Started working on do-notation

Author: sobolevn

README.md

@@ -9,16 +9,17 @@ Monads for `python` made simple and safe.
 
 - Provides primitives to write declarative business logic
 - Fully typed with annotations and checked with `mypy`,
-  making your code type-safe as well
+  allowing you to write type-safe code as well
+- No operator overloading or other unpythonic stuff that makes your eyes bleed
 
 
 ## Inspirations
 
 This module is heavily based on:
 
-- https://github.com/dry-rb/dry-monads
-- https://github.com/dbrattli/OSlash
-- https://bitbucket.org/jason_delaat/pymonad
+- [dry-rb/dry-monads](https://github.com/dry-rb/dry-monads)
+- [Ø](https://github.com/dbrattli/OSlash)
+- [pymonad](https://bitbucket.org/jason_delaat/pymonad)
 
 
 ## License

dry_monads/do_notation.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+
+from functools import wraps
+from typing import TYPE_CHECKING, Callable, TypeVar
+
+from dry_monads.primitives.exceptions import UnwrapFailedError
+
+if TYPE_CHECKING:
+    from dry_monads.primitives.monad import Monad  # noqa: Z435, F401
+
+_MonadType = TypeVar('_MonadType', bound='Monad')
+
+
+def do_notation(
+    function: Callable[..., _MonadType],
+) -> Callable[..., _MonadType]:
+    """Decorator to enable 'do-notation' context."""
+    @wraps(function)
+    def decorator(*args, **kwargs) -> _MonadType:
+        try:
+            return function(*args, **kwargs)
+        except UnwrapFailedError as exc:
+            return exc.halted_monad
+    return decorator

dry_monads/either.py

@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
 
-from typing import Callable
+from typing import Callable, NoReturn
 
+from dry_monads.primitives.exceptions import UnwrapFailedError
 from dry_monads.primitives.monad import Monad, NewValueType, ValueType
 
 
@@ -36,6 +37,10 @@ def value_or(self, default_value: NewValueType) -> NewValueType:
         """Returns the value if we deal with 'Right' or default if 'Left'."""
         return default_value
 
+    def unwrap(self) -> NoReturn:
+        """Raises an exception, since it does not have a value inside."""
+        raise UnwrapFailedError(self)
+
 
 class Right(Either[ValueType]):
     """
@@ -74,6 +79,10 @@ def value_or(self, default_value: NewValueType) -> ValueType:
         """Returns the value if we deal with 'Right' or default if 'Left'."""
         return self._inner_value
 
+    def unwrap(self) -> ValueType:
+        """Returns the unwrapped value from the inside of this monad."""
+        return self._inner_value
+
 
 # Useful aliases for end users:
 

dry_monads/primitives/exceptions.py

@@ -0,0 +1,10 @@
+# -*- coding: utf-8 -*-
+
+
+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."""
+        super().__init__()
+        self.halted_monad = monad

dry_monads/primitives/monad.py

@@ -36,7 +36,7 @@ def fmap(self, function):  # pragma: no cover
 
         And returns a new functor value.
         """
-        raise NotImplementedError
+        raise NotImplementedError()
 
     @abstractmethod
     def bind(self, function):  # pragma: no cover
@@ -45,12 +45,22 @@ def bind(self, function):  # pragma: no cover
 
         And returns a new monad.
         """
-        raise NotImplementedError
+        raise NotImplementedError()
 
     @abstractmethod
-    def value_of(self, default_value):
+    def value_or(self, default_value):
         """Forces to unwrap value from monad or return a default."""
-        raise NotImplementedError
+        raise NotImplementedError()
+
+    @abstractmethod
+    def unwrap(self) -> ValueType:
+        """
+        Custom magic method to unwrap inner value from monad.
+
+        Should be redefined for ones that actually have values.
+        And for ones that raise an exception for no values.
+        """
+        raise NotImplementedError()
 
     def __str__(self) -> str:
         """Converts to string."""

setup.cfg

@@ -63,9 +63,12 @@ line_length = 79
 # The mypy configurations: http://bit.ly/2zEl9WI
 python_version = 3.6
 
+# We have disabled this checks due to some problems with `mypy` type
+# system, it does not look like it will be fixed soon.
+# disallow_any_explicit = True
+# disallow_any_generics = True
+
 check_untyped_defs = True
-disallow_any_explicit = True
-disallow_any_generics = True
 disallow_untyped_calls = True
 ignore_errors = False
 ignore_missing_imports = True