解析错误在哪里

Question

在此代码中

neighbours :: CityMap -> District -> [District]
neighbours (CM (_,rs)) b = mapMaybe neighbour rs
    where neighbour (p,q)
        | b == p    = Just q --parse error (possibly incorrect indentation or mismatched brackets)
        | b == q    = Just p
        | otherwise = Nothing

我先解析«|»

Answer 1

守卫必须比他们所属的功能名称缩进，例如：

neighbours :: CityMap -> District -> [District]
neighbours (CM (_,rs)) b = mapMaybe neighbour rs
    where neighbour (p,q)
           | b == p    = Just q 
           | b == q    = Just p
           | otherwise = Nothing

这是因为在哪里，你要定义一个（本地）函数neighbour，它必须遵循布局规则;如果守卫在左边，则不是neighbour定义的延续。你会在一个看起来像这样的文件中得到同样的错误：

  neighbour (p,q)
| b == p   = Just q