Additional Syntax

By now you can already build some actual projects. And you should do it! Before we dive into the next unit, let's learn some more practical syntax so you have the real-world tools you need need to build that next killer dapp.

Binary Operators

The first new syntax will be some binary operators. Binary operators take two operands, which is why they're called bi -nary. Most of these are for arithmetic. Consider these examples.

new c2f, f2c in {

   * Converts Celcius temperatures to Farenheit. The conversion
   * multiply by 9/5 and add 32
  contract c2f(celcius, return) = {
    return!((*celcius * 9 / 5) + 32)

  contract f2c(farenheit, return) = {
    Nil // TODO you complete this contract

  new result in {
    // 0 C should be 32 F
    c2f!(0, *result)
    // 100 C should be 212 F
    c2f!(100, *result)


Your turn to implement the f2c contract. You can use the same two test cases in reverse to make sure you're getting the right result.

The final binary operator you should know is ++ which is used for "concatenation" or connecting two smaller things to make a bigger thing. The operator works for lists, which we'll learn about in the next unit, as well as strings which we already know about.

new greeter in {
  contract greeter(name, return) = {
    return!("Hello there, " ++ *name)
  new result in {
    greeter!("Joshy", *result)|
    greeter!("Tom", *result)

What would the code result!("I" ++ "<3" ++ "rholang") output?

  • I <3 rholang
  • ["I", "<3", "rholang"]
  • I<3rholang
  • I++<3++rholang

Receiving Processes?

We always send __ and receive __.

  • processes, names
  • processes, processes
  • names, names
  • names, processes
  • no such restriction

That was just a nice refresher from last unit. Hope your memory is holding up so far. If you've been writing your own rholang code, you may have found yourself really wishing you could receive processes directly so you didn't have to type all those *s. This is a common situation, and luckily rholang has a nice solution. We do always have to receive names, but we can bind them to name syntax like @myValue. Since @myValue is a name, myValue must be a process.

This syntax allows us to do things like `for (@number <- someChan){double!(2 * number)}

What code could be parred with the previous code to leave the number 24 on double?

  • @number!(12)
  • someChan!(12)
  • @number!(24)
  • double!(48)


Revisit the telephone game from lesson 3 and show that we could have used the @message pattern so message would be a process.

What should replace the ... in for(@x <- @y){result!(...)} to make the program valid?

  • @x
  • x
  • *x

Ifs and Conditions

In nearly every programming language the program's behavior can vary depending on some condition. For example I run on the trails if it is nice out, but stick to the roads in if it is rainy. In rholang the syntax is to do that is.

if ( /* condition */ ) {
  Nil // Do this if condition is true
else {
  Nil // Do this if condition is false

The situations where you will use if are limitless and include guessing a secret word correctly, setting the high score in a video game, determining which poker hand is higher, and calculating the winner of an election. This example contract shows how you might check the status of a bank account.

new result in {
  for (@balance <= @"signTest") {
    if (balance > 0) {
      result!("Account in good standing.")
    } else {
      result!("Account overdrawn.")


The accounting program has a problem. It says that accounts with a balance of zero are overdrawn. But really zero should be in good standing. You can fix this using the "greater than or equal" operator, >=. Make sure you add a few tests to make sure it works.

Comparison Operators

Now that you know how to use the if/else construct, there are lots of good comparison operators at your disposal.

  • a > b Is a greater than b?
  • a < b Is a less than b?
  • a == b Is a equal to b?
  • a <= b Is a less than or equal to b?
  • a >= b Is a greater than or equal to b?
  • a != b Is a unequal to b?

These operators work on both numbers and text strings. Text strings are sorted lexicographically, which is a lot like alphabetically. But be careful! If you try to compare a number to a string, it will just be another stopped process.

Which of these are true?

  • 4 < 3
  • "b" < "a"
  • 5 <= 6
  • "hello" != "hello"


Write a rholang program that requires the user to send in their name. In most cases the contract will simply reply with "hello", but if their name is the same as yours, it will tell them so.

Boolean Operators

Rholang also has the classic Boolean operators AND, OR, and NOT. The syntax is

  • a and b true when both a and b are true
  • a or b true when either a or b is true
  • not a true when a is false

What would result!(true and true) output?

  • true
  • false
  • neither; that's invalid syntax

What would result!(not true) output?

  • true
  • false
  • neither; that's invalid syntax

What would result!((not not true) or false) output?

  • true
  • false
  • neither; that's invalid syntax


Write a contract that tells a caller whether they are eligible to vote. In order to vote you must be a certain age and of a certain country. You can pick the age and country. To use the contract, I would par in canIVote!("Nigeria", 30)".


The contract above only works for one specific country. Use what we learned about factories to enable creating many of these eligibility checkers. To create a new checker for Canada, where the voting age is 18 par in checkerFactory!(canadaChecker, "Canada", 18). Then a 41-year-old Estonian would check whether he can vote in Canada with canadaChecker!("Estonia", 41). Spoiler alert: He cannot vote in Canada.

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