Here's a simple example of a program that uses envparse's parser:

module Main (main) where

import Control.Monad (unless)
import Env

data Hello = Hello { name :: String, quiet :: Bool }

hello :: IO Hello
hello = Env.parse (header "envparse example") $
  Hello <$> var (str <=< nonempty) "NAME"  (help "Target for the greeting")
        <*> switch                 "QUIET" (help "Whether to actually print the greeting")
 
main :: IO ()
main = do
  Hello { name, quiet } <- hello
  unless quiet $
    putStrLn ("Hello, " ++ name ++ "!")

The NAME environment variable is mandatory and contains the name of the person to greet. QUIET, on the other hand, is an optional boolean flag, false by default, that decides whether the greeting should be silent.

If the NAME variable is undefined in the environment then running the program will result in the following help text:

envparse example
 
Available environment variables:
 
  NAME                   Target for the greeting
  QUIET                  Whether to actually print the
                         greeting
 
Parsing errors:
 
  NAME is unset

---

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