Type Aliases

Elm allows you to create a type alias. An alias is just a shorter name for some other type. It looks like this:

type alias User =
  { name : String
  , bio : String

So rather than having to type out this record type all the time, we can just say User instead. For example, you can shorten type annotations like this:

hasDecentBio : User -> Bool
hasDecentBio user =
  String.length user.bio > 80

That would be { name : String, bio : String } -> Bool without the type annotation. The main point of type aliases is to help us write shorter and clearer type annotations. This becomes more important as your application grows. Say we have a updateBio function:

updateBio : String -> User -> User
updateBio bio user =
  { user | bio = bio }

First, think about the type signature without a type alias! Now, imagine that as our application grows we add more fields to represent a user. We could add 10 or 100 fields to the User type alias, and we do not need any changes to our updateBio function. Nice!

Record Constructors

When you create a type alias specifically for a record, it also generates a record constructor. So if we define a User type alias in elm repl we could start building records like this:

> type alias User = { name : String, bio : String }

> User "Tom" "Friendly Carpenter"
{ name = "Tom", bio = "Friendly Carpenter" }

The arguments are in the order they appear in the type alias declaration. This can be pretty handy.

And again, this is only for records. Making type aliases for non-record types will not result in a constructor.

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