ghc pattern match

Foo.hs

{-# LANGUAGE PatternSynonyms #-}

module Foo where

newtype FooBar = FooBar String deriving (Eq, Show)

pattern Whoa :: FooBar
pattern Whoa = FooBar "whoa"

pattern Meah :: FooBar
pattern Meah = FooBar "meah"

{-# COMPLETE Whoa, Meah, FooBar #-}

Main.hs

module Main where

import Foo
import System.Environment
import Control.Monad

main :: IO ()
main = do
  as <- getArgs

  forM_ as $ \x ->
    case FooBar x
      of Whoa -> putStrLn "Whoa!"
         -- Meah -> putStrLn "Meah!"
         FooBar huh -> putStrLn huh

  print Whoa
  print (Whoa == FooBar "whoa")