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OptionT

funstruct.experimental.monadtransformer.option_t

OptionT — adds optionality to any monad F.

OptionT[F, A] wraps F[Option[A]]. bind short-circuits on Nothing, map transforms the value inside Some.

Examples:

>>> from funstruct.experimental.monadtransformer.option_t import OptionT
>>> from funstruct.monad.either import Either, Right, Left
>>> from funstruct.monad.option import Some, Nothing

Wrapping Either — a computation that can fail OR be absent:

>>> OptionT(Right(Some(1))).map(lambda x: x + 10).run()
Right(Some(11))
>>> OptionT(Right(Nothing())).map(lambda x: x + 10).run()
Right(Nothing())
>>> OptionT(Left("db error")).map(lambda x: x + 10).run()
Left('db error')

bind chains — short-circuits on Nothing OR Left:

>>> OptionT(Right(Some(1))).bind(
...     lambda x: OptionT(Right(Some(x * 10)))
... ).run()
Right(Some(10))
>>> OptionT(Right(Nothing())).bind(
...     lambda x: OptionT(Right(Some(x * 10)))
... ).run()
Right(Nothing())

handle_error_with — recover from Nothing (not from Left):

>>> OptionT(Right(Nothing())).handle_error_with(
...     lambda: OptionT(Right(Some(99)))
... ).run()
Right(Some(99))

lift_f — bring F[A] into OptionT (wraps in Some):

>>> OptionT.lift_f(Right(42)).run()
Right(Some(42))

OptionT

Bases: MonadTransformer, Generic[_F, _A]

OptionT: F[Option[A]].

Adds optionality/absence to any monad F. bind delegates to F's bind, then pattern-matches the Option inside: Some(v) continues, Nothing short-circuits.

Haskell: MaybeT m a Scala: OptionT[F[_], A]

Source code in funstruct/experimental/monadtransformer/option_t.py 
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class OptionT(MonadTransformer, Generic[_F, _A]):
    """OptionT: ``F[Option[A]]``.

    Adds optionality/absence to any monad F. bind delegates to F's bind,
    then pattern-matches the Option inside: Some(v) continues, Nothing
    short-circuits.

    Haskell: ``MaybeT m a``
    Scala:   ``OptionT[F[_], A]``
    """

    def __init__(self, run) -> None:
        self._run = run

    def run(self):
        """Unwrap to get F[Option[A]]."""
        return self._run

    def bind(self, f: Callable[[_A], OptionT[_F, _B]]) -> OptionT[_F, _B]:
        """FlatMap: unwrap F, match Option, chain on Some.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT(Right(Some(1))).bind(
        ...     lambda x: OptionT(Right(Some(x + 10)))
        ... ).run()
        Right(Some(11))
        >>> OptionT(Right(Nothing())).bind(
        ...     lambda x: OptionT(Right(Some(x + 10)))
        ... ).run()
        Right(Nothing())
        """

        def _handle(opt):
            match opt:
                case Some(v):
                    return f(v).run()
                case _:
                    return self._run.__class__.pure(Nothing())

        return OptionT(self._run.bind(_handle))

    def map(self, f: Callable[[_A], _B]) -> OptionT[_F, _B]:
        """Transform the value inside Some inside F.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some
        >>> OptionT(Right(Some(5))).map(lambda x: x * 2).run()
        Right(Some(10))
        """

        def _map_opt(opt):
            match opt:
                case Some(v):
                    return Some(f(v))
                case _:
                    return opt

        return OptionT(self._run.map(_map_opt))

    def handle_error_with(self, f: Callable[[], OptionT[_F, _A]]) -> OptionT[_F, _A]:
        """Recover from Nothing: if inner is Nothing, use fallback.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT(Right(Nothing())).handle_error_with(
        ...     lambda: OptionT(Right(Some(99)))
        ... ).run()
        Right(Some(99))
        >>> OptionT(Right(Some(1))).handle_error_with(
        ...     lambda: OptionT(Right(Some(99)))
        ... ).run()
        Right(Some(1))
        """

        def _handle(opt):
            match opt:
                case Nothing():
                    return f().run()
                case _:
                    return self._run.__class__.pure(opt)

        return OptionT(self._run.bind(_handle))

    def fold(self, on_nothing: Callable[[], _B], on_some: Callable[[_A], _B]) -> _F:
        """Eliminate the Option inside F, returning F[B].

