Haskell打印对

Question

我是Haskell的新手。 我得到了newtype pair which overloads plus operator

代码：

newtype Pair a b = Pair (a,b)  deriving (Eq,Show)

instance (Num a,Num b) => Num (Pair a b) where
   Pair (a,b) + Pair (c,d) = Pair (a+c,b+d)
   Pair (a,b) * Pair (c,d) = Pair (a*c,b*d)
   Pair (a,b) - Pair (c,d) = Pair (a-c,b-d)
   abs    (Pair (a,b)) = Pair (abs a,    abs b) 
   signum (Pair (a,b)) = Pair (signum a, signum b) 
   fromInteger i = Pair (fromInteger i, fromInteger i)

main = do print Pair (1, 3)

当我尝试使用ghc --make编译文件时，收到以下错误消息

BigNumber.hs:11:11:
    Couldn't match expected type `(t1, t2) -> t0'
                with actual type `IO ()'
    The function `print' is applied to two arguments,
    but its type `((a0, b0) -> Pair a0 b0) -> IO ()' has only one
    In a stmt of a 'do' block: print Pair (1, 3)
    In the expression: do { print Pair (1, 3) }

我的目标是创建一个文件，对某种newtype执行某些操作，然后打印出结果。

Answer 1

将main更改为

main = do print (Pair (1, 3))

或

main = do print $ Pair (1, 3)

或（main由单个表达式组成的最佳选项）

main = print $ Pair (1, 3)

您必须向print提供一个参数，而不是2。