optional.mojo

# ===----------------------------------------------------------------------=== #
# Copyright (c) 2024, Modular Inc. All rights reserved.
#
# Licensed under the Apache License v2.0 with LLVM Exceptions:
# https://llvm.org/LICENSE.txt
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ===----------------------------------------------------------------------=== #
"""Defines Optional, a type modeling a value which may or may not be present.

Optional values can be thought of as a type-safe nullable pattern.
Your value can take on a value or `None`, and you need to check
and explicitly extract the value to get it out.

```mojo
from collections import Optional
var a = Optional(1)
var b = Optional[Int](None)
if a:
    print(a.value()[])  # prints 1
if b:  # bool(b) is False, so no print
    print(b.value()[])
var c = a.or_else(2)
var d = b.or_else(2)
print(c)  # prints 1
print(d)  # prints 2
```
"""

from utils import Variant


# TODO(27780): NoneType can't currently conform to traits
@value
struct _NoneType(CollectionElement):
    pass


# ===----------------------------------------------------------------------===#
# Optional
# ===----------------------------------------------------------------------===#


@value
struct Optional[T: CollectionElement](CollectionElement, Boolable):
    """A type modeling a value which may or may not be present.

    Optional values can be thought of as a type-safe nullable pattern.
    Your value can take on a value or `None`, and you need to check
    and explicitly extract the value to get it out.

    Currently T is required to be a `CollectionElement` so we can implement
    copy/move for Optional and allow it to be used in collections itself.

    ```mojo
    from collections import Optional
    var a = Optional(1)
    var b = Optional[Int](None)
    if a:
        print(a.value()[])  # prints 1
    if b:  # bool(b) is False, so no print
        print(b.value()[])
    var c = a.or_else(2)
    var d = b.or_else(2)
    print(c)  # prints 1
    print(d)  # prints 2
    ```

    Parameters:
        T: The type of value stored in the Optional.
    """

    # Fields
    # _NoneType comes first so its index is 0.
    # This means that Optionals that are 0-initialized will be None.
    alias _type = Variant[_NoneType, T]
    var _value: Self._type

    # ===-------------------------------------------------------------------===#
    # Life cycle methods
    # ===-------------------------------------------------------------------===#

    fn __init__(inout self):
        """Construct an empty Optional."""
        self._value = Self._type(_NoneType())

    fn __init__(inout self, owned value: T):
        """Construct an Optional containing a value.

        Args:
            value: The value to store in the optional.
        """
        self._value = Self._type(value^)

    fn __init__(inout self, value: NoneType):
        """Construct an empty Optional.

        Args:
            value: Must be exactly `None`.
        """
        self = Self()

    # ===-------------------------------------------------------------------===#
    # Operator dunders
    # ===-------------------------------------------------------------------===#

    fn __is__(self, other: NoneType) -> Bool:
        """Return `True` if the Optional has no value.

        It allows you to use the following syntax: `if my_optional is None:`

        Args:
            other: The value to compare to (None).

        Returns:
            True if the Optional has no value and False otherwise.
        """
        return not self.__bool__()

    fn __isnot__(self, other: NoneType) -> Bool:
        """Return `True` if the Optional has a value.

        It allows you to use the following syntax: `if my_optional is not None:`.

        Args:
            other: The value to compare to (None).

        Returns:
            True if the Optional has a value and False otherwise.
        """
        return self.__bool__()

    # ===-------------------------------------------------------------------===#
    # Trait implementations
    # ===-------------------------------------------------------------------===#

    fn __bool__(self) -> Bool:
        """Return true if the Optional has a value.

        Returns:
            True if the optional has a value and False otherwise.
        """
        return not self._value.isa[_NoneType]()

    fn __invert__(self) -> Bool:
        """Return False if the optional has a value.

        Returns:
            False if the optional has a value and True otherwise.
        """
        return not self

    # ===-------------------------------------------------------------------===#
    # Methods
    # ===-------------------------------------------------------------------===#

    @always_inline
    fn value(
        self: Reference[Self, _, _]
    ) -> Reference[T, self.is_mutable, self.lifetime]:
        """Retrieve a reference to the value of the Optional.

        This check to see if the optional contains a value.
        If you call this without first verifying the optional with __bool__()
        eg. by `if my_option:` or without otherwise knowing that it contains a
        value (for instance with `or_else`), the program will abort

        Returns:
            A reference to the contained data of the option as a Reference[T].
        """
        if not self[].__bool__():
            abort(".value() on empty Optional")

        return self[].unsafe_value()

    @always_inline
    fn unsafe_value(
        self: Reference[Self, _, _]
    ) -> Reference[T, self.is_mutable, self.lifetime]:
        """Unsafely retrieve a reference to the value of the Optional.

