Example (7): Signature for Hello World

Well it just occurred to me: how about this one line protocol? Everybody has written one or more "hello world" programs in your pet languages and here I give the signature for a hello world in scribble:

protocol GreetWorld {

  participant You, World;
  channel chWorld @ World;

  chWorld.greetings(string) from You to World;

}

Note this is a signature, so it does not specify what you would say: there are no values mentioned. So it is called GreetWorld rather than HelloWorld. It says that you will send a greeting with a string --- perhaps "hello" ---- to the world through channel chWorld.

As we have seen we can have a simpler version which does not use the operator "greetings":

protocol GreetWorld {

  participant You, World;
  channel chWorld @ World;

  chWorld.string from You to World;

}

or perhaps you wish to use the formatted string as in C in which case we can write:

import formattedString;
protocol GreetWorld {

  participant You, World;
  channel chWorld @ World;

  chWorld.formattedString from You to World;

}

Well we have not introduced "import" yet: this is the keyword to import data types and protocols from outside, usually from files deposited in the same directory as this protocol is defined. So it declares that we use a non-primitive data type called "formattedString" in this protocol. We shall later illustrate how we can use diverse data formats in our scribbling, both in signature/models and in behavioural description.

So in this way we can send a formatted string such as "hello world/n" from you to the world --- for example consider channel "chWorld" is bound to a port of a printer then your message will be printed there, after travelling through that vast expanse. This is the hello world in the era of communication-centred programming (more variations on the same theme later).