# Restrict records using Opaque Types

{% hint style="info" %}
This topic has been covered by [Elm Patterns](https://sporto.github.io/elm-patterns/advanced/opaque-types.html)
{% endhint %}

{% hint style="info" %}
If you're writing a package use a [pipeline API](https://orasund.gitbook.io/elm-cookbook/recipes-1/designing-a-api/create-upwards-compatible-apis) instead.
{% endhint %}

{% tabs %}
{% tab title="Problem" %}
{% code title="Main.elm" %}

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

{-| adds a Rating to a Movie,
Only allows ratings between 1 and 5.
-}     
addRating : Int -> Movie -> Movie
addRating rating movie =
    { movie | rating = rating}
```

{% endcode %}
{% endtab %}

{% tab title="Solution" %}
{% code title="Movie.elm" %}

```
module Movie exposing (Movie,fromTitle,addRating)

type Movie =
    Movie 
        { title : String
        , rating : Int
        }

{-| adds a Rating to a Movie,
Only allows ratings between 1 and 5.
-}     
addRating : Int -> Movie -> Maybe Movie
addRating rating (Movie movie) =
    if 1 <= rating && rating <= 5 then 
        Just <| Movie { movie | rating = rating}
    else
        Nothing

{-| Constructs a Movie from a Title
-}
fromTitle : String -> Movie
fromTitle title =
  Movie
    { title = title
    , rating = 0
    }
```

{% endcode %}

{% code title="Main.elm" %}

```
import Movie exposing (Movie)

newMovie : String -> Int -> Movie
newMovie title rating =
    let
        movie = Movie.fromTitle title
    in
    movie
    |> Movie.addRating rating
    |> Maybe.withDefault movie
```

{% endcode %}
{% endtab %}
{% endtabs %}

## Question

How can I only allow specific states for my type?

## Answer

Place the type into a new File and don't expose the constructor: `exposing (Movie)` instead of `exposing(Movie(..))`.\
Then write your own constructors.

{% hint style="info" %}
We can unwrap function parameters like this:

```
addRating rating (Movie movie) =
    ...
```

{% endhint %}

## Further reading

* 📖**Book:** [Beginning Elm](https://elmprogramming.com/commands.html#opaque-type) by Pawan Poudel
* 📄**Article:** [Advanced Types in Elm - Opaque Types](https://medium.com/@ckoster22/advanced-types-in-elm-opaque-types-ec5ec3b84ed2) by Charlie Koster
* 👥**Thread:** [Post-update invariant repair: good idea or bad?](https://discourse.elm-lang.org/t/post-update-invariant-repair-good-idea-or-bad/3715/2)