Applying Maybes When Updating A Model - elm

Question

I want to update my model this way:
updatedModel =
if model.firstChord && model.secondChord then
{ model | firstChord = {}, secondChord = {} }
else
model
the firstChord and the secondChord both are of the type Chord:
type alias Chord =
{ root : Maybe Pitch
, third : Maybe Pitch
, fifth : Maybe Pitch
}
The pitch type looks like:
-- pitch
type alias Pitch = ( PitchName, PitchLevel )
-- pitch name
type alias PitchName = String
-- pitch level
type alias PitchLevel = Int
My initial Model has these fields:
{ firstChord =
{ root = ( "C", 3 )
, third = ( "E", 3 )
, fifth = ( "G", 3 )
}
, secondChord =
{ root = ( "F", 3 )
, third = ( "A", 3 )
, fifth = ( "C", 4 )
}
I like to have optional pitch values.
How can I update my model giving it a value OR nothing?
Thanks.

Answer 1

Not quite sure what are you looking for. I guest you would like to have a Maybe Chord like this.
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

Answer 2

When you have a Maybe a type, such as Maybe Pitch you have two ways to set its value: Nothing or Just a. So instead of this:
{ firstChord =
{ root = ( "C", 3 )
, third = ( "E", 3 )
, fifth = ( "G", 3 )
}
, secondChord =
{ root = ( "F", 3 )
, third = ( "A", 3 )
, fifth = ( "C", 4 )
}
...you need to do this:
{ 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 )
}
Of course, you then use Nothing so indicate no value:
firstChord =
{ root = Just ( "C", 3 )
, third = Just ( "E", 3 )
, fifth = Nothing
}