data 定义
we can use the same name for data type and value constructor, This has no special meaning, like Person in the following:
data Point = Point Float Float deriving(Show)
Value constructors are actually **functions that ultimately return a value of a data type. **
record syntax
当一种data type包含很多字段的时候，用旧的方式定义会完全搞不懂什么是什么，recoard syntax省事多了
并且这样会声称字段对应的方法 model, year
>> data Car = Car { model :: String, year :: int } deriving Show
>> car = Car { model = "Honda", year = 1988}
>> model car
Honda
>> car
Car { model = "Honda", year = 1988}
type parameter and type constructor
给data type加参数可以生成不同的data type，Maybe可以生成Maybe [Char] Maybe Int之类
data Maybe a = Nothing | Just a
-- a 只是定义了类型, 跟参数完全没关系
-- ZipList Int之类的是个整体，而不是真的有具体的值ZipList 1
data ZipList a = ZipList { getZipList :: [a] }
newtype
newtype比data高效，因为data要重新包装，而newtype只包含一个值，只是值的类型被重新定义了一下
data ZipList a = ZipList { ZipList :: [a]}
newtype ZipList a = ZipList { ZipList :: [a]}
