Haskell語法例如

Question

我試圖做一對Printable的實例，但我無法弄清楚正確的語法。 我打印是這樣的：

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

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

和我的對實例是：

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

，其在編制，給了我一個No instance for (Printable a) arising from a use of ‘toString’。 我在做什麼錯？

Answer 1

你需要a和b是的Printable實例：

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