# -*- coding: utf-8 -*-
from typing import Any, Callable, ClassVar, Generic, TypeVar, Union
from typing_extensions import final
from returns.context.requires_context import RequiresContext
from returns.primitives.container import BaseContainer
from returns.primitives.types import Immutable
from returns.result import Failure, Result, Success
# Context:
_EnvType = TypeVar('_EnvType', contravariant=True)
# Result:
_ValueType = TypeVar('_ValueType', covariant=True)
_NewValueType = TypeVar('_NewValueType')
_ErrorType = TypeVar('_ErrorType', covariant=True)
_NewErrorType = TypeVar('_NewErrorType')
# Helpers:
_FirstType = TypeVar('_FirstType')
[docs]class RequiresContextResult(
BaseContainer,
Generic[_EnvType, _ValueType, _ErrorType],
):
"""
The ``RequiresContextResult`` combinator.
See :class:`returns.context.requires_context.RequiresContext` for more docs.
This is just a handy wrapper around ``RequiresContext[env, Result[a, b]]``
which represents a context-dependent pure operation
that might fail and return :class:`returns.result.Result`.
It has several important differences from the regular ``Result`` classes.
It does not have ``Success`` and ``Failure`` subclasses.
Because, the computation is not yet performed.
And we cannot know the type in advance.
So, this is a thin wrapper, without any changes in logic.
Why do we need this wrapper? That's just for better usability!
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Success, Result
>>> def function(arg: int) -> Result[int, str]:
... return Success(arg + 1)
>>> # Without wrapper:
>>> assert RequiresContext.from_value(Success(1)).map(
... lambda result: result.bind(function),
... )(...) == Success(2)
>>> # With wrapper:
>>> assert RequiresContextResult.from_success(1).bind_result(
... function,
... )(...) == Success(2)
This way ``RequiresContextResult`` allows to simply work with:
- raw values and pure functions
- ``RequiresContext`` values and pure functions returning it
- ``Result`` and functions returning it
Imporatant implementation detail:
due it is meaning, ``RequiresContextResult``
cannot have ``Success`` and ``Failure`` subclasses.
We only have just one type. That's by design.
Different converters are also not supported for this type.
Use converters inside the ``RequiresContext`` context, not outside.
See also:
https://dev.to/gcanti/getting-started-with-fp-ts-reader-1ie5
https://en.wikipedia.org/wiki/Lazy_evaluation
https://bit.ly/2R8l4WK
https://bit.ly/2RwP4fp
"""
#: This field has an extra 'RequiresContext' just because `mypy` needs it.
_inner_value: Callable[
['RequiresContextResult', _EnvType],
Result[_ValueType, _ErrorType],
]
#: A convinient placeholder to call methods created by `.from_value()`.
empty: ClassVar[Any] = object()
def __init__(
self,
inner_value: Callable[[_EnvType], Result[_ValueType, _ErrorType]],
) -> None:
"""
Public constructor for this type. Also required for typing.
Only allows functions of kind ``* -> *``
and returning :class:`returns.result.Result` instances.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success
>>> str(RequiresContextResult(lambda deps: Success(deps + 1)))
'<RequiresContextResult: <function <lambda> at ...>>'
"""
super().__init__(inner_value)
def __call__(self, deps: _EnvType) -> Result[_ValueType, _ErrorType]:
"""
Evaluates the wrapped function.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success
>>> def first(lg: bool) -> RequiresContextResult[float, int, str]:
... # `deps` has `float` type here:
... return RequiresContextResult(
... lambda deps: Success(deps if lg else -deps),
... )
...
>>> instance = first(False)
>>> assert instance(3.5) == Success(-3.5)
In other things, it is a regular python magic method.
"""
return self._inner_value(deps)
[docs] def map( # noqa: A003
self, function: Callable[[_ValueType], _NewValueType],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]':
"""
Composes successful container with a pure function.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> assert RequiresContextResult.from_success(1).map(
... lambda x: x + 1,
... )(...) == Success(2)
>>> assert RequiresContextResult.from_failure(1).map(
... lambda x: x + 1,
... )(...) == Failure(1)
"""
return RequiresContextResult(lambda deps: self(deps).map(function))
[docs] def bind(
self,
function: Callable[
[_ValueType],
'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]',
],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]':
"""
Composes this container with a function returning the same type.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> def first(lg: bool) -> RequiresContextResult[float, int, int]:
... # `deps` has `float` type here:
... return RequiresContextResult(
... lambda deps: Success(deps) if lg else Failure(-deps),
... )
...
