 -*-
+
+from typing import TYPE_CHECKING, TypeVar, Union
+
+if TYPE_CHECKING:  # pragma: no cover
+    from dry_monads.either import Either  # noqa: Z435, F401
+    from dry_monads.primitives.monad import Monad  # noqa: Z435, F401
+
+# We need to have this ugly type because there is no other way around it:
+MonadType = TypeVar('MonadType', bound=Union['Monad', 'Either'])

tests/test_do_notation/test_do_notation.py

@@ -7,7 +7,7 @@
 @do_notation
 def _example1(number: int) -> Either[int, str]:
     first = Success(1).unwrap()
-    second = Success(number).unwrap() if number % 2 else Failure('E').unwrap()
+    second = Success(number).unwrap() if number else Failure('E').unwrap()
     return Success(first + second)
 
 
@@ -24,5 +24,5 @@ def test_do_notation_success():
 
 def test_do_notation_failure():
     """Ensures that do notation works well for Failure."""
-    assert _example1(6) == Failure('E')
+    assert _example1(0) == Failure('E')
     assert _example2(0) == Failure(0)