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT(Right(Some(5))).fold(lambda: 0, lambda x: x * 2)
        Right(10)
        >>> OptionT(Right(Nothing())).fold(lambda: 0, lambda x: x * 2)
        Right(0)
        """
        return self._run.map(lambda opt: opt.fold(on_nothing, on_some))

    def get_or_else(self, default: _A) -> _F:
        """Extract the Some value or return default, inside F.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT(Right(Some(42))).get_or_else(0)
        Right(42)
        >>> OptionT(Right(Nothing())).get_or_else(0)
        Right(0)
        """
        return self._run.map(lambda opt: opt.get_or_else(default))

    def filter(self, f: Callable[[_A], bool]) -> OptionT[_F, _A]:
        """Filter the value inside Some; becomes Nothing if predicate fails.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT(Right(Some(10))).filter(lambda x: x > 5).run()
        Right(Some(10))
        >>> OptionT(Right(Some(3))).filter(lambda x: x > 5).run()
        Right(Nothing())
        """
        return OptionT(self._run.map(lambda opt: opt.filter(f)))

    def and_then(self, other: OptionT) -> OptionT:
        """Kleisli composition: value from self becomes input for other's run."""
        return self.bind(lambda _: other)

    @classmethod
    def do(cls, gen_fn) -> Callable[..., OptionT]:
        """Do-notation via generators. Short-circuits on Nothing. Returns a callable.

        >>> from funstruct.monad.either import Right
        >>> from funstruct.monad.option import Some
        >>> def pipeline():
        ...     x = yield OptionT(Right(Some(1)))
        ...     y = yield OptionT(Right(Some(x + 10)))
        ...     return x + y
        >>> OptionT.do(pipeline)().run()
        Right(Some(12))
        """

        def _thunk(*args, **kwargs):
            def _run_do():
                gen = gen_fn(*args, **kwargs)
                try:
                    first = next(gen)
                except StopIteration:
                    raise ValueError("do block must yield at least once")

                def step(opt):
                    match opt:
                        case Nothing():
                            return first.run().__class__.pure(Nothing())
                        case Some(value):
                            try:
                                next_t = gen.send(value)
                                return next_t.run().bind(step)
                            except StopIteration as e:
                                return first.run().__class__.pure(Some(e.value))

                return first.run().bind(step)

            return cls(_run_do())

        return _thunk

    @classmethod
    def pure(cls, value, monad: type) -> OptionT:
        """Lift a plain value into OptionT.

        >>> from funstruct.monad.either import Either, Right
        >>> from funstruct.monad.option import Some
        >>> OptionT.pure(42, Either).run()
        Right(Some(42))
        """
        return cls(monad.pure(Some(value)))

    @classmethod
    def none(cls, monad: type) -> OptionT:
        """Construct an OptionT holding Nothing.

        >>> from funstruct.monad.either import Either, Right
        >>> from funstruct.monad.option import Nothing
        >>> OptionT.none(Either).run()
        Right(Nothing())
        """
        return cls(monad.pure(Nothing()))

    @classmethod
    def lift_f(cls, fa: _F) -> OptionT:
        """Lift F[A] into OptionT — wraps value in Some.

        Use when you have the outer monad but not the inner Option.

        Haskell equivalent: ``lift :: m a -> OptionT m a``

        >>> from funstruct.monad.either import Right, Left
        >>> from funstruct.monad.option import Some
        >>> OptionT.lift_f(Right(42)).run()
        Right(Some(42))
        >>> OptionT.lift_f(Left("err")).run()
        Left('err')
        """
        return cls(fa.map(lambda a: Some(a)))

    @classmethod
    def from_option(cls, option: Option[_A], monad: type) -> OptionT:
        """Lift Option[A] into OptionT — wraps in the outer monad.

        Use when you have the inner Option but not the outer monad.

        >>> from funstruct.monad.either import Either, Right
        >>> from funstruct.monad.option import Some, Nothing
        >>> OptionT.from_option(Some(42), Either).run()
        Right(Some(42))
        >>> OptionT.from_option(Nothing(), Either).run()
        Right(Nothing())
        """
        return cls(monad.pure(option))

    def __repr__(self) -> str:
        return f"OptionT({repr(self._run)})"

run()

Unwrap to get F[Option[A]].

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def run(self):
    """Unwrap to get F[Option[A]]."""
    return self._run

bind(f)

FlatMap: unwrap F, match Option, chain on Some.

from funstruct.monad.either import Right from funstruct.monad.option import Some, Nothing OptionT(Right(Some(1))).bind( ... lambda x: OptionT(Right(Some(x + 10))) ... ).run() Right(Some(11)) OptionT(Right(Nothing())).bind( ... lambda x: OptionT(Right(Some(x + 10))) ... ).run() Right(Nothing())

