• decline-derive
  • Installation
  • Getting started
  • Contributing

Experimental library to quickly derive Decline CLI interfaces, to quickly prototype CLI applications.

It is driven by types (with enums and case classes representing groups of subcommands and groups of parameters respectively, see example below), and can handle anything Decline has built-in support for, and some extra handling of Option[..] and List[..] types. The library is inspired by clap from Rust, but mainly because it's the most recent example I came across - deriving CLI parsers with macros is a long standing tradition in Scala.

This library does not aim to cover every possible case representable with Decline - in fact in only covers the most basic of cases. For complicated CLIs I would still recommend using the approach that Decline library recommends.

Acknowledgements:

• August Nagro's post on how to read annotations from macros
• Scala 3 documentation page on macro-driven derivation

Latest version:

• SBT: libraryDependencies += "com.indoorvivants" %%% "decline-derive" % "<latest version>"
• Scala CLI: //> using dep com.indoorvivants::decline-derive::<latest version>
• Mill: ivy"com.indoorvivants::decline-derive::<latest version>"

import decline_derive.*

enum CLI derives CommandApplication:
  case Index(location: String, @Name("lit") isLit: Boolean)
  case Run(@Positional("files") files: List[String])
  case Bootstrap(config: String = "config.toml")

@main def helloDecline(args: String*) =
    println(CommandApplication.parse[CLI](args))

Notice how we're using @Name("lit") to customise certain aspects of generated Decline parser. Default parameter values turn them into optional in the generated CLI

For more configuration options, see tests, annotations.

If you see something that can be improved in this library – please contribute! Turning users into contributors and maintainers is one of the purest joys of OSS.

This library was largely put together on board of a plane during a very short flight, so it might have some rough edges.

This is a standard Scala CLI project, with a Makefile for some useful commands.

Here are some useful commands:

• make test – run tests. Note that this command runs tests for all three platforms - which might be unnecessarily slow for development purposes. Quickest feedback loop is achieved by just running scala-cli test *.scala
• make check-docs – verify that snippets in README.md (this file) compile
• make pre-ci – format the code so that it passes CI check
• make run-example – run example from README