Using Either

Using the Either Type

Either is a monad that represents either one thing or another thing.

An Either has two subclasses that hold values: Left and Right. We can think of Try as a type of Either where the Left value is always a Failure. Either is a more generic version of Try, where the left value can be anything, and not just a Failure class.

We can create an Either in three ways:

>>> from pyeffects.Either import *
>>> Either.of(123)
>>> Right("abc")
>>> Left("abc")

We can also check if an Either is Left or Right.

>>> from pyeffects.Either import *
>>> v = Right("abc")
>>> v.is_right()
>>> v.is_left()

Let’s say we want to convert a string to an integer, and then double the value if it is a valid integer. We can map successful values as Right and invalid values to Left.

>>> from pyeffects.Either import *
>>> from pyeffects.Try import *
>>> def to_int(s):
...   val = Try.of(lambda: int(s))
...   return Right(val) if val.is_success() else Left("age is invalid")
>>> to_int("5")
>>> value = to_int("not an integer")
>>> value
Left(age is invalid)
>>> value.is_right()
>>> value.is_left()
>>> to_int("not an integer").map(lambda v: v * 2)  # Failure does not map
Left(age is invalid)
>>> to_int("5").map(lambda v: v * 2)  # Map success and double value

map and flat_map: Either is a monad and we can use flat_map and map with them.

>>> from pyeffects.Either import *
>>> hello = Right("Hello")
>>> world = Right("World")
>>> hello.flat_map(lambda h: w: h + " " + w + "!"))
Right(Hello World!)

If we try to map or flat_map on a Left, we get back a Left.

>>> from pyeffects.Either import *
>>> left_value = Left("Hello")
>>> left_value.flat_map(lambda v: Right(len(v)))

left and right: We can retrieve the value of an Either by using left and right, for Left and Right values.

>>> from pyeffects.Either import *
>>> left_value = Left(123)
>>> right_value = Right("abc")
>>> left_value.left()
>>> right_value.right()