how-to-update-nested-records.md

How to update nested Records?

{% hint style="warning" %} Not best practice: Create an opaque type instead. {% endhint %}

{% tabs %} {% tab title="Problem" %}

type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }

rate : Int -> Model -> Model
rate newRating model =
    { model 
    | currentMovie = 
        --{ model.currentMovie | rating = newRating }
        Debug.todo "the line above does not compile"
    }

{% endtab %}

{% tab title="Solution" %}

type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }

rate : Int -> Model -> Model
rate newRating ({currentMovie} as model) =
    { model 
    | currentMovie = 
        {currentMovie | rating = newRating }
    }

{% endtab %}

{% tab title="Alternative Solution" %}

type alias Movie = 
    { title : String
    , rating : Int
    }
  
type alias Model = 
    { currentMovie : Movie
    }

rate : Int -> Model -> Model
rate rating ({currentMovie} as model) =
    model
    |> updateMovie (addRating rating)

updateMovie : (Movie -> Movie) -> Model -> Model
updateMovie fun ({currentMovie} as model) =
    { model | currentMovie = fun currentMovie}
   
addRating : Int -> Movie -> Movie
addRating rating movie =
    { movie | rating = rating}

{% hint style="success" %} This is the a better solution {% endhint %} {% endtab %} {% endtabs %}

Question

How can I update a nested record field?

Answer

First get the nested record, then update it:

let
    currentMovie = model.currentMovie
in
{ currentMovie | rating = newRating}

{% hint style="info" %} For function parameters we can use the keyword as to bind fields of a record to a variable with the same name:

rate newRating ({currentMovie} as model) =
    ...

{% endhint %}

Further reading

• 📄Article: Updating nested records in Elm by Wouter In t Velt