>>> def second(
... number: int,
... ) -> RequiresContextResult[float, str, int]:
... # `deps` has `float` type here:
... return RequiresContextResult(
... lambda deps: Success('>=' if number >= deps else '<'),
... )
...
>>> assert first(True).bind(second)(1) == Success('>=')
>>> assert first(False).bind(second)(2) == Failure(-2)
"""
return RequiresContextResult(
lambda deps: self(deps).bind(
lambda inner: function(inner)(deps), # type: ignore
),
)
[docs] def bind_result(
self,
function: Callable[[_ValueType], Result[_NewValueType, _ErrorType]],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]':
"""
Binds ``Result`` returning function to current container.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure, Result
>>> def function(number: int) -> Result[int, str]:
... if number > 0:
... return Success(number + 1)
... return Failure('<0')
...
>>> assert RequiresContextResult.from_success(1).bind_result(
... function,
... )(RequiresContextResult.empty) == Success(2)
>>> assert RequiresContextResult.from_success(0).bind_result(
... function,
... )(RequiresContextResult.empty) == Failure('<0')
>>> assert RequiresContextResult.from_failure(':(').bind_result(
... function,
... )(RequiresContextResult.empty) == Failure(':(')
"""
return RequiresContextResult(lambda deps: self(deps).bind(function))
[docs] def bind_context(
self,
function: Callable[
[_ValueType],
RequiresContext[_EnvType, _NewValueType],
],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]':
"""
Binds ``RequiresContext`` returning function to current container.
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Success, Failure
>>> def function(arg: int) -> RequiresContext[str, int]:
... return RequiresContext(lambda deps: len(deps) + arg)
...
>>> assert function(2)('abc') == 5
>>> assert RequiresContextResult.from_success(2).bind_context(
... function,
... )('abc') == Success(5)
>>> assert RequiresContextResult.from_failure(2).bind_context(
... function,
... )('abc') == Failure(2)
"""
return RequiresContextResult(
lambda deps: self(deps).map(
lambda inner: function(inner)(deps), # type: ignore
),
)
[docs] def fix(
self, function: Callable[[_ErrorType], _NewValueType],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]':
"""
Composes failed container with a pure function.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success
>>> assert RequiresContextResult.from_success(1).fix(
... lambda x: x + 1,
... )(...) == Success(1)
>>> assert RequiresContextResult.from_failure(1).fix(
... lambda x: x + 1,
... )(...) == Success(2)
"""
return RequiresContextResult(lambda deps: self(deps).fix(function))
[docs] def alt(
self, function: Callable[[_ErrorType], _NewErrorType],
) -> 'RequiresContextResult[_EnvType, _ValueType, _NewErrorType]':
"""
Composes failed container with a pure function.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> assert RequiresContextResult.from_success(1).alt(
... lambda x: x + 1,
... )(...) == Success(1)
>>> assert RequiresContextResult.from_failure(1).alt(
... lambda x: x + 1,
... )(...) == Failure(2)
"""
return RequiresContextResult(lambda deps: self(deps).alt(function))
[docs] def rescue(
self,
function: Callable[
[_ErrorType],
'RequiresContextResult[_EnvType, _ValueType, _NewErrorType]',
],
):
"""
Composes this container with a function returning the same type.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> def rescuable(arg: str) -> RequiresContextResult[str, str, str]:
... if len(arg) > 1:
... return RequiresContextResult(
... lambda deps: Success(deps + arg),
... )
... return RequiresContextResult(
... lambda deps: Failure(arg + deps),
... )
...
>>> assert RequiresContextResult.from_success('a').rescue(
... rescuable,
... )('c') == Success('a')
>>> assert RequiresContextResult.from_failure('a').rescue(
... rescuable,
... )('c') == Failure('ac')
>>> assert RequiresContextResult.from_failure('aa').rescue(
... rescuable,
... )('b') == Success('baa')
"""
return RequiresContextResult(
lambda deps: self(deps).rescue(
lambda inner: function(inner)(deps), # type: ignore
),
)
[docs] def value_or(
self, default_value: _FirstType,
) -> Callable[[_EnvType], Union[_ValueType, _FirstType]]:
"""
Returns a callable that either returns a success or default value.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> assert RequiresContextResult.from_success(1).value_or(2)(
... RequiresContextResult.empty,
... ) == 1
>>> assert RequiresContextResult.from_failure(1).value_or(2)(
... RequiresContextResult.empty,
... ) == 2
"""
return lambda deps: self(deps).value_or(default_value)
[docs] def unwrap(self) -> Callable[[_EnvType], _ValueType]:
"""
Returns a callable that unwraps success value or raises exception.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> assert RequiresContextResult(
... lambda _: Success(1),
... ).unwrap()(RequiresContextResult.empty) == 1
.. code::
>>> RequiresContextResult(
... lambda _: Failure(1),
... ).unwrap()(RequiresContextResult.empty)
Traceback (most recent call last):
...
