Allow options via environment variables

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
haskell
Domain
cli

Research direction

Start with the issue's testOption example and review how option precedence and help text are currently defined. Compare the requested PROGNAME_TEST behavior with the proposed output, including command-line overrides. Done means the library supports the documented environment-variable behavior and its help presentation is settled.

Written by the indexing model from the issue text.

Description

testOption :: Parser String
testOption = strOption
     ( long "test"
    <> env "PROGNAME_TEST"
    <> short 't'
    <> metavar "VAL"
    <> help "For testing purposes only")
    <|> pure "default value"

I don't know if the environment variable choice should display in the help text, but it could look something like this:

Usage: progname [-t|--test VAL]

Available options:
  -h,--help                Show this help text
  -t,--test VAL            For testing purposes only

Environment variables:
  PROGNAME_TEST=VAL        For testing purposes only. Overridden by -t,--test

If you don't think this should be a responsibility of this library, what would be a good alternative to getting environment variable options that are possibly overridden by command line options?

Awesome library by the way

Dominant language
Haskell
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