Pat1.hs

{-# LANGUAGE PatternSynonyms, ViewPatterns, TypeFamilies #-}
module Pat1 (Vec(Nil,(:>)), pattern (:>)) where

import qualified Control.Lens as Lens hiding (pattern (:>))

newtype Vec a = Vec {unvec :: [a]}

pattern Nil :: Vec a
pattern Nil = Vec []

pattern (:>) :: a -> Vec a -> Vec a
pattern (:>) x xs <- ((\ys -> (head $ unvec ys,Vec . tail $ unvec ys)) -> (x,xs))
  where
    (:>) x xs = Vec (x:unvec xs)

infixl 5 :>

type instance Lens.Index (Vec a) = Int