TypeAliases.md

Type aliases

A type alias doesn't define a type, it simply provides a convenient alternative name for another type or record shape.

We will define a Ship and a Person, but the compiler sees them as the same thing, which is a record with a name field.

type alias Ship = { name : String }
type alias Person = { name : String }

We'll create a Ship and a Person, but in fact they are both just a record with a name field. The compiler only sees them as this.

vessel : Ship
vessel = { name = "HMS Splendid" }

captain : Person
captain = { name = "Captain Robert Q. Peabody" }

Here are two functions which seem to take a Ship and a Person as arguments, but in fact they are really just taking a record with a name field.

launch : Ship -> String
launch s =
  "I name this ship " ++ s.name

greet : Person -> String
greet p =
  "Hello there, " ++ p.name

Here we launch a ship and greet a person. As you'd expect, both functions compile fine and give the expected output.

launchShip = launch vessel
greetPerson = greet captain

But these next two functions also compile and execute okay. That's because Ship and Person are not types, they're just aliases for a record with a name field.

launchPerson = launch captain
greetShip = greet vessel

We can copy all the code above into an Elm module called TypeAliases.elm. And then here's what it looks like when we use the functions in the REPL:

> import TypeAliases exposing (..)
> launchShip
"I name this ship HMS Splendid" : String
> greetPerson
"Hello there, Captain Robert Q. Peabody" : String
> launchPerson
"I name this ship Captain Robert Q. Peabody" : String
> greetShip
"Hello there, HMS Splendid" : String
>