Skip to main content

Practical functional programming for Python 3.8+

Project description


PyPI Python package Upload Python Package codecov

Python :heart: F#

FSlash (F/) 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 meaningful and productive 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.

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

Better Python with F#

FSlash 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.

FSlash 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.

Getting Started

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

$ pip3 install fslash


  • 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.

I named the project FSlash since it's 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.


  • 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

FSlash 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.
  • List - an immutable list type.
  • Computational Expressions: 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.


OSlash 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 fslash.core import pipe

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

assert value == gn(fn(x))

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

from fslash.core import pipe

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

assert value == gn(fn(x))

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

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


Functions may even be composed directly into custom operators:

from fslash.core import compose

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

ys = custom(xs)


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 fslash.core import Some, Nothing, Option

def keep_positive(a: int) -> Option[int]:
    if a > 0:
        return Some(a)
        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 OSlash are implemented as coroutines (enhanced generators) using yield, yield from and return to consume or generate optional values:

from import option
from fslash.core import Some

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 fslash.core import Some, Nothing
from import 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:


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 fslash.core import Result, Ok, Error, pipe
from import result

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

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


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 F/ you pipe the result so it flows from one operator to the next:
ys = seq.of(xs).pipe( 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 F/ you use option as the module to access module functions such as and the name Option for the type itself.

>>> from fslash.core import Option, option
>>> Option
<class 'fslash.core.option.Option'>
>>> option
<module 'fslash.core.option' from '/Users/dbrattli/Developer/Github/FSlash/fslash/core/'>

Common Gotchas and Pitfalls

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

The FSlash List type has the same name as the builtin List type in Python

You can easily import the FSlash list module with e.g a different name:

from fslash.collections import List as FList, list as flist

... or you can rename the standard Python list:

from builtins import list as pylist
from typing import List as PyList

FSlash 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 FSlash. If you think the function is also usable for others, then please open a PR to include it with FSlash.


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



Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

FSlash-0.17.0.tar.gz (51.6 kB view hashes)

Uploaded source

Built Distribution

FSlash-0.17.0-py3-none-any.whl (49.2 kB view hashes)

Uploaded py3

Supported by

AWS AWS Cloud computing Datadog Datadog Monitoring Facebook / Instagram Facebook / Instagram PSF Sponsor Fastly Fastly CDN Google Google Object Storage and Download Analytics Huawei Huawei PSF Sponsor Microsoft Microsoft PSF Sponsor NVIDIA NVIDIA PSF Sponsor Pingdom Pingdom Monitoring Salesforce Salesforce PSF Sponsor Sentry Sentry Error logging StatusPage StatusPage Status page