Using Futures

Using the Future monad

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The Future monad represents the result of an asynchronous computation that runs on another thread.

We use Future.run to run a function asynchronously. Let’s look at an example:

>>> import time
>>> from pyeffects.Future import *
>>> import time
>>> def delayed_result():
...   time.sleep(0.1)
...   return 100
...
>>> result = Future.run(delayed_result).map(lambda v: v + 1)
>>> result.is_done()
False
>>> time.sleep(0.2)
>>> result.is_done()
True
>>> result.get()
101

Initially, the Future has no value, and runs your function in the background on another thread. Once the function completes, the return value is supplied to the Future and can be retrieved using the get() method.

We can also create an immediate future from a value:

>>> from pyeffects.Future.import *
>>> val = Future.of(5).map(lambda v: v * v)
>>> val
Future(Success(25))

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map and flat_map: Future can also use flat_map and map functions to chain operations.

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

If we try to map or flat_map on a Future that has failed, we get back a failed Future

>>> from pyeffects.Future import *
>>> hello = Future.of("Hello")
>>> failed = Future.run(lambda: int("Hello World"))  # Can't convert a string to int
>>> hello.flat_map(lambda h: world.map(lambda w: h + " " + w + "!"))
Future(Failure(invalid literal for int() with base 10: 'Hello World'))

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on_complete can be used when we want to call a function when the future has completed.

>>> from pyeffects.Future import *
>>> Future.of("Hello World!").on_complete(lambda s: print(s))
Success(Hello World!)