The unit type

The empty tuple () is called the unit type in Elm. It is so prevalent that it deserves some explanation.

Consider a type alias with a type variable (represented by a):

type alias Message a =
    { code : String
    , body : a
    }

You can make a function that expects a Message with the body as a String like this:

readMessage : Message String -> String
readMessage message =
    ...

Or a function that expects a Message with the body as a List of Integers:

readMessage : Message (List Int) -> String
readMessage message =
    ...

But what about a function that doesn't need a value in the body? We use the unit type for indicating that the body should be empty:

readMessage : Message () -> String
readMessage message =
    ...

This function takes Message with an empty body. This is not the same as any value, just an empty one.

So the unit type is commonly used as a placeholder for an empty value.

Task

A real world example of this is the Task type. When using Task, you will see the unit type very often.

A typical task has an error and a result:

Task error result

• Sometimes we want a task where the error can be safely ignored: Task () result
• Or the result is ignored: Task error ()
• Or both: Task () ()