Functors, Applicatives, And Monads in Python

OSlash (Ø) is a library for learning and understanding functional programming in Python 3.12+. It re-implements
concepts from Learn You a Haskell for Great Good! using Python with modern type annotations. OSlash unifies
functional and object-oriented paradigms by grouping related functions within classes. Objects are never used
for storing values or mutable data; data exists only within function closures.

✨ What's New in 1.0

OSlash 1.0 is a complete modernization for Python 3.12+:

Modern Type System: Fully type-checked with Pyright in strict mode
PEP 695 Syntax: Clean type parameters (class Maybe[T]: instead of Generic[T])
Modern Tooling: Built with uv, formatted with ruff, validated with pre-commit hooks
Production Status: Stable release ready for educational use

Type Safety: OSlash is fully type-checked with Pyright in strict mode, providing excellent IDE support and catching errors at development time. It leverages Python 3.12's PEP 695 type parameter syntax for clean, ergonomic generic types.

OSlash is intended to be a tutorial. For practical functional programming in Python in production environments you
should consider using Expression instead.

Install

# Using pip
pip install oslash

# Or using uv (recommended)
uv add oslash

The project currently contains implementations for:

Abstract Base Classes

Functor, for stuff that can be mapped
Applicative, for callable stuff
Monoid, for associative stuff
Monad, for monadic stuff

And Some Monads

Identity, boxed stuff in its simplest form
Maybe (Just | Nothing), for optional stuff
Either (Right | Left), for possible failures
List, purely functional list of stuff
IO Action, for impure stuff
Writer, for logging stuff
Reader, for callable stuff
State, for stateful computations of stuff
Cont, for continuation of stuff

Monadic functions

>>, for sequencing monadic actions
lift, for mapping a function over monadic values
join, for removing one level of monadic structure
compose, for composing monadic functions

Utility functions

compose, for composing 0 to n functions

But why?

Yes, I know there are other projects out there like PyMonad,
fn.py. I'm simply doing this in order to better understand the
book. It's so much easier to learn when you implement things yourself. The code may be
similar to PyMonad in structure, but is quite different in implementation.

Why is the project called OSlash? OSlash is the Norwegian character called Oslash.
Initially I wanted to create a project that used Ø and ø (unicode) for the project name and modules. It didn't work out
well, so I renamed it to OSlash.

Examples

Haskell:

> fmap (+3) (Just 2)
Just 5

> (+3) <$> (Just 2)
Just 5

Python:

>>> Just(2).map(lambda x: x+3)
Just 5

>>> (lambda x: x+3) % Just(2)
Just 5

IO Actions:

from oslash import put_line, get_line

main = put_line("What is your name?") | (lambda _:
    get_line() | (lambda name:
    put_line("What is your age?") | (lambda _:
    get_line() | (lambda age:
    put_line("Hello " + name + "!") | (lambda _:
    put_line("You are " + age + " years old"))))))

if __name__ == "__main__":
    main()

Tutorials

Functors, Applicatives, And Monads In Pictures in Python.
Three Useful Monads (in progress)
Using Either monad in Python

dbrattli/OSlash