通过点函数更新Elm-lang记录中的字段？ [英] Update a field in an Elm-lang record via dot function?

问题描述

是否可以通过函数（或其他方式）更新Elm记录中的字段而无需显式指定确切的字段名称？

Is it possible to update a field in an Elm record via a function (or some other way) without explicitly specifying the precise field name?

示例：

> fields = { a = 1, b = 2, c = 3 }
> updateField fields newVal fieldToUpdate = { fields | fieldToUpdate <- newVal }
> updateField fields 5 .a -- does not work

UPDATE：

要添加一些上下文，我试图干燥以下代码：

UPDATE:

To add some context, I'm trying to DRY up the following code:

UpdatePhraseInput contents ->
  let currentInputFields = model.inputFields
  in { model | inputFields <- { currentInputFields | phrase <- contents }}

UpdatePointsInput contents ->
  let currentInputFields = model.inputFields
  in { model | inputFields <- { currentInputFields | points <- contents }}

如果我可以叫神话般的 updateInput 函数是这样的：

Would be really nice if I could call a mythical updateInput function like this:

UpdatePhraseInput contents -> updateInput model contents .phrase
UpdatePointsInput contents -> updateInput model contents .points

推荐答案

滚动您自己的更新功能

是的，尽管可能不如从字段中获取 。但是想法是相同的，您编写了一个仅使用记录更新语法的函数：

Rolling your own update function

Yes, though perhaps not as nicely as getting from a field. But the idea is the same, you write a function that simply uses the record update syntax:

setPhrase r v = { r | phrase <- v }
setPoints r v = { r | points <- v }
updInputFields r f = { r | inputFields <- f r.inputFields }

然后您可以编写：

UpdatePhraseInput contents -> updInputFields model (flip setPhrase contents)
UpdatePointsInput contents -> updInputFields model (flip setPoints contents)

焦点库

当您组合 field 和 fieldSet 时，您会得到类似 焦点 。尽管该库不仅可以处理记录，还可以处理更多的事情。这是使用 Focus 的示例：

The Focus library

When you combine field and fieldSet, you get something like a Focus. Although that library works for more things than just records. Here's an example of what this would look like using Focus:

phrase = Focus.create .phrase (\upd r -> { r | phrase <- upd r.phrase })
points = Focus.create .points (\upd r -> { r | points <- upd r.points })
inputFields = Focus.create .inputFields (\upd r -> { r | inputFields <- upd r.inputFields})

然后您可以编写：

UpdatePhraseInput contents -> Focus.set (inputFields => phrase) contents model
UpdatePointsInput contents -> Focus.set (inputFields => points) contents model

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