Musings about FP and CS

A log of my journey through FP and CS

Declarative CLI parsing in JS

by Clement Delafargue on February 25, 2015

Tagged as: javascript.

I’m currently working on a CLI application written in javascript. As with any kind of programs, the UI plays a very important part.

A good application is consistent and easy to discover by its users. The two means of achieving that with a CLI app are proper help output and autocompletion.

While there are lots of cli parsing libraries available in node js, very few of them handle help correctly, and none of them provide any help regarding autocompletion.

On the other hand in the Haskell world, we have optparse-applicative which makes this problems disappear: it allows you to declare your parsers in a declarative and composable fashion, generates all the help output and even manages shell completion.

CLI Parse

That’s why I’ve released CLI Parse. It does all this (and a bit more).

The code is available: CLI Parse on github.

$ npm install cliparse
var testCli = cliparse.cli({
  name: "testCli",
  description: "Simple CLI written for the sake of the example",
  args: [ cliparse.argument("value") ],
  helpCommand: false,
}, function(x) { console.log(x.args[0]); });

Here’s a laundry list of nice features:

  • out of the box bash and zsh completion
  • easy-to-manipulate parsers
  • nested commands syntax (à la git)
  • custom data parsers
  • custom autocompletion

Since the application is described in a pure data structure, you can manipulate it as you want (you can even let your users extend your app, like git, it will be fully integrated, even in the autocompletion).


It’s already usable, but as always there is always room for improvement:

  • native ZSH completion
  • variadic arguments
  • cleaner help output

Help / PRs more than welcome. The code is quite easy to follow, so don’t be shy :-)

Hear me rant (in French)

I’ve given a talk about CLI Parse in February: slides (English)

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