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

它抱怨道：

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？我还想将“show方法”添加到Shape并覆盖它，以便我可以在屏幕上（打印）代表Shape。

Answer 1

您需要添加show：

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

将数据表示为字符串

1 个答案: