通过点函数更新 Elm-lang 记录中的字段？

Question

是否可以通过函数（或其他方式）更新 Elm 记录中的字段而不明确指定精确的字段名称？

示例：

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

更新：

为了添加一些上下文，我正在尝试整理以下代码：

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

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

如果我能像这样调用神话般的 updateInput 函数就太好了：

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

Answer 1

滚动你自己的更新函数

是的，虽然可能不如从田地里得到好。但是思路是一样的，你写一个简单的使用记录更新语法的函数：

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

那你可以这样写：

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

`Focus` 图书馆

当您组合 field 和 fieldSet 时，您会得到类似于 Focus 的内容。尽管该图书馆的用途不仅仅是记录。这是使用 Focus:

的示例

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})

那你可以这样写：

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

Answer 2

理想情况下，您可以定义一个可以 select 更新哪个字段的函数，但您不能。

参见 This article，它解释了为什么目前无法在 elm 中功能性地更新记录中的字段：

"Seasoned functional programmers will surely have noticed that many of these concepts sound a lot like lenses, and Elm actually already has a lens-like library authored by Evan himself, called Focus. This, however, does not actually solve the problem: it requires manual description of setters just like the purely function based approach does."

...并且以下 comment from the the creator of Elm explaining why proposed support for this 未添加到最近对记录的更改中：

"I considered that and decided against it. I know that proposal. It's a language feature proposal, not a simple syntax thing"

通过点函数更新 Elm-lang 记录中的字段？

Update a field in an Elm-lang record via dot function?

functional-programming

elm

更新：

滚动你自己的更新函数

`Focus` 图书馆

通过点函数更新 Elm-lang 记录中的字段？

Update a field in an Elm-lang record via dot function?

functional-programming

elm

更新：

滚动你自己的更新函数

Focus 图书馆

`Focus` 图书馆