newtype keyword.

newtype-keyword.hs

@@ -0,0 +1,29 @@
+-- typeclass declaration using 'data' keyword
+data ZipList1 a = ZipList1 [a]
+
+-- typeclass declaration using record syntax
+data ZipList2 a = ZipList2 { getZipList2 :: [a] }
+
+-- typeclass declaration using 'newtype' keyword. It's faster than 'data' for types that just wrap an existent type. It can only have one value constructor.
+newtype ZipList3 a = ZipList3 { getZipList3 :: [a] }
+
+newtype CharList = CharList { getCharList :: [Char] } deriving (Eq, Show)
+
+a1 = CharList "this will be shown!"  
+a2 = CharList "benny" == CharList "benny"  
+a3 = CharList "benny" == CharList "oisters"
+
+newtype Pair b a = Pair { getPair :: (a,b) }
+
+instance Functor (Pair c) where  
+    fmap f (Pair (x,y)) = Pair (f x, y)
+
+b1 = getPair $ fmap (*100) (Pair (2,3))  
+b2 = getPair $ fmap reverse (Pair ("london calling", 3))
+
+-- try it by calling helloMe undefined to see the difference between 'data' and 'newtype' keywords
+-- data CoolBool = CoolBool { getCoolBool :: Bool }
+newtype CoolBool = CoolBool { getCoolBool :: Bool }
+
+helloMe :: CoolBool -> String  
+helloMe (CoolBool _) = "hello"