Restrict records using Opaque Types

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This topic has been covered by Elm Patterns​
If you're writing a package use a pipeline API instead.
Problem
Solution
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}
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
    }
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
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.
We can unwrap function parameters like this:
addRating rating (Movie movie) =
    ...
Further reading
📖Book: Beginning Elm by Pawan Poudel
📄Article: Advanced Types in Elm - Opaque Types by Charlie Koster
👥Thread: Post-update invariant repair: good idea or bad?​

Non empty lists using Zippers

Write safer functions using Phantom Types

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