inter-hub-haskell/docs/ihp-overview.md

# IHP Framework Overview

> Reference notes for implementing the Interaction Hub Framework (IHF) using IHP.
> Based on IHP v1.5.0 (released March 2026).

---

## What IHP Is

**IHP** (Integrated Haskell Platform) is a batteries-included, full-stack web framework built on Haskell and Nix. Its goal is rapid application development with robust, type-safe code.

- **Language:** Haskell (GHC 9.10 default; GHC 9.12 experimental in v1.5)
- **Paradigm:** Functional, strongly typed, server-rendered with optional realtime
- **Creator:** [digitally induced](https://github.com/digitallyinduced) (Hamburg). Open-sourced 2020, in production since 2017
- **Current version:** v1.5.0 (March 25, 2026) — largest release to date (1,051 commits)
- **License:** MIT

### v1.5 Headline Changes

| Change | Impact |
|--------|--------|
| `postgresql-simple` → `hasql` driver | Up to 50% lower query latency |
| Dev server RAM: 4 GB → 500–800 MB | Practical on smaller machines |
| Session middleware 3×, URL gen 5×, rendering 2× faster | Overall snappier |
| `typedSql` quasiquoter | Compile-time SQL type checking against live dev DB |
| `fetchPipelined` | Multiple queries in one network round-trip |
| Composite primary key support | Needed for join-table models |
| Integration test mode | Temporary Postgres DB per test run |
| 15+ modules on Hackage separately | `ihp-mail`, `ihp-datasync`, etc. |

### Design Philosophy

- Type errors at compile time, not runtime
- Single command (`devenv up`) starts a fully self-contained environment — Postgres included, managed by Nix. No Docker, no Kubernetes required
- Optimized for AI-assisted development — the type system automatically verifies generated code

---

## Core Architecture

### MVC-Influenced Structure

| Layer | IHP Location | Role |
|-------|-------------|------|
| Model | `Application/Schema.sql` + generated types | Schema, query builder, relationships |
| Controller | `Web/Controller/<Name>.hs` | Action handlers, parameter binding, DB calls |
| View | `Web/View/<Name>/<Action>.hs` | HSX templates, `View` typeclass |
| Routing | `Web/Routes.hs` + `Web/FrontController.hs` | URL ↔ action mapping |
| Types | `Web/Types.hs` | All controller action constructors |
| Helpers | `Application/Helper/Controller.hs`, `Application/Helper/View.hs` | Shared logic |

Multi-application support: a single project can contain multiple sub-apps (`Web/`, `Admin/`). `new-application admin` generates an `Admin/` subtree with routes auto-prefixed `/admin/`.

### Type-Safe URL / Action System

The IHP router always maps HTTP requests to **data constructors** defined in `Web/Types.hs`:

```haskell
data WidgetsController
    = WidgetsAction
    | NewWidgetAction
    | ShowWidgetAction     { widgetId :: !(Id Widget) }
    | CreateWidgetAction
    | EditWidgetAction     { widgetId :: !(Id Widget) }
    | UpdateWidgetAction   { widgetId :: !(Id Widget) }
    | DeleteWidgetAction   { widgetId :: !(Id Widget) }
    deriving (Eq, Show, Data)
```

- URLs generated from values, not strings: `pathTo ShowWidgetAction { widgetId = someId }`
- Compile-time guarantee: broken links are type errors, not 404s
- `urlTo` generates absolute URLs (protocol + domain)

### Routing

Defined in `Web/Routes.hs`, registered in `Web/FrontController.hs`.

**AutoRoute** (most common): `instance AutoRoute WidgetsController` — IHP generates RESTful routes from action name prefixes:

| Prefix | HTTP Method |
|--------|------------|
| `Delete` | DELETE |
| `Create`, `Update` | POST/PATCH |
| anything else | GET |

**Custom routes:** implement `CanRoute` (attoparsec parser URL → action) and `HasPath` (reverse). `customRoutes` overrides individual AutoRoute entries. Supports SEO slugs like `/widgets/my-slug`.

HTTP method override for HTML forms: pass `_method=DELETE` (or `PATCH`) as a hidden field.

---

## Development Workflow

### Installation

```bash
# 1. Install Determinate Nix (with Flakes + lazy-trees)
curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install

# 2. Install ihp-new
nix profile install nixpkgs#ihp-new

# 3. Install direnv and hook into shell
nix profile add nixpkgs#direnv
echo 'eval "$(direnv hook bash)"' >> ~/.bashrc   # or zshrc
```

### Creating a Project

```bash
ihp-new my-project
cd my-project
devenv up
```

First startup: 10–15 minutes (downloads GHC, Postgres, all Haskell deps via Nix binary cache). Subsequent starts are fast (under 30s).