returns.primitives.exceptions.UnwrapFailedError
"""
return lambda deps: self(deps).unwrap()
[docs] def failure(self) -> Callable[[_EnvType], _ErrorType]:
"""
Returns a callable that unwraps failure value or raises exception.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> assert RequiresContextResult(
... lambda _: Failure(1),
... ).failure()(RequiresContextResult.empty) == 1
.. code::
>>> RequiresContextResult(
... lambda _: Success(1),
... ).failure()(RequiresContextResult.empty)
Traceback (most recent call last):
...
returns.primitives.exceptions.UnwrapFailedError
"""
return lambda deps: self(deps).failure()
[docs] @classmethod
def lift(
cls,
function: Callable[[_ValueType], _NewValueType],
) -> Callable[
['RequiresContextResult[_EnvType, _ValueType, _ErrorType]'],
'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]',
]:
"""
Lifts function to be wrapped in a conatiner for better composition.
In other words, it modifies the function's
signature from: ``a -> b`` to:
``RequiresContextResult[env, a, err]``
-> ``RequiresContextResult[env, b, err]``
Works similar to :meth:`~RequiresContextResult.map`,
but has inverse semantics.
This is how it should be used:
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success
>>> def function(arg: int) -> str:
... return str(arg) + '!'
...
>>> unit = RequiresContextResult.from_success(1)
>>> deps = RequiresContextResult.empty
>>> assert RequiresContextResult.lift(function)(
... unit,
... )(deps) == Success('1!')
See also:
- https://wiki.haskell.org/Lifting
- https://github.com/witchcrafters/witchcraft
- https://en.wikipedia.org/wiki/Natural_transformation
"""
return lambda container: container.map(function)
[docs] @classmethod
def lift_result(
cls,
function: Callable[[_ValueType], Result[_NewValueType, _ErrorType]],
) -> Callable[
['RequiresContextResult[_EnvType, _ValueType, _ErrorType]'],
'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]',
]:
"""
Lifts function from ``Result`` for better composition.
In other words, it modifies the function's
signature from: ``a -> Result[b, c]`` to:
``RequiresContextResult[env, a, c]``
-> ``RequiresContextResult[env, b, c]``
Similar to :meth:`~RequiresContextResult.lift`,
but works with other type.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure, Result
>>> def function(arg: int) -> Result[str, int]:
... if arg > 0:
... return Success(str(arg) + '!')
... return Failure(arg)
...
>>> deps = RequiresContextResult.empty
>>> assert RequiresContextResult.lift_result(function)(
... RequiresContextResult.from_success(1),
... )(deps) == Success('1!')
>>> assert RequiresContextResult.lift_result(function)(
... RequiresContextResult.from_success(0),
... )(deps) == Failure(0)
>>> assert RequiresContextResult.lift_result(function)(
... RequiresContextResult.from_failure('nope'),
... )(deps) == Failure('nope')
"""
return lambda container: container.bind_result(function)
[docs] @classmethod
def lift_context(
cls,
function: Callable[
[_ValueType],
RequiresContext[_EnvType, _NewValueType],
],
) -> Callable[
['RequiresContextResult[_EnvType, _ValueType, _ErrorType]'],
'RequiresContextResult[_EnvType, _NewValueType, _ErrorType]',
]:
"""
Lifts function from ``RequiresContext`` for better composition.
In other words, it modifies the function's
signature from: ``a -> RequiresContext[env, b]`` to:
``RequiresContextResult[env, a, c]``
-> ``RequiresContextResult[env, b, c]``
Similar to :meth:`~RequiresContextResult.lift`,
but works with other type.
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Success, Failure
>>> def function(arg: int) -> RequiresContext[str, int]:
... return RequiresContext(lambda deps: len(deps) + arg)
...
