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
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type alias Movie = 
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    { title : String
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    , rating : Int
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    }
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​
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{-| adds a Rating to a Movie,
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Only allows ratings between 1 and 5.
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-}     
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addRating : Int -> Movie -> Movie
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addRating rating movie =
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    { movie | rating = rating}
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Movie.elm
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module Movie exposing (Movie,fromTitle,addRating)
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​
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type Movie =
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    Movie 
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        { title : String
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        , rating : Int
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        }
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​
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{-| adds a Rating to a Movie,
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Only allows ratings between 1 and 5.
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-}     
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addRating : Int -> Movie -> Maybe Movie
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addRating rating (Movie movie) =
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    if 1 <= rating && rating <= 5 then 
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        Just <| Movie { movie | rating = rating}
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    else
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        Nothing
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​
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{-| Constructs a Movie from a Title
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-}
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fromTitle : String -> Movie
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fromTitle title =
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  Movie
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    { title = title
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    , rating = 0
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    }
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Main.elm
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import Movie exposing (Movie)
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​
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newMovie : String -> Int -> Movie
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newMovie title rating =
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    let
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        movie = Movie.fromTitle title
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    in
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    movie
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    |> Movie.addRating rating
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    |> Maybe.withDefault movie
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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:
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addRating rating (Movie movie) =
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    ...
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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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