Functional Programming — A Two-Track Companion Course

Two parallel learning tracks, reinforcing each other:

Track	Focus	Language	Goal
Haskell	Functional programming — types, purity, monads, type-driven design	Haskell	Learn to think in pure functional terms
Nix	Functional devops — reproducible builds, declarative systems, NixOS	Nix (the language) + NixOS	Apply functional principles to infrastructure

Why pair them?

Haskell and Nix share deep DNA: both are purely functional, both treat configuration and programs as evaluable values, both prioritise reproducibility. The Haskell track teaches you the ideas; the Nix track shows those ideas at work on real systems — packages, services, entire operating systems.

Haskell concept	Nix counterpart
Pure functions	Nix derivations (same inputs → same output)
Immutability	The /nix/store (content-addressed, never mutated)
Type classes	mkDerivation, callCabal2nix, etc. (combinators)
Laziness	Nix's lazy evaluation of derivations
Monadic IO	The Nix evaluator + builtins.derivation
Algebraic data types	Nix attrsets + the rec keyword

Structure

functional-programming/
├── README.md          ← you are here
├── haskell/
│   ├── HASKELL_OVERVIEW.md   ← start here
│   ├── PATTERNS.md
│   ├── ALLIUM_RESEARCH.md
│   ├── allium-hs/            ← standalone experiment
│   └── lesson-1 .. lesson-5  ← the Haskell course
└── nix/
    └── README.md             ← placeholder (course not yet started)

How to use this repository

Each lesson is self-contained — cd into it, run nix develop, follow LESSON.md. No global state, no setup beyond having Nix installed.

The recommended order:

Haskell track

1. Lesson 1 — Your first TUI: pure state, pattern matching, the Elm architecture
2. Lesson 2 — Async, BChan, HTTP, JSON (talking to DeepSeek)
3. Lesson 3 — Types-first design: a decoupled agent protocol over NDJSON
4. Lesson 3a — Refactor with view models and command types
5. Lesson 4 — Testing pure functions: Hspec + QuickCheck
6. Lesson 5 — Real streaming via SSE (the protocol's Token/Done finally used)

Nix track

Planned. Will cover: Nix the language, derivations, flakes, NixOS modules, declarative services, dev shells, and Haskell-on-Nix integration. Each lesson mirrors the Haskell worklog style.

Prerequisites

• Nix installed (with flakes enabled)
• A terminal that supports Unicode (for the TUI lessons)
• For lessons 2+: a DeepSeek API key in ~/.pi/agent/auth.json (or modify the backend to use another OpenAI-compatible endpoint)

Tooling

Every lesson ships a flake.nix providing:

• GHC (Haskell compiler) or nix itself
• cabal-install (build tool)
• haskell-language-server (IDE backend)
• ghcid (instant feedback loop)
• Hoogle (offline type search)

Run nix develop in any lesson directory. That's it.