>>> assert RequiresContextResult.lift_context(function)(
... RequiresContextResult.from_success(2),
... )('abc') == Success(5)
>>> assert RequiresContextResult.lift_context(function)(
... RequiresContextResult.from_failure(0),
... )('abc') == Failure(0)
"""
return lambda container: container.bind_context(function)
[docs] @classmethod
def from_result(
cls, inner_value: Result[_ValueType, _ErrorType],
) -> 'RequiresContextResult[Any, _ValueType, _ErrorType]':
"""
Creates new container with ``Result`` as a unit value.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success, Failure
>>> deps = RequiresContextResult.empty
>>> assert RequiresContextResult.from_result(
... Success(1),
... )(deps) == Success(1)
>>> assert RequiresContextResult.from_result(
... Failure(1),
... )(deps) == Failure(1)
"""
return RequiresContextResult(lambda _: inner_value)
[docs] @classmethod
def from_typecast(
cls, container: RequiresContext[
_EnvType, Result[_NewValueType, _NewErrorType],
],
) -> 'RequiresContextResult[_EnvType, _NewValueType, _NewErrorType]':
"""
You might end up with ``RequiresContext[Result]`` as a value.
This method is designed to turn it into ``RequiresContextResult``.
It will save all the typing information.
It is just more useful!
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Success, Failure
>>> assert RequiresContextResult.from_typecast(
... RequiresContext.from_value(Success(1)),
... )(RequiresContextResult.empty) == Success(1)
>>> assert RequiresContextResult.from_typecast(
... RequiresContext.from_value(Failure(1)),
... )(RequiresContextResult.empty) == Failure(1)
"""
return RequiresContextResult(container)
[docs] @classmethod
def from_successful_context(
cls, inner_value: RequiresContext[_EnvType, _FirstType],
) -> 'RequiresContextResult[_EnvType, _FirstType, Any]':
"""
Creates new container from ``RequiresContext`` as a success unit.
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Success
>>> assert RequiresContextResult.from_successful_context(
... RequiresContext.from_value(1),
... )(...) == Success(1)
"""
return RequiresContextResult(lambda deps: Success(inner_value(deps)))
[docs] @classmethod
def from_failed_context(
cls, inner_value: RequiresContext[_EnvType, _FirstType],
) -> 'RequiresContextResult[_EnvType, Any, _FirstType]':
"""
Creates new container from ``RequiresContext`` as a failure unit.
.. code:: python
>>> from returns.context import RequiresContext
>>> from returns.result import Failure
>>> assert RequiresContextResult.from_failed_context(
... RequiresContext.from_value(1),
... )(...) == Failure(1)
"""
return RequiresContextResult(lambda deps: Failure(inner_value(deps)))
[docs] @classmethod
def from_success(
cls, inner_value: _FirstType,
) -> 'RequiresContextResult[Any, _FirstType, Any]':
"""
Creates new container with ``Success(inner_value)`` as a unit value.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Success
>>> assert RequiresContextResult.from_success(1)(...) == Success(1)
"""
return RequiresContextResult(lambda _: Success(inner_value))
[docs] @classmethod
def from_failure(
cls, inner_value: _FirstType,
) -> 'RequiresContextResult[Any, Any, _FirstType]':
"""
Creates new container with ``Failure(inner_value)`` as a unit value.
.. code:: python
>>> from returns.context import RequiresContextResult
>>> from returns.result import Failure
>>> assert RequiresContextResult.from_failure(1)(...) == Failure(1)
"""
return RequiresContextResult(lambda _: Failure(inner_value))
[docs]@final
class ContextResult(Immutable, Generic[_EnvType]):
"""
Helpers that can be used to work with ``RequiresContextResult`` container.
Related to :class:`returns.context.Context`, refer there for the docs.
"""
[docs] @classmethod
def ask(cls) -> RequiresContextResult[_EnvType, _EnvType, Any]:
"""
Is used to get the current dependencies inside the call stack.
Similar to :meth:`returns.context.Context.ask`,
but returns ``Result`` instead of a regular value.
Please, refer to the docs there to know how to use it.
One important note that is worth doublicating here:
you might need to provide ``_EnvType`` explicitly,
so ``mypy`` will know about it statically.
.. code:: python
>>> from returns.context import ContextResult
>>> from returns.result import Success
>>> assert ContextResult[int].ask().map(str)(1) == Success('1')
"""
return RequiresContextResult(Success)
# Aliases:
#: Alias for a popular case when ``Result`` has ``Exception`` as error type.
RequiresContextResultE = RequiresContextResult[
_EnvType, _ValueType, Exception,
]
#: Alias to save you some typing. Uses original name from Haskell.
ReaderResult = RequiresContextResult[_EnvType, _ValueType, _ErrorType]
#: Alias to save you some typing. Has ``Exception`` as error type.
ReaderResultE = RequiresContextResult[_EnvType, _ValueType, Exception]