# 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(123)
>>> Right("abc")
Right(abc)
>>> Left("abc")
Left(abc)
```

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

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

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")
Right(5)
>>> value = to_int("not an integer")
>>> value
Left(age is invalid)
>>> value.is_right()
False
>>> value.is_left()
True
>>> 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
Right(10)
```

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: world.map(lambda 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(Hello)
```

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()
123
>>> right_value.right()
'abc'
```