如何 List.sum 一个列表（可能是浮点数）？

Question

我正在尝试将字符串转换为浮点数并找到它们的总和：

> String.split "," "0.2,0.3,3.1" 
> List.map String.toFloat ["0.2","0.3","3.1"]
> List.sum [Just 0.2,Just 0.3,Just 3.1]

但我收到这些编译器消息：

This argument is a list of type:

    List (Maybe Float)

But `sum` needs the 1st argument to be:

    List number

Hint: Use Maybe.withDefault to handle possible errors. Longer term, it is
usually better to write out the full `case` though!

我怎样才能得到这些值的总和？

Answer 1

您已经创建了一个List (Maybe Float)，但您需要一个List Float(number是一种特殊类型，可以是Int或Float)。这意味着您需要处理列表中的值为 的情况Nothing。错误消息建议使用Maybe.withDefault，它看起来像这样：

"0.2,0.3,3.1"
|> String.split ","
|> List.map String.toFloat
|> List.map (Maybe.withDefault 0)
|> List.sum

或者，List.filterMap设计为采用返回 aMaybe并删除Nothing值的函数。在这种情况下，String.toFloat是那种功能，所以你可以改为：

"0.2,0.3,3.1"
|> String.split ","
|> List.filterMap String.toFloat
|> List.sum