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Sending and Standard Out

Say Hello

"Person waiving hello"

There is a long-standing tradition in programming that your first program should say "Hello World". Here's the simplest rholang code to put that text on the screen.



Make the program print "Rholang rocks!" instead of "Hello World".

WTH is stdout?

Channels are like mailboxes for sending messages

The heart of rholang is communicating on channels. Channels are communication lines that you use to send and receive messages. To send a message on a channel, you use the ! character.

Redo this diagram!

We created the channel stdout on the first line of the program with new stdout. You'll create lots of channels as you learn rholang. We also gave our channel a special power by including (rho:io:stdout). More on that later, but for now just know that you need that part in parentheses to make text actually appear on the screen.

Using other channels

Sent messages wait to be received here in "message purgatory"... JK, it's called the "tuplespace"

You can actually send messages on lots of channels, not just stdout. But unlike stdout they won't display on the screen because we won't add any special powers to them.


So where do the other channels go then? Nowhere! Not yet anyway. The messages just sit there waiting for someone (or some process) to receive them. We'll learn how to receive messages in the next lesson. The place where messages sit in the meantime is called the "tuplespace".

Make sure your message is sitting in the tuplespace. You should see some text like this depending on which developer environment you use.

Storage Contents:
 @{"RandoChannel"}!("This won't be on the screen") | for( x0, x1 <= @{Unforgeable(0x01)} ) { Nil } | for( x0, x1, x2, x3 <= @{"secp256k1Verify"} ) { Nil } | for( x0, x1 <= @{"sha256Hash"} ) { Nil } | for( x0, x1 <= @{Unforgeable(0x03)} ) { Nil } | for( x0, x1, x2, x3 <= @{"ed25519Verify"} ) { Nil } | for( x0, x1 <= @{"blake2b256Hash"} ) { Nil } | for( x0 <= @{Unforgeable(0x02)} ) { Nil } | for( x0 <= @{Unforgeable(0x00)} ) { Nil } | for( x0, x1 <= @{"keccak256Hash"} ) { Nil }

Doing two things at once

Rather than following an ordered list, all ingredients are added concurrently.  Looks delicions

In rholang we don't tell the computer to do one thing, then another, then a third. Rather we tell it all the things to do, and it does them "concurrently," or all at once.


The | is pronounced "parallel", or "par" for short.


Send the message "1 large pepperoni please" on a channel called "pizza shop".


Send "Hi Mom" on the channel "Mom's Phone".


Print two messages, "Rick" and "Morty", on the screen in one program.


What will stdout!("Programming!") print to the screen?

  • Programming!
  • stdout!
  • Nothing

What channel does what!("Up") send a message on?

  • Up
  • what
  • what
  • stdout

Which does rholang do first in

  • prints "Dogs"
  • prints "Cats"
  • Neither. They are concurrent


There is also a special channel called rho:io:stderr. Check out what happens when you send to it. (what's the difference?)