使用elemIndex将MaybeInt转换为Int

Question

我有一个表（字符串列表），并有一个“选择”方法来删除某些行。 Select获取列名和一个值，并删除该列中没有该值的所有行。我使用了elemIndex，并收到类型不匹配错误（可能是Int-> Int）。

  type Field = String
  type Row   = [Field]
  type Table = [Row]

  select :: Field -> Field -> Table -> Table
  select column value table@(header:rows)
      = header : (filter rowHasValue rows)
      where columnIndex     = elemIndex column header
            rowHasValue row = (row !! columnIndex) == value;

我尝试创建另一个函数，但是在哪里得到解析错误

maybeValue :: Field -> Field -> Int
maybeValue column header = case maybeValue of
                                Just n -> n
                                Nothing ->
    where maybeValue = elemIndex column header;

确切的错误消息：

Main.hs:85:39: error:
    * Couldn't match expected type `Int' with actual type `Maybe Int'
    * In the second argument of `(!!)', namely `columnIndex'
      In the first argument of `(==)', namely `(row !! columnIndex)'
      In the expression: (row !! columnIndex) == value
   |
85 |             rowHasValue row = (row !! columnIndex) == value