How to have parametrizable "methods" in Elm data-structures - data-structures

Question

I'm stuck refactoring a large data-structure in Elm. I know how I would implement this in OO languages, but have no experience in a functional setting. I can't express well what my problem is, because I can only frame it in OOP terms which don't apply. So I go by way of example, which is a simplified version of the module I'm refactoring.
Suppose I have this type:
type alias Book = { title : String, text : String }
and I have two Books:
englishBook = { title = "An English Book", text = "This book is an Eglish book." }
frenchBook = { title = "Un livre Francais", text = "Ce livre est un livre Francais." }
There is an associated index function to compute which words are in a Book:
index = String.words >> Set.fromList
Here's already my first problem. When index takes a String, the user of the module must know how to take the text from the book. Instead, my habits say that the function should do this for us. So index could behave like a method and take a Book as first argument: index = .text >> String.words >> Set.fromList. But that also feels weird.
That's not the end of it though, because the index generator should be parametrizable. Depending on the book, it should do different things. So I could add the index function like this:
englishBook = { title = "...", text = "...", index = englishIndex }
frenchBook = { title = "...", text = "...", index = frenchIndex }
now each book has the function to build its index. But still the caller has to supply the record when it wants the index:
wordsInEnglishBook = englishBook.index englishBook.text
which is not a nice solution to me because it burdens the caller with internals of the module. Well what if that part is encapsulated with a constructor?
book title text index = { title = title, text = text, index = \_ -> index text }
Now I've come full circle and have implemented a method. So what is the idiomatic solution for this in Elm?

Answer 1

You can use a custom type to represent the language then pattern match on the language to perform your index.
type Language
= English
| French
type alias Book =
{ title : String
, text : String
, language : Language
}
wordsInBook : Book -> Set String
wordsInBook { language, text } =
case language of
English ->
doSomethingWithEnglish text
French ->
doSomethingWithFrench text
or
type Book
= Book Language Data
type Language
= English
| French
type alias Data =
{ title : String
, text : String
}
wordsInBook : Book -> Set String
wordsInBook (Book language data) =
case language of
English ->
doSomethingWithEnglish data
French ->
doSomethingWithFrench data