Create upwards compatible APIs

This topic has been covered by Elm Patterns

Problem

Movie.elm

type Movie =
    Movie 
        { title : String
        , rating : Int
        , -- adding a field will destroy the function below
        }

new : String -> Int -> Movie
new title rating =
  Movie
    { title = title
    , rating = clamp 1 5 rating
    }

Solution

Movie.elm

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

fromTitle : String -> Movie -> Movie
fromTitle title =
    Movie 
        { title : title
        , rating : 0
        , director : Nothing
        }

withRating : Int -> Movie -> Movie
withRating rating (Movie movie) =
    { movie | rating = clamp 1 5 rating }

withDirector : String -> Movie -> Movie
withDirector director (Movie movie) =
    { movie | director = director }

Question

How can I define my type, such that I can add features without breaking the API?

Answer

First write a constructor fromTitle that only uses as few arguments as possible. Next add partial constructors for every feature of your type: withRating and withDirector.

Now creating a new Movie can be done like this:

Movie.fromTitle "Life of Brian"
|> Movie.withDirector "Terry Jones"
|> Movie.withRating 5

Further Reading

• 📄Article: With* Functions in Elm by Charlie Koster
• 🎥Video: Robot Buttons from Mars by Brian Hicks
• ❗Example: NoRedInk/elm-json-decode-pipeline
• ❗Example: Chadtech/random-pipeline
• ❗Example: Particle Type from BrianHicks/elm-particle
• ❗Example: Request Type from dillonkearns/elm-graph
• ❗Example: Image Type from Orasund/pixelengine