haskell - 在哪里解析错误

Question

在这段代码中

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

我首先解析了«|»

Answer 1

守卫的缩进必须比它们所属的函数的名称更进一步，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 where 之后，您定义了一个（本地）函数neighbour，它也必须遵循布局规则；如果守卫在更左边，它不是定义的延续neighbour。您会在如下所示的文件中收到相同的错误：

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