sobolevn · Jan 27, 2021
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -8,6 +8,11 @@ See [0Ver](https://0ver.org/).
 
 ## 0.16.0 WIP
 
+## Features
+
+- Adds `LazyIO` container
+- Marks `IO` as `@final`
+
 ### Misc
 
 - Makes `_Nothing` a singleton

diff --git a/returns/contrib/mypy/_consts.py b/returns/contrib/mypy/_consts.py
@@ -6,8 +6,8 @@
 #: Set of full names of our decorators.
 TYPED_DECORATORS: Final = frozenset((
     'returns.result.safe',
-    'returns.io.impure',
-    'returns.io.impure_safe',
+    'returns.io.io.impure',
+    'returns.io.ioresult.impure_safe',
     'returns.maybe.maybe',
     'returns.future.future',
     'returns.future.asyncify',

diff --git a/returns/contrib/pytest/plugin.py b/returns/contrib/pytest/plugin.py
@@ -127,7 +127,8 @@ def _trace_function(
     arg: Any,
 ) -> None:
     is_desired_type_call = (
-        event == 'call' and frame.f_code is trace_type.__code__
+        event == 'call' and
+        frame.f_code is trace_type.__code__
     )
     if is_desired_type_call:
         current_call_stack = inspect.stack()
@@ -152,7 +153,7 @@ def containers_to_patch(cls) -> tuple:
             RequiresContextFutureResult,
         )
         from returns.future import FutureResult
-        from returns.io import _IOFailure, _IOSuccess
+        from returns.io.ioresult import _IOFailure, _IOSuccess
         from returns.result import _Failure, _Success
 