        This doesn't check to see if the optional contains a value.
        If you call this without first verifying the optional with __bool__()
        eg. by `if my_option:` or without otherwise knowing that it contains a
        value (for instance with `or_else`), you'll get garbage unsafe data out.

        Returns:
            A reference to the contained data of the option as a Reference[T].
        """
        debug_assert(self[].__bool__(), ".value() on empty Optional")
        return self[]._value[T]

    @always_inline
    fn _value_copy(self) -> T:
        """Unsafely retrieve the value out of the Optional.

        Note: only used for Optionals when used in a parameter context
        due to compiler bugs.  In general, prefer using the public `Optional.value()`
        function that returns a `Reference[T]`.
        """

        debug_assert(self.__bool__(), ".value() on empty Optional")
        return self._value[T]

    fn take(inout self) -> T:
        """Move the value out of the Optional.

        The caller takes ownership over the new value, which is moved
        out of the Optional, and the Optional is left in an empty state.

        This check to see if the optional contains a value.
        If you call this without first verifying the optional with __bool__()
        eg. by `if my_option:` or without otherwise knowing that it contains a
        value (for instance with `or_else`), you'll get garbage unsafe data out.

        Returns:
            The contained data of the option as an owned T value.
        """
        if not self.__bool__():
            abort(".take() on empty Optional")
        return self.unsafe_take()

    fn unsafe_take(inout self) -> T:
        """Unsafely move the value out of the Optional.

        The caller takes ownership over the new value, which is moved
        out of the Optional, and the Optional is left in an empty state.

        This check to see if the optional contains a value.
        If you call this without first verifying the optional with __bool__()
        eg. by `if my_option:` or without otherwise knowing that it contains a
        value (for instance with `or_else`), the program will abort!

        Returns:
            The contained data of the option as an owned T value.
        """
        debug_assert(self.__bool__(), ".unsafe_take() on empty Optional")
        return self._value.unsafe_replace[_NoneType, T](_NoneType())

    fn or_else(self, default: T) -> T:
        """Return the underlying value contained in the Optional or a default value if the Optional's underlying value is not present.

        Args:
            default: The new value to use if no value was present.

        Returns:
            The underlying value contained in the Optional or a default value.
        """
        if self.__bool__():
            return self._value[T]
        return default


# ===----------------------------------------------------------------------===#
# OptionalReg
# ===----------------------------------------------------------------------===#


@register_passable("trivial")
struct OptionalReg[T: AnyRegType](Boolable):
    """A register-passable optional type.

    This struct optionally contains a value. It only works with trivial register
    passable types at the moment.

    Parameters:
        T: The type of value stored in the Optional.
    """

    # Fields
    alias _mlir_type = __mlir_type[`!kgen.variant<`, T, `, i1>`]
    var _value: Self._mlir_type

    # ===-------------------------------------------------------------------===#
    # Life cycle methods
    # ===-------------------------------------------------------------------===#

    fn __init__(inout self):
        """Create an optional with a value of None."""
        self = Self(None)

    fn __init__(inout self, value: T):
        """Create an optional with a value.

        Args:
            value: The value.
        """
        self._value = __mlir_op.`kgen.variant.create`[
            _type = Self._mlir_type, index = Int(0).value
        ](value)

    fn __init__(inout self, value: NoneType):
        """Create an optional without a value from a None literal.

        Args:
            value: The None value.
        """
        self._value = __mlir_op.`kgen.variant.create`[
            _type = Self._mlir_type, index = Int(1).value
        ](__mlir_attr.false)

    # ===-------------------------------------------------------------------===#
    # Operator dunders
    # ===-------------------------------------------------------------------===#

    fn __is__(self, other: NoneType) -> Bool:
        """Return `True` if the Optional has no value.

        It allows you to use the following syntax: `if my_optional is None:`

        Args:
            other: The value to compare to (None).

        Returns:
            True if the Optional has no value and False otherwise.
        """
        return not self.__bool__()

    fn __isnot__(self, other: NoneType) -> Bool:
        """Return `True` if the Optional has a value.

        It allows you to use the following syntax: `if my_optional is not None:`

        Args:
            other: The value to compare to (None).

        Returns:
            True if the Optional has a value and False otherwise.
        """
        return self.__bool__()

    # ===-------------------------------------------------------------------===#
    # Trait implementations
    # ===-------------------------------------------------------------------===#

    fn __bool__(self) -> Bool:
        """Return true if the optional has a value.

        Returns:
            True if the optional has a value and False otherwise.
        """
        return __mlir_op.`kgen.variant.is`[index = Int(0).value](self._value)

    # ===-------------------------------------------------------------------===#
    # Methods
    # ===-------------------------------------------------------------------===#

    @always_inline
    fn value(self) -> T:
        """Get the optional value.

        Returns:
            The contained value.
        """
        return __mlir_op.`kgen.variant.take`[index = Int(0).value](self._value)