# Haskell optparse-applicative

This note covers `33-haskell-optparse-cli/`, which parses a small deployment CLI with `optparse-applicative`.

---

## 1. Why This Is Different from the Parser-Combinator Example

`12-haskell-parser-combinators/` builds a parser for a custom command language.

This example solves a different problem: parsing a conventional CLI with:

- subcommands,
- long options,
- repeated options, and
- typed option readers.

That is a common real-world Haskell task, and `optparse-applicative` is the standard tool for it.

---

## 2. What the Pure Parser Gives You

The example exposes:

```haskell
runCliParser :: [String] -> Either String Command
```

Internally it uses `execParserPure`, which keeps the parsing logic testable without invoking `IO` or shelling out.

That is the main teaching point here: command-line parsing can still be structured as a pure transformation from argument vectors to typed commands.

---

## 3. What Makes the Example Non-Trivial

The parser handles two shapes of command:

- `validate`, which parses typed environment, replica, and track options, and
- `promote`, which parses repeated `--owner` flags plus a `--dry-run` switch.

That is enough surface area to show why a dedicated CLI parser library is useful.

---

## 4. Commands to Try

```bash
cd 33-haskell-optparse-cli

nix develop
cabal run
cabal run -- validate --service api --env production --replicas 3 --track stable
cabal run -- promote --service api --from-tag blue --to-tag green --owner platform --owner security --dry-run
cabal test

nix build
./result/bin/mini-cli validate --service api --env production --replicas 3 --track stable

nix run . -- validate --service api --env production --replicas 3 --track stable
nix flake check
```