PatternSynonymImport.hs

module PatternSynonymsImport where

-- one can use either an unqualified import
import PatternSynonymsTest
-- alternatively one can use explicit imports (this requires enabling the
-- PatternSynonyms language extension):
-- import PatternSynonymsTest (Option(..), pattern None)

maybeToOption :: Maybe a -> Option a
maybeToOption Nothing = None
maybeToOption (Just a) = Some a

optionToMaybe :: Option a -> Maybe a
optionToMaybe None = Nothing
optionToMaybe (Some a) = Just a

PatternSynonymTest.hs

{-# LANGUAGE PatternSynonyms #-}

module PatternSynonymsTest
  ( Option(..)
  , pattern None
  ) where

data Option a
    = Some a
    | NoneMsg String
    deriving (Show, Eq)

pattern None <- NoneMsg _ where
  None = NoneMsg ""