Expression 0.19.0

Practical functional programming for Python 3.8+

Expression

Expressions evaluates to a value. Statements do something.

Expression aims to be a solid library for practical functional programming in Python 3.8+. By practical we mean that the goal of the library if to enable you to do productive and meaningful functional programming in Python instead of being a Monad tutorial.

Python is a multi-paradigm programming language that also supports functional programming constructs such as functions, higher-order functions, lambdas, and in many ways favors composition over inheritance.

Better Python with F#

Expression tries to make a better Python by providing several functional features inspired by F# into Python. This serves two purposes:

• Make it easier for Python programmers to learn F# by starting out in a programming language they already know. Then get inspired to try out F# by itself.
• Make it easier for F# developers to use Python when needed, and re-use many of the concepts and abstractions that they already know and love.

Expression will enable you to work with Python along with F# using many of the same programming concepts and abstractions. This enables concepts such as Railway oriented programming (ROP) for better and predictable error handling. Pipelining for workflows, computational expressions, etc.

F# is a functional programming language for .NET that is succinct (concise, readable and type-safe) and kind of Pythonic. F# looks a lot more like Python than C# and F# can also do a lot of things better than Python:

• Strongly typed, if it compiles it usually works
• Type inference, the compiler deduces types during compilation
• Expression based language

Getting Started

You can install the latest expression from PyPI by running pip (or pip3). Note that expression only works for Python 3.8+.

$ pip3 install expression

Why

• I love F#, and know F# quite well. I'm the creator of projects such as Oryx, Fable.Reaction and Feliz.ViewEngine
• I love Python, and know Python really well. I'm the creator of both RxPY and OSlash, two functional style libraries for Python.

For a long time I'm been wanting to make a "bridge" between these two languages and got inspired to write this library after watching "F# as a Better Python" - Phillip Carter - NDC Oslo 2020. Doing a transpiler like Fable for Python is one option, but a Python library may give a lower barrier and a better introduction to existing Python programmers.

Expression is an F# inspired version of my previously written OSlash monad tutorial where I ported a number of Haskell abstractions to Python. I never felt that OSlash was really practically usable in Python, but F# is much closer to Python than Haskell, so it makes more sense to try and make a functional library inspired by F# instead.

Goals

• The resulting code should look and feel like Python. We want to make a better Python, not some obscure DSL or academic Monad tutorial
• Provide pipelining and pipe friendly methods. Compose all the things!
• Dot-chaining on objects as an alternative syntax to pipes.
• Avoid currying, not supported in Python by default and not a well known concept by Python programmers.
• Avoid operator (|, >>, etc) overloading, this usually confuses more than it helps.
• Use type-hints for all functions and methods.
• Code should pass strict static type checking by tools such as mypy and pylance. Pylance is awesome, use it!

Supported features

Expression will never provide you with all the features of F# and .NET. We are providing a few of the features we think are useful, and will add more on-demand as we go along.

• Option - for optional stuff and better None handling.
• Result - for better error handling and enables railway-oriented programming in Python.
• Sequence - a better itertools and fully compatible with Python iterables.
• FrozenList - a frozen and immutable list type.
• Effects: - light weight computational expressions for Python. This is actually amazing stuff.
  • option - an optional world for working with optional values.
  • result - an error handling world for working with result values.
• Mailbox Processor: for lock free programming using the Actor model.
• Cancellation Token: for cancellation of asynchronous (and synchronous) workflows.
• Disposable: For resource management.

Pattern matching is provided by Pampy, while we wait for PEP 634 and structural pattern matching for Python.

Pipelining

Expression provides a pipe function similar to |> in F#. We don't want to overload any Python operators e.g | so pipe is a plain old function taking N-arguments and thus lets you pipe a value though any number of functions.

from expression.core import pipe

gn = lambda g: g * y
fn = lambda x: x + z
value = pipe(
    x,
    fn,
    gn
)

assert value == gn(fn(x))

Expression objects also have a pipe method so you can dot chain pipelines directly on the object:

from expression.core import pipe

gn = lambda g: g * y
fn = lambda x: x + z
value = x.pipe(
    fn,
    gn
)

assert value == gn(fn(x))

So for example with sequences you may create sequence transforming pipelines:

ys = xs.pipe(
    seq.map(lambda x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0)
)

