Using Option

Using the Option class

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A simple monad might be a container that holds either one element or zero elements.

This is the Option class. Option has two subclasses: Some and Empty. If the Option contains a value, it is Some(value), otherwise it is Empty.

We can create an Option in three ways:

>>> from pyeffects.Option import *
>>> Option.of(123)
Some(123)
>>> Some("abc")
Some(abc)
>>> empty
Empty()

We can also check if an Option is empty of not.

>>> from pyeffects.Option import *
>>> v = Some("abc")
>>> v.is_defined()
True
>>> v.is_empty()
False

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map and flat_map: Options are monads, so we can use flat_map and map with them.

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

If we try to map or flat_map on an Empty, we get back an Empty,

>>> from pyeffects.Option import *
>>> hello = Some("Hello")
>>> world = empty
>>> hello.flat_map(lambda h: world.map(lambda w: h + " " + w + "!"))
Empty()

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get and get_or_else: We can retrieve the value of an Option by using get. However, this is unsafe. If the Option is Empty, it will throw an exception.

>>> from pyeffects.Option import *
>>> v = empty
>>> v.get()
TypeError: get cannot be called on this class

Because of this, we try to avoid using .get wherever possible. Instead, we can use get_or_else. This returns the contents of the Option if available, or a default value if it’s not.

>>> from pyeffects.Option import *
>>> v1 = Some("Hello")
>>> v2 = empty
>>> v1.get_or_else("World")
'Hello'
>>> v2.get_or_else("World")
'World'

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or_else_supply and or_else: We can also supply a value if our ‘Option’ is empty using a function or another Option, using or_else_supply and or_else respectively.

>>> from pyeffects.Option import *
>>> empty.or_else_supply(lambda: "Hello World!")
'Hello World!'
>>> empty.or_else(Some("Hello World!"))
Some(Hello World!)

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foreach can be used when we want to apply a function, but unlike with map, we don’t care about the return value.

>>> from pyeffects.Option import *
>>> Some("Hello World!").foreach(lambda s: print(s))
'Hello World!'