Fable.Python: F# Advent 2025

Write F#, run Python - a practical guide to Fable.Python

This is a comprehensive guide to Fable.Python, written as literate F# that transpiles to Python and generates its own documentation.

1. Introduction - What is Fable.Python and why use it
2. For Python Developers - F# concepts explained for Pythonistas
3. Getting Started - Setup, your first project, hello world
4. Interop - Using existing Python libraries and Fable.Python bindings
5. Bindings - Creating your own type-safe bindings for Python libraries
6. Compatibility - Supported F# features and limitations
7. Fable v5 - What's new in Fable v5 for Python
8. Libraries - Existing ecosystem (Thoth.Json, AsyncRx, Siren, etc.) (coming soon)
9. Pydantic - Pydantic interop with Decorate and ClassAttributes
10. Units of Measure - Compile-time dimensional analysis
11. Testing - XUnit and Fable.Pyxpecto (coming soon)
12. Fabletext: The Strange Loop - The self-documenting finale

This guide is self-documenting: each chapter is an .fs file with embedded Markdown comments. Fabletext (the final chapter) processes these files to generate the documentation you're reading - including itself.

The chain:

1. Write F# with embedded Markdown (chapters/*.fs)
2. Compile to Python with Fable
3. Run Fabletext (F# compiled to Python) to extract documentation
4. Output: the Markdown you're reading

The blog post is its own proof of concept.

# Install dependencies
just setup
just restore
npm install

# Build everything (F# → Python → Markdown)
just all

# Or generate just the blogpost
just blogpost

just setup      # Install Fable and Python dependencies (uv)
just restore    # Restore NuGet packages
just build      # Compile F# to Python with Fable
just generate   # Convert chapters to individual markdown files
just blogpost   # Generate concatenated blogpost.md for publishing
just format     # Format F# (fantomas) and Python (ruff) files
just lint       # Lint Python (ruff) and Markdown (markdownlint)
just all        # Full pipeline: restore, build, generate, format, lint
just clean      # Remove generated files

chapters/
├── introduction.fs       # What is Fable.Python
├── python.fs             # F# for Python developers
├── getting-started.fs    # Setup and first project
├── interop.fs            # Using Python libraries
├── bindings.fs           # Creating bindings
├── compatibility.fs      # F# feature support
├── fable-v5.fs           # What's new in Fable v5
├── pydantic.fs           # Pydantic interop
└── units-of-measure.fs   # Dimensional analysis
tools/
├── fabletext.fs          # Fabletext converter (F#)
└── fabletext.fsproj
output/
├── chapters/             # Generated Python from chapters
└── tools/
    └── fabletext.py      # Generated converter (Python)
docs/
├── introduction.md       # Individual chapter docs
├── python.md
├── ...
└── blogpost.md           # Concatenated for Hashnode

Defined in justfile:

chapters := "introduction python getting-started interop bindings compatibility fable-v5 pydantic units-of-measure"

To add a new chapter, just add the file and update this list.

• Fable 5 (alpha) - F# to Python compiler
• uv - Python dependency management
• just - Command runner
• ruff - Python formatter/linter
• fantomas - F# formatter
• markdownlint - Markdown linter

• Thoth.Json.Python - Type-safe JSON
• AsyncRx - Reactive extensions
• Fable.Giraffe - Web framework
• Feliz.ViewEngine - HTML DSL
• Siren - Mermaid diagrams
• Fable.Pyxpecto - Testing
• ARCtrl - Real-world multi-target library

• Fable
• Fable.Python Documentation
• Fable.Python GitHub
• BINDINGS_GUIDE.md - Comprehensive binding patterns

Part of F# Advent 2025