Restrict records using Opaque Types

This topic has been covered by Elm Patterns

If you're writing a package use a pipeline API instead.

Problem

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}

Solution

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?