The next part are the messages in our application:
type Msg
= NoOp
We have only listed one message that our program can receive, which we’ve named NoOp (meaning “no operation”). Our program doesn’t yet have any features that would require responding to a message, so we’re just providing a dummy message here as a placeholder.
A type declaration defines a union type in Elm (also known as an algebraic data type , or ADT). Note that this is different from type alias, which simply gives a new, more convenient name to some existing type (as we did with our Model above).
Our message type above defines only a single message (NoOp), but in general a union type defines a list of possible values. Here’s a quick example:
type Answer = Yes | No
Here we’re adding a whole new type of value to Elm, called an Answer. In our program, any value with a type of Answer is guaranteed to have one of two values: Yes or No.
To learn more about union types, check out the relevant section of the official Elm language tutorial.