左右如何阅读价值 [英] Either Right Left how to read value

问题描述

我试图读取一些CSV文件，我用一些例子来说明。这是我得到的

I am trying to read CSV file which I succeded with some examples. Here is what I've got

*Main System.IO> let result=parseCSV contents
*Main System.IO> result
Right [["Name","Value","Amount"],["Rob","1","10"],["Bob","1.42","15.3"],["Tom","452.2","23.1"]]

但是，如果我尝试从这个数组我得到一个错误

But If I try to read value from this array I get an error

*Main System.IO> head result

<interactive>:21:6:
    Couldn't match expected type `[a0]'
            with actual type `Either ParseError [[String]]'
In the first argument of `head', namely `result'
In the expression: head result
In an equation for `it': it = head result

那么我怎样才能摆脱正确的和实际使用列表？

So how can I get Rid of the Right and actually use the list ?

*Main System.IO> :t result
result :: Either ParseError [[String]]

推荐答案

要摆脱 Right ，您需要决定在发生错误时应该怎么做。如果你只是想得到一个空的结果，你可以使用：

To get rid of the Right, you need to decide what to do when you get an error instead. If you just want to get an empty result you could use:

justTheResult :: [[[Char]]]
justTheResult = either (const []) id result

如果你想要提出错误，可以使用：

If you want an error to be raised, you could use:

justTheResult :: [[[Char]]]
justTheResult = either (const $ error "Couldn't parse CSV") id result

如果您想稍后保留该错误，结果，你可以使用任一的 Functor（E或e）实例：

If you want to keep the error for later, but do something with the result, you could instead use Either's Functor (Either e) instance:

errorOrNumberOfRows :: Either ParseError Int
errorOrNumberOfRows = fmap length result

现在你将在同一个地方访问 Int 长度

Now you'll be in the same place accessing that Int length

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