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 use 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