Elm：自定义类型的访问值

Question

我在Elm中有一个自定义类型来处理错误分支。基本上，我有一个输入，它给出String s，我需要将其转换为Int s。

type Seconds
    = Error Int
    | Valid Int

type alias Model =
    { timeBetweenExercises : Seconds
    , roundsSequenceOne : Seconds
    , roundsSequenceTwo : Seconds
    , roundsSequenceThree : Seconds
    , repititionsPerExercise : Seconds
    , secondsPerExercise : Seconds
    , totalTrainingDuration : Seconds
    }


init : Model
init =
    { timeBetweenExercises = Valid 3
    , roundsSequenceOne = Valid 3
    , roundsSequenceTwo = Valid 3
    , roundsSequenceThree = Valid 3
    , repetitionsPerExercise = Valid 6
    , secondsPerExercise = Valid 6
    , totalTrainingDuration = Valid 6
    }

我从Evan's "Life of a file" talk得到了关于自定义类型的想法。我想记住出现错误时的数字（例如，用户输入的是字符串而不是数字）。这是尝试我的更新功能：

update : Msg -> Model -> Model
update msg model =
    case msg of
        TimeBetweenExercisesChanged newTime ->
            case String.toInt newTime of
                Nothing ->
                    { model | timeBetweenExercises = Error model.timeBetweenExercises }

                Just time ->
                    { model | timeBetweenExercises = Valid time }

我的问题是，由于model.timeBetweenExercises的类型为Seconds，所以编译器对我大吼大叫。有没有办法我只能获取自定义类型的Int值？

Answer 1

在我看来，该模型是错误的，有两个原因：

1. 如果您具有所有变体通用的值，通常更方便地将其上移或下移。因此，您可能会拥有：

2. 您的更新功能表明Error状态实际上没有描述其持有的值，因为如果无效，您只是将newTime丢掉，而对于Error情况则使用旧时间。] >

基于此，我建议使用以下模型：

type alias TimeInput =
    { seconds: Int
    . state: TimeInputState
    }

type TimeInputState = Error | Valid

type alias Model =
    { timeBetweenExercises : TimeInput
      ...
    }
并将您的更新功能更改为此：

update : Msg -> Model -> Model
update msg model =
    case msg of
        TimeBetweenExercisesChanged newTime ->
            case String.toInt newTime of
                Nothing ->
                    { model
                        | timeBetweenExercises = 
                            { seconds: timeBetweenExercises.seconds
                            , state: Error
                            }
                    }

                Just time ->
                    { model
                        | timeBetweenExercises = 
                            { seconds: time
                            , state: Valid
                            }
                    }
``