         return (

diff --git a/returns/io/__init__.py b/returns/io/__init__.py
@@ -0,0 +1,7 @@
+from returns.io.io import IO as IO
+from returns.io.io import impure as impure
+from returns.io.ioresult import IOFailure as IOFailure
+from returns.io.ioresult import IOResult as IOResult
+from returns.io.ioresult import IOResultE as IOResultE
+from returns.io.ioresult import IOSuccess as IOSuccess
+from returns.io.ioresult import impure_safe as impure_safe
diff --git a/returns/io/io.py b/returns/io/io.py
@@ -0,0 +1,228 @@
+from functools import wraps
+from typing import TYPE_CHECKING, Callable, TypeVar
+
+from typing_extensions import final
+
+from returns.interfaces.specific import io
+from returns.primitives.container import BaseContainer, container_equality
+from returns.primitives.hkt import Kind1, SupportsKind1, dekind
+
+if TYPE_CHECKING:
+    from returns.io.ioresult import IOResult
+
+_ValueType = TypeVar('_ValueType', covariant=True)
+_NewValueType = TypeVar('_NewValueType')
+
+# Result related:
+_ErrorType = TypeVar('_ErrorType', covariant=True)
+_NewErrorType = TypeVar('_NewErrorType')
+
+# Helpers:
+_FirstType = TypeVar('_FirstType')
+_SecondType = TypeVar('_SecondType')
+
+
+@final
+class IO(
+    BaseContainer,
+    SupportsKind1['IO', _ValueType],
+    io.IOLike1[_ValueType],
+):
+    """
+    Explicit container for impure function results.
+
+    We also sometimes call it "marker" since once it is marked,
+    it cannot be ever unmarked.
+    There's no way to directly get its internal value.
+
+    Note that ``IO`` represents a computation that never fails.
+
+    Examples of such computations are:
+
+    - read / write to localStorage
+    - get the current time
+    - write to the console
+    - get a random number
+
+    Use ``IOResult[...]`` for operations that might fail.
+    Like DB access or network operations.
+
+    See also:
+        - https://dev.to/gcanti/getting-started-with-fp-ts-io-36p6
+        - https://gist.github.com/chris-taylor/4745921
+
+    """
+
+    _inner_value: _ValueType
+
+    #: Typesafe equality comparison with other `Result` objects.
+    equals = container_equality
+
+    def __init__(self, inner_value: _ValueType) -> None:
+        """
+        Public constructor for this type. Also required for typing.
+
+        .. code:: python
+
+          >>> from returns.io import IO
+          >>> assert str(IO(1)) == '<IO: 1>'
+
+        """
+        super().__init__(inner_value)
+
+    def map(  # noqa: WPS125
+        self,
+        function: Callable[[_ValueType], _NewValueType],
+    ) -> 'IO[_NewValueType]':
+        """
+        Applies function to the inner value.
+
+        Applies 'function' to the contents of the IO instance
+        and returns a new IO object containing the result.
+        'function' should accept a single "normal" (non-container) argument
+        and return a non-container result.
+
+        .. code:: python
+
+          >>> def mappable(string: str) -> str:
+          ...      return string + 'b'
+
+          >>> assert IO('a').map(mappable) == IO('ab')
+
+        """
+        return IO(function(self._inner_value))
+
+    def apply(
+        self,
+        container: Kind1['IO', Callable[[_ValueType], _NewValueType]],
+    ) -> 'IO[_NewValueType]':
+        """
+        Calls a wrapped function in a container on this container.
+
+        .. code:: python
+
+          >>> from returns.io import IO
+          >>> assert IO('a').apply(IO(lambda inner: inner + 'b')) == IO('ab')
+
+        Or more complex example that shows how we can work
+        with regular functions and multiple ``IO`` arguments:
+
+        .. code:: python
+
+          >>> from returns.curry import curry
+
+          >>> @curry
+          ... def appliable(first: str, second: str) -> str:
+          ...      return first + second
+
+          >>> assert IO('b').apply(IO('a').apply(IO(appliable))) == IO('ab')
+
+        """
+        return self.map(dekind(container)._inner_value)  # noqa: WPS437
+
+    def bind(
+        self,
+        function: Callable[[_ValueType], Kind1['IO', _NewValueType]],
+    ) -> 'IO[_NewValueType]':
+        """
+        Applies 'function' to the result of a previous calculation.
+
+        'function' should accept a single "normal" (non-container) argument
+        and return ``IO`` type object.
+
+        .. code:: python
+
+          >>> def bindable(string: str) -> IO[str]:
+          ...      return IO(string + 'b')
+
+          >>> assert IO('a').bind(bindable) == IO('ab')
+
+        """
+        return dekind(function(self._inner_value))
+
+    #: Alias for `bind` method. Part of the `IOLikeN` interface.
+    bind_io = bind
+
+    @classmethod
+    def from_value(cls, inner_value: _NewValueType) -> 'IO[_NewValueType]':
+        """
+        Unit function to construct new ``IO`` values.
+
+        Is the same as regular constructor:
+
+        .. code:: python
+
+          >>> from returns.io import IO
+          >>> assert IO(1) == IO.from_value(1)
+
+        Part of the :class:`returns.interfaces.applicative.ApplicativeN`
+        interface.
+        """
+        return IO(inner_value)
+
+    @classmethod
+    def from_io(cls, inner_value: 'IO[_NewValueType]') -> 'IO[_NewValueType]':
+        """
+        Unit function to construct new ``IO`` values from existing ``IO``.
+
+        .. code:: python
+
+          >>> from returns.io import IO
+          >>> assert IO(1) == IO.from_io(IO(1))
+
+        Part of the :class:`returns.interfaces.specific.IO.IOLikeN` interface.
+
+        """
+        return inner_value
+
+    @classmethod
+    def from_ioresult(
+        cls,
+        inner_value: 'IOResult[_NewValueType, _NewErrorType]',
+    ) -> 'IO[Result[_NewValueType, _NewErrorType]]':
+        """
+        Converts ``IOResult[a, b]`` back to ``IO[Result[a, b]]``.
+
+        Can be really helpful for composition.
+
+        .. code:: python
+
+            >>> from returns.io import IO, IOSuccess
+            >>> from returns.result import Success
+            >>> assert IO.from_ioresult(IOSuccess(1)) == IO(Success(1))
+
+        Is the reverse of :meth:`returns.io.IOResult.from_typecast`.
+        """
+        return IO(inner_value._inner_value)  # noqa: WPS437
+
+
+# Helper functions:
+
+def impure(
+    function: Callable[..., _NewValueType],
+) -> Callable[..., IO[_NewValueType]]:
+    """
+    Decorator to mark function that it returns :class:`~IO` container.
+
+    If you need to mark ``async`` function as impure,
+    use :func:`returns.future.future` instead.
+    This decorator only works with sync functions. Example:
+
+    .. code:: python
+
+      >>> from returns.io import IO, impure
+
+      >>> @impure
+      ... def function(arg: int) -> int:
+      ...     return arg + 1  # this action is pure, just an example
+      ...
+
+      >>> assert function(1) == IO(2)
+
+    Requires our :ref:`mypy plugin <mypy-plugins>`.
+
+    """
+    @wraps(function)
+    def decorator(*args, **kwargs):
+        return IO(function(*args, **kwargs))
+    return decorator