haskell - 将数据表示为字符串

Question

我有以下简单的代码：

data Shape = Circle Float Float Float | Rectangle Float Float Float Float deriving (Show)
surface :: Shape -> Float
surface (Circle _ _ r) = pi * r ^ 2

main = putStrLn $ surface $ Circle 10 20 30

它抱怨：

Couldn't match expected type `String' with actual type `Float'
In the second argument of `($)', namely `surface $ Circle 10 20 30'

如何摆脱错误？我还想“添加”show方法Shape并覆盖它，以便我可以Shape在屏幕上（打印）表示我想要的任何内容。

Answer 1

您需要添加显示：

main = putStrLn $ show $ surface $ Circle 10 20 30

如果您想要自己的 Show 方法，请不要派生 Show：

data Shape = Circle Float Float Float
           | Rectangle Float Float Float Float

instance Show Shape where   
  show (Circle _ _ r) = show r   
  show (Rectangle r _ _ _) = show r

main = putStrLn $ show $ Circle 10 20 30