Elm 在更新模型时应用可能情况

我想通过以下方式更新我的模型：

updatedModel =
  if model.firstChord && model.secondChord then
    { model | firstChord = {}, secondChord = {} }
  else
    model

第一和弦和第二和弦都属于和弦类型：

type alias Chord =
{   root : Maybe Pitch
,   third : Maybe Pitch
,   fifth : Maybe Pitch
}

音高类型如下所示：

-- pitch
type alias Pitch = ( PitchName, PitchLevel )

-- pitch name
type alias PitchName = String

-- pitch level
type alias PitchLevel = Int

我的初始模型包含以下字段：

{ firstChord =
    {   root = ( "C", 3 )
    ,   third = ( "E", 3 )
    ,   fifth = ( "G", 3 )
    }
, secondChord =
    {   root = ( "F", 3 )
    ,   third = ( "A", 3 )
    ,   fifth = ( "C", 4 )
    }

我喜欢有可选的音高值

我如何更新我的模型给它一个值或什么都没有

---

谢谢。

不太清楚你在找什么。我想你会喜欢这样的和弦

type Pitch =
    Pitch String Int

type alias Chord =
    { root: Maybe Pitch
    , third: Maybe Pitch
    , fifth: Maybe Pitch
    }

type alias Model =
    { firstChord: Maybe Chord
    , secondChord: Maybe Chord
    }

init: Model
init =
    { firstChord =
        { root = Pitch "C" 3
        , third = Pitch "E" 3
        , fifth = Pitch  "G" 3
        }
    , secondChord =
        { root = Pitch "F" 3
        , third = Pitch "A" 3
        , fifth = Pitch "C" 4
        }
    }

update: Model -> Model
update model =
    case (model.firstChord, model.secondChord) of
        (Just first, Just second) ->
            { model | firstChord = Nothing, secondChord = Nothing}
        _ ->
            model

---

不太清楚你在找什么。我想你会喜欢这样的和弦

type Pitch =
    Pitch String Int

type alias Chord =
    { root: Maybe Pitch
    , third: Maybe Pitch
    , fifth: Maybe Pitch
    }

type alias Model =
    { firstChord: Maybe Chord
    , secondChord: Maybe Chord
    }

init: Model
init =
    { firstChord =
        { root = Pitch "C" 3
        , third = Pitch "E" 3
        , fifth = Pitch  "G" 3
        }
    , secondChord =
        { root = Pitch "F" 3
        , third = Pitch "A" 3
        , fifth = Pitch "C" 4
        }
    }

update: Model -> Model
update model =
    case (model.firstChord, model.secondChord) of
        (Just first, Just second) ->
            { model | firstChord = Nothing, secondChord = Nothing}
        _ ->
            model

---

当你有一个

可能是一个
类型，比如
可能是音高
时，你有两种方法来设置它的值：
无
或
只是一个
。因此，与此相反：

{ firstChord =
    {   root = ( "C", 3 )
    ,   third = ( "E", 3 )
    ,   fifth = ( "G", 3 )
    }
, secondChord =
    {   root = ( "F", 3 )
    ,   third = ( "A", 3 )
    ,   fifth = ( "C", 4 )
    }

…您需要这样做：

{ firstChord =
    {   root = Just ( "C", 3 )
    ,   third = Just ( "E", 3 )
    ,   fifth = Just ( "G", 3 )
    }
, secondChord =
    {   root = Just ( "F", 3 )
    ,   third = Just ( "A", 3 )
    ,   fifth = Just ( "C", 4 )
    }

当然，然后使用
Nothing
，因此表示无值：

firstChord =
    {   root = Just ( "C", 3 )
    ,   third = Just ( "E", 3 )
    ,   fifth = Nothing
    }

当你有一个
可能是一个
类型，比如
可能是音高
时，你有两种方法来设置它的值：
无
或
只是一个
。因此，与此相反：

{ firstChord =
    {   root = ( "C", 3 )
    ,   third = ( "E", 3 )
    ,   fifth = ( "G", 3 )
    }
, secondChord =
    {   root = ( "F", 3 )
    ,   third = ( "A", 3 )
    ,   fifth = ( "C", 4 )
    }

…您需要这样做：

{ firstChord =
    {   root = Just ( "C", 3 )
    ,   third = Just ( "E", 3 )
    ,   fifth = Just ( "G", 3 )
    }
, secondChord =
    {   root = Just ( "F", 3 )
    ,   third = Just ( "A", 3 )
    ,   fifth = Just ( "C", 4 )
    }

当然，然后使用
Nothing
，因此表示无值：

firstChord =
    {   root = Just ( "C", 3 )
    ,   third = Just ( "E", 3 )
    ,   fifth = Nothing
    }

为什么不
键入PitchName=A | B | C |……|G
。和
type Pitch=Pitch PitchName PitchLevel
。然后
somePitch=pitchg3
如果第一和弦和第二和弦是Chord类型，那么
firstChord={}
将是非法的，因为{}不是Chord类型。为什么不
typepitchname=A | B | C |……|G
。和
type Pitch=Pitch PitchName PitchLevel
。然后
somePitch=Pitch g3
如果firstChord和secondChord是Chord类型，那么
firstChord={}
将是非法的，因为{}不是Chord类型。