Tempo Programming Language
Language Guide
Functions

Functions

All choreographies start with a function defined over a set of roles.

Tempo
func@(A,B,C) hello() {
  let hello: String@B = await A->B "Hello";
  let greeting: String@C = await B->C (hello + ", World!");
}

Before you can call a function, its roles must first be instantiated (similarly to generic type parameters). You instantiate a function by substituting its roles written funcName@(A,B,C), where funcName is the name of the function and (A,B,C) are the roles to substitute with. All roles in a function substitution must be unique. When a function has been instantiated, you can call it like a regular function funcName@(A,B,C)(arg1, arg2).

Tempo
func@(Sender, Receiver) say(message: String@Sender) String@Receiver {
  return await Sender->Receiver message;
}

func@(A,B,C) hello() {
  let hello: String@B = say@(A,B)("Hello");
  let greeting: String@C = say@(B,C)(hello + " World!");
}

Return

A function can return, only if all roles of the function has knowledge about the return statement.

Closures

You can pass around functions as closures after they have been instantiated.

Tempo
func@(Sender, Receiver) say(message: String@Sender) String@Receiver {
  return await Sender->Receiver message;
}

func@(A,B) main() {
  let f: func@(A,B)(String@A)String@B = say@(A,B);
  let hello: String@B = f("hello");
}

You can also capture variables with a closure like so.

Tempo
func@(A,B) main() {
  let count: Int@A = 0;
  let counter: func@(A,B)()Int@B = func@(A,B) () Int@B {
    count = count + 1;
    return await A->B count;
  };

  counter(); // 1@B
  counter(); // 2@B
  counter(); // 3@B
}
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CommunicationExpressions