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def bind(self, f: Callable[[_A], OptionT[_F, _B]]) -> OptionT[_F, _B]:
    """FlatMap: unwrap F, match Option, chain on Some.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT(Right(Some(1))).bind(
    ...     lambda x: OptionT(Right(Some(x + 10)))
    ... ).run()
    Right(Some(11))
    >>> OptionT(Right(Nothing())).bind(
    ...     lambda x: OptionT(Right(Some(x + 10)))
    ... ).run()
    Right(Nothing())
    """

    def _handle(opt):
        match opt:
            case Some(v):
                return f(v).run()
            case _:
                return self._run.__class__.pure(Nothing())

    return OptionT(self._run.bind(_handle))

map(f)

Transform the value inside Some inside F.

from funstruct.monad.either import Right from funstruct.monad.option import Some OptionT(Right(Some(5))).map(lambda x: x * 2).run() Right(Some(10))

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def map(self, f: Callable[[_A], _B]) -> OptionT[_F, _B]:
    """Transform the value inside Some inside F.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some
    >>> OptionT(Right(Some(5))).map(lambda x: x * 2).run()
    Right(Some(10))
    """

    def _map_opt(opt):
        match opt:
            case Some(v):
                return Some(f(v))
            case _:
                return opt

    return OptionT(self._run.map(_map_opt))

handle_error_with(f)

Recover from Nothing: if inner is Nothing, use fallback.

from funstruct.monad.either import Right from funstruct.monad.option import Some, Nothing OptionT(Right(Nothing())).handle_error_with( ... lambda: OptionT(Right(Some(99))) ... ).run() Right(Some(99)) OptionT(Right(Some(1))).handle_error_with( ... lambda: OptionT(Right(Some(99))) ... ).run() Right(Some(1))

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def handle_error_with(self, f: Callable[[], OptionT[_F, _A]]) -> OptionT[_F, _A]:
    """Recover from Nothing: if inner is Nothing, use fallback.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT(Right(Nothing())).handle_error_with(
    ...     lambda: OptionT(Right(Some(99)))
    ... ).run()
    Right(Some(99))
    >>> OptionT(Right(Some(1))).handle_error_with(
    ...     lambda: OptionT(Right(Some(99)))
    ... ).run()
    Right(Some(1))
    """

    def _handle(opt):
        match opt:
            case Nothing():
                return f().run()
            case _:
                return self._run.__class__.pure(opt)

    return OptionT(self._run.bind(_handle))

fold(on_nothing, on_some)

Eliminate the Option inside F, returning F[B].

from funstruct.monad.either import Right from funstruct.monad.option import Some, Nothing OptionT(Right(Some(5))).fold(lambda: 0, lambda x: x * 2) Right(10) OptionT(Right(Nothing())).fold(lambda: 0, lambda x: x * 2) Right(0)

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def fold(self, on_nothing: Callable[[], _B], on_some: Callable[[_A], _B]) -> _F:
    """Eliminate the Option inside F, returning F[B].

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT(Right(Some(5))).fold(lambda: 0, lambda x: x * 2)
    Right(10)
    >>> OptionT(Right(Nothing())).fold(lambda: 0, lambda x: x * 2)
    Right(0)
    """
    return self._run.map(lambda opt: opt.fold(on_nothing, on_some))

get_or_else(default)

Extract the Some value or return default, inside F.

from funstruct.monad.either import Right from funstruct.monad.option import Some, Nothing OptionT(Right(Some(42))).get_or_else(0) Right(42) OptionT(Right(Nothing())).get_or_else(0) Right(0)

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def get_or_else(self, default: _A) -> _F:
    """Extract the Some value or return default, inside F.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT(Right(Some(42))).get_or_else(0)
    Right(42)
    >>> OptionT(Right(Nothing())).get_or_else(0)
    Right(0)
    """
    return self._run.map(lambda opt: opt.get_or_else(default))

filter(f)

Filter the value inside Some; becomes Nothing if predicate fails.

from funstruct.monad.either import Right from funstruct.monad.option import Some, Nothing OptionT(Right(Some(10))).filter(lambda x: x > 5).run() Right(Some(10)) OptionT(Right(Some(3))).filter(lambda x: x > 5).run() Right(Nothing())

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def filter(self, f: Callable[[_A], bool]) -> OptionT[_F, _A]:
    """Filter the value inside Some; becomes Nothing if predicate fails.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT(Right(Some(10))).filter(lambda x: x > 5).run()
    Right(Some(10))
    >>> OptionT(Right(Some(3))).filter(lambda x: x > 5).run()
    Right(Nothing())
    """
    return OptionT(self._run.map(lambda opt: opt.filter(f)))

and_then(other)

Kleisli composition: value from self becomes input for other's run.

