64 lines
1.7 KiB
Markdown
64 lines
1.7 KiB
Markdown
# Haskell optparse-applicative
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This note covers `33-haskell-optparse-cli/`, which parses a small deployment CLI with `optparse-applicative`.
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---
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## 1. Why This Is Different from the Parser-Combinator Example
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`12-haskell-parser-combinators/` builds a parser for a custom command language.
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This example solves a different problem: parsing a conventional CLI with:
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- subcommands,
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- long options,
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- repeated options, and
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- typed option readers.
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That is a common real-world Haskell task, and `optparse-applicative` is the standard tool for it.
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---
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## 2. What the Pure Parser Gives You
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The example exposes:
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```haskell
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runCliParser :: [String] -> Either String Command
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```
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Internally it uses `execParserPure`, which keeps the parsing logic testable without invoking `IO` or shelling out.
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That is the main teaching point here: command-line parsing can still be structured as a pure transformation from argument vectors to typed commands.
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---
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## 3. What Makes the Example Non-Trivial
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The parser handles two shapes of command:
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- `validate`, which parses typed environment, replica, and track options, and
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- `promote`, which parses repeated `--owner` flags plus a `--dry-run` switch.
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That is enough surface area to show why a dedicated CLI parser library is useful.
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---
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## 4. Commands to Try
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```bash
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cd 33-haskell-optparse-cli
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nix develop
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cabal run
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cabal run -- validate --service api --env production --replicas 3 --track stable
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cabal run -- promote --service api --from-tag blue --to-tag green --owner platform --owner security --dry-run
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cabal test
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nix build
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./result/bin/mini-cli validate --service api --env production --replicas 3 --track stable
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nix run . -- validate --service api --env production --replicas 3 --track stable
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nix flake check
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```
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