Create small command line program that parses arguments

Before starting my take-home assignment, the instructions advised me to create a
"Hello, world" program in the language of my choice. Since I'm choosing Haskell,
I created this example as my starter boilerplate.

scratch/brilliant/.ghci

@@ -0,0 +1,2 @@
+:set prompt "> "
+:set -Wall

scratch/brilliant/Main.hs

@@ -0,0 +1,45 @@
+{-# LANGUAGE RecordWildCards #-}
+--------------------------------------------------------------------------------
+module Main where
+--------------------------------------------------------------------------------
+import Options.Applicative
+import Data.Semigroup ((<>))
+
+import qualified System.Environment as Env
+--------------------------------------------------------------------------------
+
+data CommandArgs = CommandArgs
+  { hello :: String
+  , quiet :: Bool
+  , enthusiasm :: Int
+  } deriving (Eq, Show)
+
+parseArgs :: Parser CommandArgs
+parseArgs =
+  CommandArgs <$> strOption
+                  ( long "hello"
+                 <> metavar "TARGET"
+                 <> help "Target for the greeting" )
+              <*> switch
+                  ( long "quiet"
+                 <> short 'q'
+                 <> help "Whether to be quiet" )
+              <*> option auto
+                  ( long "enthusiasm"
+                 <> help "How enthusiastic to greet"
+                 <> showDefault
+                 <> value 1
+                 <> metavar "INT" )
+
+main :: IO ()
+main = do
+  args <- execParser opts
+  greet args
+  where
+    opts = info (parseArgs <**> helper)
+      ( fullDesc
+     <> progDesc "Print a greeting for TARGET"
+     <> header "header - a test for optparse-applicative" )
+
+greet :: CommandArgs -> IO ()
+greet CommandArgs{..} = putStrLn $ "Hello, " ++ hello ++ replicate enthusiasm '!'

scratch/brilliant/shell.nix

@@ -0,0 +1,9 @@
+let
+  pkgs = import /home/wpcarro/nixpkgs {};
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [
+      optparse-applicative
+    ]))
+  ];
+}