### Dev Server

`devenv up` starts everything:
- Application server on `http://localhost:8000`
- IHP IDE + Schema Designer on `http://localhost:8001`
- Postgres (managed by Nix; no system Postgres needed)
- Live reloading (typically sub-50ms after save)

### Project File Structure

```
my-project/
├── Application/
│   ├── Schema.sql              # Single source of truth for all DB types
│   ├── Migration/              # <timestamp>-description.sql files
│   ├── Helper/
│   │   ├── Controller.hs       # Shared controller helpers
│   │   └── View.hs             # Shared view helpers
│   └── Script/                 # One-off scripts / cron job binaries
├── Web/
│   ├── Types.hs                # ALL controller action constructors
│   ├── Routes.hs               # AutoRoute instance declarations
│   ├── FrontController.hs      # WAI entry; dispatch; auth init; default layout
│   ├── Controller/             # One file per controller
│   ├── View/                   # One dir per controller, one file per action
│   │   └── Layout.hs           # Default layout (Html -> Html)
│   └── Component/              # Server-Side Components (optional)
├── Config/
│   ├── Config.hs               # Env vars, secrets, feature flags
│   └── nix/
│       └── hosts/
│           └── production/     # Declarative NixOS server config
├── Test/                       # Integration tests
├── static/                     # CSS, JS, images
├── flake.nix                   # Nix flake — all deps declared here
├── App.cabal                   # Cabal package definition
├── Main.hs                     # Entry point
└── Makefile                    # Build targets
```

### Adding Dependencies

In `flake.nix`:
```nix
ihp = {
  enable = true;
  projectPath = ./.;
  packages = [ pkgs.imagemagick ];    # native deps
  haskellPackages = p: [
    p.ihp
    p.aeson
    p.your-library
  ];
};
```

### Code Generators

Navigate to `http://localhost:8001/Generators` or right-click a table in Schema Designer → "Generate Controller". Scaffolds controllers, views, routes, and type entries to match the table's fields. Also available: Migration, Job, Mail, Script generators.

### Testing

v1.5: integration test mode creates a temporary Postgres DB automatically per test run. Tests live in `Test/`.

---

## Deployment

### Primary: NixOS / `deploy-to-nixos`

The entire server config (nginx, Let's Encrypt, systemd, app config) lives declaratively in `Config/nix/hosts/production/` — version-controlled and reproducible.

```bash
deploy-to-nixos production
```

Runs `nixos-rebuild` remotely over SSH. AWS EC2: NixOS AMI, `t3a.small` min (`t3a.medium` recommended), 60 GiB root disk, ports 22/80/443.

**Required env vars:**

| Variable | Purpose |
|----------|---------|
| `IHP_SESSION_SECRET` | Session encryption key (`new-session-secret` to generate) |
| `DATABASE_URL` | Postgres connection string |
| `IHP_BASEURL` | External URL (e.g., `https://example.com`) |

**Production features built-in:**
- Systemd watchdog (heartbeat 30s; auto-restart after 60s)
- Socket activation (queues requests during restarts — zero-downtime deploys)
- Sentry integration via `ihp-sentry` (IHP Pro)

### Docker (IHP Pro)

```bash
nix build .#unoptimized-docker-image --option sandbox false
cat result | podman load
```

Env vars: same as above + `IHP_ASSET_VERSION` (cache-busting) + `IHP_REQUEST_LOGGER_IP_ADDR_SOURCE=FromHeader` (behind load balancer). Minimal Docker images lack CA certificates — copy `caCertificates` and set `SSL_CERT_FILE`.

### Bare Metal Binary

```bash
nix build .#optimized-prod-server     # full optimisation
nix build .#unoptimized-prod-server   # faster build, for staging
```

Runtime: `IHP_ENV=Production`, `DATABASE_URL`, `PORT` (default 8000). GHC RTS tunable via `GHCRTS`.

### CSS/JS Bundling

```bash
make static/prod.js static/prod.css
```

Dev uses individual files; production uses bundled/minified versions. Background jobs require deploying the `RunJobs` binary alongside the main app.

---

## Key Links

- [IHP Homepage](https://ihp.digitallyinduced.com/)
- [IHP Guide (full docs)](https://ihp.digitallyinduced.com/Guide/)
- [GitHub: digitallyinduced/ihp](https://github.com/digitallyinduced/ihp)
- [ihp-boilerplate](https://github.com/digitallyinduced/ihp-boilerplate) — template used by `ihp-new`
- [v1.5 release announcement](https://discourse.haskell.org/t/ihp-v1-5-has-been-released/13846)