fptools.either¶

class pythonic_fp.fptools.either.Either(value: L)¶

class pythonic_fp.fptools.either.Either(value: L, side: EitherBool)

class pythonic_fp.fptools.either.Either(value: R, side: EitherBool)

Either Monad

Data structure semantically containing either a left or a right value, but not both.

Implements a left biased Either Monad.

Either(value: +L, LEFT) produces a left Either
Either(value: +L, RIGHT) produces a right Either

In a Boolean context

A left Either is “truthy”
A right Either is “falsy”

Two Either objects compare as equal when

both are left values or both are right values whose values
are the same object
compare as equal

Immutable, an Either does not change after being created. Therefore map & bind return new instances.

Warning

The contained value need not be immutable, therefore not hashable if value is mutable.

Note

Either(value: +L, side: Left): Either[L, R] -> left: Either[L, R] Either(value: +R, side: Right): Either[L, R] -> right: Either[L, R]

__init__(value: L) → None¶
__init__(value: L, side: EitherBool) → None
__init__(value: R, side: EitherBool) → None

__hash__() → int¶: Return hash(self).

__repr__() → str¶: Return repr(self).

__str__() → str¶: Return str(self).

__len__() → int¶: An Either always contains just one value.

__eq__(other: object) → bool¶: Return self==value.

get() → L¶

Get value if a left.

Warning

Unsafe method get. Will raise ValueError if Either is a right. Best practice is to first check the Either in a boolean context.

Returns:: its value if a Left
Raises:: ValueError – if not a left

get_left() → MayBe¶

Get value of Either if a left. Safer version of get method.

Returns:: MayBe[L]

get_right() → MayBe¶

Get value of Either if a right.

Returns:: MayBe[R]

map_right(f: Callable[[R], V]) → Either[L, V]¶

Construct new Either with a different right.

Parameters:: f – function to map a right value
Returns:: a new Either if a right, otherwise itself

map(f: Callable[[L], U]) → Either[U, R]¶

Map over if a left value. Return new instance.

Parameters:: f – function to map a left value
Returns:: a new Either if a left, otherwise itself

bind(f: Callable[[L], Either[U, R]]) → Either[U, R]¶

Flatmap over the left value, propagate right values.

Parameters:: f – function to flatmap a left value
Returns:: a new Either if a left, otherwise itself

map_except(f: Callable[[L], U], fallback_right: R) → Either[U, R]¶

Map over if a left value - with fallback upon exception.

if Either is a left then map f over its value
- if f returns normally, then return a left Either[U, R]
- if f raises an exception, return right Either[U, R]
if Either is a right
- return new Either(right=self._right): Either[+U, +R]

Warning

Swallows exceptions.

bind_except(f: Callable[[L], Either[U, R]], fallback_right: R) → Either[U, R]¶

Flatmap Either with function f with a fallback right if exception is thrown.

Warning

Swallows exceptions.

Parameters:: fallback_right – fallback value if exception thrown
Returns:: a successfully mapped left, a propagated right, or a right with fallback value

static sequence(sequence_xor_uv: Sequence[Either]) → Either[Sequence, V]¶

Sequence a sequence subtype of Sequence[Either[U, V]]``

If the iterated Either values are all lefts, then return an Either of an iterable of the left values. Otherwise return a right Either containing the first right encountered.

class pythonic_fp.fptools.either.EitherBool¶

class pythonic_fp.fptools.either.EitherBool(witness: object)

Boolean-like type used by Either constructor.

A “truthy” value passed to constructor produces the unique LEFT value.
A “falsy” value passed to constructor produces the unique RIGHT value.

Parameters:: witness – Determines truthiness of the SBool.
Returns:: The truthy or falsy SBool class instance.

__repr__() → str¶: Return repr(self).

pythonic_fp.fptools.either.LEFT = TRUTH¶: Singleton value signaling Either constructor to produce a LEFT Either, the default.

pythonic_fp.fptools.either.RIGHT = LIE¶: Singleton value signaling Either constructor to produce a RIGHT Either.