Source code in funstruct/experimental/monadtransformer/option_t.py 
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def and_then(self, other: OptionT) -> OptionT:
    """Kleisli composition: value from self becomes input for other's run."""
    return self.bind(lambda _: other)

do(gen_fn) classmethod

Do-notation via generators. Short-circuits on Nothing. Returns a callable.

from funstruct.monad.either import Right from funstruct.monad.option import Some def pipeline(): ... x = yield OptionT(Right(Some(1))) ... y = yield OptionT(Right(Some(x + 10))) ... return x + y OptionT.do(pipeline)().run() Right(Some(12))

Source code in funstruct/experimental/monadtransformer/option_t.py 
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@classmethod
def do(cls, gen_fn) -> Callable[..., OptionT]:
    """Do-notation via generators. Short-circuits on Nothing. Returns a callable.

    >>> from funstruct.monad.either import Right
    >>> from funstruct.monad.option import Some
    >>> def pipeline():
    ...     x = yield OptionT(Right(Some(1)))
    ...     y = yield OptionT(Right(Some(x + 10)))
    ...     return x + y
    >>> OptionT.do(pipeline)().run()
    Right(Some(12))
    """

    def _thunk(*args, **kwargs):
        def _run_do():
            gen = gen_fn(*args, **kwargs)
            try:
                first = next(gen)
            except StopIteration:
                raise ValueError("do block must yield at least once")

            def step(opt):
                match opt:
                    case Nothing():
                        return first.run().__class__.pure(Nothing())
                    case Some(value):
                        try:
                            next_t = gen.send(value)
                            return next_t.run().bind(step)
                        except StopIteration as e:
                            return first.run().__class__.pure(Some(e.value))

            return first.run().bind(step)

        return cls(_run_do())

    return _thunk

pure(value, monad) classmethod

Lift a plain value into OptionT.

from funstruct.monad.either import Either, Right from funstruct.monad.option import Some OptionT.pure(42, Either).run() Right(Some(42))

Source code in funstruct/experimental/monadtransformer/option_t.py 
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@classmethod
def pure(cls, value, monad: type) -> OptionT:
    """Lift a plain value into OptionT.

    >>> from funstruct.monad.either import Either, Right
    >>> from funstruct.monad.option import Some
    >>> OptionT.pure(42, Either).run()
    Right(Some(42))
    """
    return cls(monad.pure(Some(value)))

none(monad) classmethod

Construct an OptionT holding Nothing.

from funstruct.monad.either import Either, Right from funstruct.monad.option import Nothing OptionT.none(Either).run() Right(Nothing())

Source code in funstruct/experimental/monadtransformer/option_t.py 
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@classmethod
def none(cls, monad: type) -> OptionT:
    """Construct an OptionT holding Nothing.

    >>> from funstruct.monad.either import Either, Right
    >>> from funstruct.monad.option import Nothing
    >>> OptionT.none(Either).run()
    Right(Nothing())
    """
    return cls(monad.pure(Nothing()))

lift_f(fa) classmethod

Lift F[A] into OptionT — wraps value in Some.

Use when you have the outer monad but not the inner Option.

Haskell equivalent: lift :: m a -> OptionT m a

from funstruct.monad.either import Right, Left from funstruct.monad.option import Some OptionT.lift_f(Right(42)).run() Right(Some(42)) OptionT.lift_f(Left("err")).run() Left('err')

Source code in funstruct/experimental/monadtransformer/option_t.py 
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@classmethod
def lift_f(cls, fa: _F) -> OptionT:
    """Lift F[A] into OptionT — wraps value in Some.

    Use when you have the outer monad but not the inner Option.

    Haskell equivalent: ``lift :: m a -> OptionT m a``

    >>> from funstruct.monad.either import Right, Left
    >>> from funstruct.monad.option import Some
    >>> OptionT.lift_f(Right(42)).run()
    Right(Some(42))
    >>> OptionT.lift_f(Left("err")).run()
    Left('err')
    """
    return cls(fa.map(lambda a: Some(a)))

from_option(option, monad) classmethod

Lift Option[A] into OptionT — wraps in the outer monad.

Use when you have the inner Option but not the outer monad.

from funstruct.monad.either import Either, Right from funstruct.monad.option import Some, Nothing OptionT.from_option(Some(42), Either).run() Right(Some(42)) OptionT.from_option(Nothing(), Either).run() Right(Nothing())

Source code in funstruct/experimental/monadtransformer/option_t.py 
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@classmethod
def from_option(cls, option: Option[_A], monad: type) -> OptionT:
    """Lift Option[A] into OptionT — wraps in the outer monad.

    Use when you have the inner Option but not the outer monad.

    >>> from funstruct.monad.either import Either, Right
    >>> from funstruct.monad.option import Some, Nothing
    >>> OptionT.from_option(Some(42), Either).run()
    Right(Some(42))
    >>> OptionT.from_option(Nothing(), Either).run()
    Right(Nothing())
    """
    return cls(monad.pure(option))