为什么我会收到"Equations for ... have different numbers of arguments"消息？

Question

以下函数编译并工作：

shares :: Maybe (Int, L.ByteString) -> Maybe Int                                                                                                                                                            
shares a =                                                                                                                                                                                                  
case a of                                                                                                                                                                                               
Nothing        -> Nothing                                                                                                                                                                           
Just (x, y)    -> Just x

但是当以以下形式重写时：

shares :: Maybe (Int, L.ByteString) -> Maybe Int                                                                                                                                                            
shares Nothing = Nothing                                                                                                                                                                                    
shares Just (x, y) = Just x

我收到错误

Equations for ‘shares’ have different numbers of arguments

我认为本质上是一样的。

Answer 1

在 Haskell 中，函数的参数用空格分隔。因此，最后一个方程有两个参数：a -> Maybe a型的Just和(Int, L.ByteString)型的(x, y)。由于你想要一个参数，它应该改为：

shares (Just (x, y)) = Just x