Hello, world!

“Hello, world!” is a common starting point used to showcase a programming language’s basic syntax.

Below is an example of “Hello, world!” written in Motoko:

// If an actor is declared with the persistent keyword, all private declarations are considered stable by default
persistent actor HelloWorld {
  // We store the greeting in a stable variable such that it gets persisted over canister upgrades.
  var greeting : Text = "Hello, ";

  // This update method modifies the greeting prefix.
  public func setGreeting(prefix : Text) : async () {
    greeting := prefix;
  };

  // This query method returns the currently persisted greeting with the given name.
  public query func greet(name : Text) : async Text {
    return greeting # name # "!";
  };
};

In this example:

1. The code begins by defining an actor named HelloWorld. In Motoko, an actor is an object capable of maintaining state and communicating with other entities via message passing.

2. It then declares the variable greeting. This is a stable variable because the actor is declared with the keyword persistent. Stable variables are used to store data that persists across canister upgrades. Read more about canister upgrades.

3. An update method named setGreeting is used to modify the canister’s state. This method specifically updates the value stored in greeting.

4. Finally, a query method named greet is defined. Query methods are read-only and return information from the canister without changing its state. This method returns the current greeting value, followed by the input text. The method body produces a response by concatenating "Hello, " with the input name, followed by an exclamation point.

Learn more about actors and basic syntax.