Composition

Functions may even be composed directly into custom operators:

from expression.core import compose

custom = compose(
    seq.map(lambda x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0)
)

ys = custom(xs)

Options

The option type is used when an actual value might not exist for a named value or variable. An option has an underlying type and can hold a value of that type Some(value), or it might not have the value Nothing.

from expression.core import Some, Nothing, Option

def keep_positive(a: int) -> Option[int]:
    if a > 0:
        return Some(a)
    else:
        return Nothing

from pampy import _

def exists(x : Option[int]) -> bool:
    return x.match(
        Some, lambda some: True,
        _, False
    )

Options as decorators for computational expressions. Computational expressions in Expression are implemented as coroutines (enhanced generators) using yield, yield from and return to consume or generate optional values:

from expression.builders import option
from expression.core import Some

@option
def fn():
    x = yield 42
    y = yield from Some(43)

    return x + y

xs = fn()

This enables "railway oriented programming" e.g if one part of the function yields from Nothing then the function is side-tracked (short-circuit) and the following statements will never be executed. The end result of the expression will be Nothing. Thus results from such an option decorated function can either be Ok(value) or Error(error_value).

from expression.core import Some, Nothing
from expression import effect

@effect.option
def fn():
    x = yield from Nothing # or a function returning Nothing

    # -- The rest of the function will never be executed --
    y = yield from Some(43)

    return x + y

xs = fn()
assert xs is Nothing

For more information about options:

• Tutorial
• API reference

Results

The Result[T, TError] type lets you write error-tolerant code that can be composed. Result works similar to Option but lets you define the value used for errors, e.g an exception type or similar. This is great when you want to know why some operation failed (not just Nothing).

from expression.core import Result, Ok, Error, pipe
from expression import effect

@effect.result
def fn():
    x = yield from Ok(42)
    y = yield from OK(10)
    return x + y

xs = fn()
assert isinstance(xs, Some)

Sequences

Contains operations for working with iterables. Thus all the functions in this module will work on normal Python iterables. Iterables are already immutable by design, so they are already perfectly suited for using with functional programming.

# Normal python way. Nested functions are hard to read since you need to
# start reading from the end of the expression.
xs = range(100)
ys = functools.reduce(lambda s, x: s + x, filter(lambda x: x > 100, map(lambda x: x * 10, xs)), 0)

# With Expression you pipe the result so it flows from one operator to the next:
ys = seq.of(xs).pipe(
    seq.map(lambda x: x * 10),
    seq.filter(lambda x: x > 100),
    seq.fold(lambda s, x: s + x, 0)
)
assert ys == zs

Notable Differences

In F# you modules are capitalized, in Python they are lowercase (PEP-8). E.g in F# Option is both a module and a type. In Python the module is option and the type is capitalized i.e Option.

Thus in Expression you use option as the module to access module functions such as option.map and the name Option for the type itself.

>>> from expression.core import Option, option
>>> Option
<class 'expression.core.option.Option'>
>>> option
<module 'expression.core.option' from '/Users/dbrattli/Developer/Github/Expression/expression/core/option.py'>

Common Gotchas and Pitfalls

A list of common problems and how you may solve it:

Expression is missing the function / operator I need

Remember that everything is a function, so you can easily implement the function yourself and use it with Expression. If you think the function is also usable for others, then please open a PR to include it with Expression.

Resources

• F# (http://fsharp.org)
• Get Started with F# (https://aka.ms/fsharphome)
• F# as a Better Python - Phillip Carter - NDC Oslo 2020 (https://www.youtube.com/watch?v=_QnbV6CAWXc)
• Pampy: Pattern Matching for Python (https://github.com/santinic/pampy)
• OSlash (https://github.com/dbrattli/OSlash)
• RxPY (https://github.com/ReactiveX/RxPY)
• PEP 342 -- Coroutines via Enhanced Generators (https://www.python.org/dev/peps/pep-0342/)
• PEP 380 -- Syntax for Delegating to a Subgenerator (https://www.python.org/dev/peps/pep-0380)
• PEP 479 -- Change StopIteration handling inside generators

How-to Contribute

You are very welcome to contribute with PRs :heart_eyes: It is nice if you can try to align the code with F# modules, functions and documentation. But submit a PR even if you should feel unsure.

Code, doc-strings and comments should also follow the Google Python Style Guide. Code is formatted using Black

License

MIT, see LICENSE.