Question

While I know there’s a few ways to do this, I’m most interested in finding the most idiomatic and functional Scala method.

Given the following trite example:

case class User(id: String)
val users = List(User("1"), User("2"), User("3"), User("4")) 

What’s the best way to create an immutable lookup Map of user.id -> User so that I can perform quick lookups by user.id.

In Java I’d probably use Google-Collection’s Maps.uniqueIndex although its unique property I care less about.

Answer 1

You can keep the users in a List and use list.find:

users.find{_.id == "3"} //returns Option[User], either Some(User("3")) or None if no such user

or if you want to use a Map, map the list of users to a list of 2-tuples, then use the toMap method:

val umap = users.map{u => (u.id, u)}.toMap

which will return an immutable Map[String, User], then you can use

umap contains "1" //return true

or

umap.get("1") //returns Some(User("1"))