在Haskell中定义新数据类型时获取错误"不是数据构造函数"

我正在尝试为方向定义新的数据类型.但是,当我在ghci中加载文件时,我收到错误"Not a data constructor:left".

这是代码片段:

data Dir =   left
           | right 
           | up
           | down 

instance Show Dir where
 show left = "left"
 show right = "right"
 show up = "up"
 show down = "down"


-- initialpos is assumed to be valid one
move::Pos->Dir->Pos
move prevPos left = ( (-1)  $ fst prevPos  ,  snd prevPos)   
move prevPos right = ( (+1) $ fst prevPos ,  snd prevPos )
move prevPos up = ( fst prevPos, (+1) $ snd prevPos  )
move prevPos down = ( fst prevPos, (-1) $ snd prevPos )

请帮我弄清楚出了什么问题.

忽略中缀数据构造函数,数据构造函数必须以大写字母开头.

data Dir = Left | Right | Up | Down