一些奇怪的错误（在我的模型记录中？）

Question

我的编译器抱怨两个值：

model.firstChord.fifth 和 model.secondChord.fifth

在此摘录中：

-- render frets
renderFret : Model -> Fret -> Html Msg
renderFret model fret =
    let
        ( pitchName, pitchLevel ) = fret.pitch
        ( firstChordRootPitchName, firstChordRootPitchLevel ) = model.firstChord.root
        ( firstChordThirdPitchName, firstChordThirdPitchLevel ) = model.firstChord.third
        ( firstChordFifthPitchName, firstChordFifthPitchLevel ) = model.firstChord.fifth
        ( secondChordRootPitchName, secondChordRootPitchLevel ) = model.secondChord.root
        ( secondChordThirdPitchName, secondChordThirdPitchLevel ) = model.secondChord.third
        ( secondChordFifthPitchName, secondChordFifthPitchLevel ) = model.secondChord.fifth
    in
        ...

它告诉我：

model.firstChord does not have a field named fifth. - The type of model.firstChord is:

Maybe Chord

Which does not contain a field named fifth.

但是我的模型有一个字段 fifth:

-- initial model
init : ( Model, Cmd Msg )
init =
    (
        {   firstChord =
            Just
            {   root = ( "C", 3 )
            ,   third = ( "E", 3 )
            ,   fifth = ( "G", 3 )
            }
        ,   secondChord =
            Just
            {   root = ( "F", 3 )
            ,   third = ( "A", 3 )
            ,   fifth = ( "C", 4 )
            }
        }
    ,
        Cmd.none
    )

和弦的类型是：

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

每个球场都有这种类型：

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

-- pitch name
type alias PitchName = String

-- pitch level
type alias PitchLevel = Int

问题出在哪里？

谢谢。

Answer 1

您还没有提供 Model，但在您的初始化函数中您已将和弦声明为 Maybe。如果编译器对此感到满意，则意味着您的模型还包含 Maybe。作为第一个解决方案，删除 Justs，但还要查看您的模型

init =
    (
        {   firstChord =
            Just
            {   root = ( "C", 3 )
            ,   third = ( "E", 3 )
            ,   fifth = ( "G", 3 )
            }
        ,   secondChord =
            Just   <-------- Here you say that secondChord is Maybe Chord
            {   root = ( "F", 3 )
            ,   third = ( "A", 3 )
            ,   fifth = ( "C", 4 )
            }
        }
    ,
        Cmd.none
    )

Answer 2

编译错误准确说明了问题所在

Maybe Chord 是 Just Chord 或 Nothing。这两个中的 None 包含一个名为 fifth 的字段。

为了完成这项工作，您需要确保 model.firstChord 和 model.secondChord 是 Just Chord:

-- render frets
renderFret : Model -> Fret -> Html Msg
renderFret model fret =
    case (model.firstChord, model.secondChord) of
        (Just firstChord, Just secondChord) ->                
            let
                ( pitchName, pitchLevel ) = fret.pitch
                ( firstChordRootPitchName, firstChordRootPitchLevel ) = firstChord.root
                ( firstChordThirdPitchName, firstChordThirdPitchLevel ) = firstChord.third
                ( firstChordFifthPitchName, firstChordFifthPitchLevel ) = firstChord.fifth
                ( secondChordRootPitchName, secondChordRootPitchLevel ) = secondChord.root
                ( secondChordThirdPitchName, secondChordThirdPitchLevel ) = secondChord.third
                ( secondChordFifthPitchName, secondChordFifthPitchLevel ) = model.secondChord.fifth
            in
                ...
        _ ->
            -- here is something when either `model.firstChord` or `model.secondChord` is `Nothing`

通过使用模式匹配 (Just firstChord, Just secondChord)、firstChord 和 secondChord 表达式似乎属于 Chord 类型，它有一个名为 fifth[= 的字段23=]

一些奇怪的错误（在我的模型记录中？）

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