Haskell 语法举例

Question

我正在尝试使一对成为 Printable 的实例，但我无法找出正确的语法。我的可打印是这样的：

class Printable a where
    toString :: a -> [Char]

instance Printable Bool where
    toString True = "true"
    toString False = "false"
instance Printable () where
    toString () = "unit type"

我的配对实例是这样的：

instance  Printable ( a, b ) where
    toString (a,b) = "(" ++ toString a ++ ","++ toString b ++ ")"

编译后，它给了我一个 No instance for (Printable a) arising from a use of ‘toString’。我做错了什么？

Answer 1

您需要 a 和 b 成为 Printable 的实例：

instance (Printable a, Printable b) => Printable ( a, b ) where
    toString (a,b) = "(" ++ toString a ++ ","++ toString b ++ ")"

Haskell 语法举例

Haskell syntax for instance

haskell

instances