Is it possible to automatically show in help which values are valid for an option?

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
haskell
Domain
cli

Research direction

Start with the example's opts parser and trace how option auto, help, value, and showDefault are rendered by execParser. Compare the current help output with the requested allowed-values text, then identify the existing help-generation tests or entry points that would need coverage; done means the behavior is specified and verified for sum-type options.

Written by the indexing model from the issue text.

Description

Given an option of a sum type (Foo in this example):

module Main where

import Options.Applicative

data Opts = Opts
    { flag :: Foo
    , ok :: Bool
    }
    deriving (Show)

data Foo = Bar | Baz | Quux deriving (Read, Show)

opts :: Parser Opts
opts =
    Opts
        <$> option auto (help "how to do it" <> long "flag" <> value Bar <> showDefault)
        <*> option auto (help "is it ok" <> long "ok" <> value True <> showDefault)

main :: IO ()
main = do
    opts' <- execParser (info (opts <**> helper) fullDesc)
    print opts'

This is the help output currently:

Usage: optest [--flag ARG] [--ok ARG]

Available options:
  --flag ARG               how to do it (default: Bar)
  --ok ARG                 is it ok (default: True)
  -h,--help                Show this help text

Instead of --flag ARG how to do it (default: Bar) I would like something like --flag ARG how to do it (default: Bar, allowed values: Foo, Bar, Baz). Can optparse-applicative do this?

Dominant language
Haskell
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957
Forks
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