gren-argparse

Declarative command-line argument parsing for Gren. Describe your CLI as a value; get back a typed command to pattern-match on. A Program package is also included, providing common functionality for a CLI program for parsing a command-line, running code, and exiting with a proper return code.

Built-in --help, --version, and ANSI-colored error messages are included.

Example of what is supported

% todo add "buy milk" --done
% todo add "buy milk" -d
% todo list

The walkthrough below builds this todo tool step by step.

How it works

You describe your CLI as a value, hand it the raw argv, and get back a typed result:

Argparse.Parser.run argv app  →  CommandParseResult YourCommand

Parsing is a pure function — no I/O, no process exits. You get a result back and decide what to print and what exit code to use.

Argparse.Program is an optional wrapper that handles the bootstrapping for you: CLI errors go to stderr, help goes to stdout, and a successful parse calls your handler. Then your handler tells Program which return code the process should exit with.

1. Define your program's options type

This is your type; the parser's job is to produce it.

type ProgramOptions
    = Add { text : String, done : Bool }
    | List

2. Describe your commands

An App ties the tool name, version, and a list of commands together.

import Argparse.Parser
import Argparse.PrettyPrinter as PP

parser : Argparse.Parser.App ProgramOptions
parser =
    { name = "todo"
    , version = "1.0.0"
    , intro = PP.words "A small todo list."
    , outro = PP.empty
    , commands =
        Argparse.Parser.defineGroup
            |> Argparse.Parser.withCommand
                { word = "add"
                , arguments =
                    Argparse.Parser.oneArg
                        { value = Argparse.Parser.stringParser
                        , help = "The task to add"
                        }
                , flags =
                    Argparse.Parser.initFlags (\done -> { done = done })
                        |> Argparse.Parser.toggle (Argparse.Parser.Both { long = "done", short = "d" }) "Mark it already done"
                , commonDescription = Just "Add a task to the list."
                , summary = "The `add` command appends a task:"
                , example = PP.words "todo add \"buy milk\""
                , builder =
                    \text flags ->
                        Add { text = text, done = flags.done }
                }
            |> Argparse.Parser.withCommand
                { word = "list"
                , arguments = Argparse.Parser.noArgs
                , flags = Argparse.Parser.noFlags
                , commonDescription = Just "Show every task."
                , summary = "The `list` command prints the tasks:"
                , example = PP.words "todo list"
                , builder = \_args _flags -> List
                }
    }

Note: You can nest groups under a prefix word with withPrefix to get git-style subcommands like todo remote add origin.

Arguments

Each command declares the positional arguments it accepts. Pick the combinator for the arity you want:

• noArgs — no positional arguments
• oneArg / twoArgs / threeArgs — exact arity
• optionalArg — zero or one (Maybe)
• zeroOrMoreArgs — zero or more
• oneOrMoreArgs — one or more

Flags

Flags are type-safe: initFlags takes a record constructor, and each toggle/flag/requiredFlag call fills in one field. The compiler checks that you've wired everything up correctly.

• toggle name desc — a Bool (true when the flag is present)
• flag name valueParser desc — an optional value, so you get a Maybe value
• requiredFlag name valueParser desc — a mandatory value, so you get the value directly (not a Maybe); if the user leaves the flag out, the parse fails and they're told it's missing. Handy for inputs a command can't run without when a positional argument would be awkward — say several unordered path options. Required flags are marked (required) in --help.

A flag's name controls which spellings the user can type:

• LongOnly "verbose" → --verbose
• ShortOnly "v" → -v
• Both { long = "verbose", short = "v" } → --verbose or -v

--help (and -h) and --version are always available — you don't need to declare them.

ValueParser

A ValueParser converts a raw string into a typed value. Two are built in: stringParser (any String) and pathParser (FileSystem.Path). Write your own in a few lines:

myParser : Argparse.Parser.ValueParser MyType
myParser =
    { label = "mytype", fn = MyType.fromString, examples = [ "example" ] }

3. Run it

Argparse.Program.run wires the parser into a Node program:

import Argparse.Program
import Node
import Stream.Log
import Task

main : Node.SimpleProgram a
main =
    Argparse.Program.run
        { parser = parser
        , onCommand =
            \env cmd ->
                (when cmd is
                    Add { text, done } ->
                        Stream.Log.line env.stdout
                            ((if done then "Added (done): " else "Added: ") ++ text)

                    List ->
                        Stream.Log.line env.stdout "(no tasks yet)"
                )
                    |> Task.map (\_ -> {})
        }

onCommand receives the Node.Environment (stdout, stderr, args, …) and your parsed command.

Exit codes

Your handler returns a Task Argparse.Program.Failure {}. Task.succeed {} is the only exit-0 path; all non-zero exits go through Task.fail:

If your command always succeeds, just end with:

|> Task.map (\_ -> {})

Need a full model/update loop? Call Argparse.Parser.run directly and handle each constructor yourself:

main : Node.SimpleProgram a
main =
    Node.defineSimpleProgram <| \env ->
        let
            args =
                Array.dropFirst 2 env.args
        in
        Node.endSimpleProgram <|
            when Argparse.Parser.run args parser is
                Argparse.Parser.UnknownCommand name ->
                    Stream.Log.line env.stderr ("Unknown command: " ++ name)
                        |> Task.andThen (\_ -> Node.setExitCode 1)

                Argparse.Parser.BadFlags err ->
                    Stream.Log.line env.stderr (PP.toString (Argparse.Parser.flagErrorPrettified err))
                        |> Task.andThen (\_ -> Node.setExitCode 1)

                Argparse.Parser.BadArguments err ->
                    Stream.Log.line env.stderr (PP.toString (Argparse.Parser.argumentErrorPrettified err))
                        |> Task.andThen (\_ -> Node.setExitCode 1)

                Argparse.Parser.HelpText doc ->
                    Stream.Log.line env.stdout (PP.toString doc)

                Argparse.Parser.Success cmd ->
                    runCommand env cmd

Choosing a runner

Argparse.Program has four entry points:

• run — the standard case: an App with one or more subcommands.
• runRoot — no subcommand word; the whole tool is one command (like greet --loud World). You pass a single Command instead of an App.
• runWithContext — like run, but runs an Init.await chain first so you can acquire permissions (FileSystem, terminal, …) before any command runs.
• runRootWithContext — runRoot plus the up-front permission acquisition.

Examples

Each example in examples/ is a self-contained app with a run.sh:

cd examples/one-level
./run.sh add "buy milk"
./run.sh --help

If you use devbox, you can build all examlpes with:

devbox run examples

Testing

The parser is pure, so you can test it directly with gren-lang/test:

cd tests
./run-tests.sh

If you use devbox, just run:

devbox run test