Some Strange Fault (In My Model Record ?) - elm

Question

My compiler complains about two values:
model.firstChord.fifth and model.secondChord.fifth
in this excerpt:
-- 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
...
it tells me:
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.
but my model has a field 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
)
The type of the chord is:
-- chords
type alias Chord =
{ root : Pitch
, third : Pitch
, fifth : Pitch
}
Each pitch has this type:
-- pitch
type alias Pitch = ( PitchName, PitchLevel )
-- pitch name
type alias PitchName = String
-- pitch level
type alias PitchLevel = Int
Where could be the problem?
Thank you.

Answer 1

Compile error says exactly what the problem is
Maybe Chord is either Just Chord or Nothing. None of these two contain a field named fifth.
In order to make this work you need to make sure that model.firstChord and model.secondChord are 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`
By using pattern matching (Just firstChord, Just secondChord), firstChord and secondChord expressions appear to be of type Chord, which has a field named fifth

Answer 2

You have not provied your Model, but in your init function you have declared the chord as a Maybe. If the compiler was happy with that, then it means your model also includes the Maybes. As a first solution, remove the Justs, but also look at your Model
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
)