Primitives for networking

XLogo has 4 primitives that allow it to communicate over a network: listentcp, executetcp, chattcp and send. In all future examples, we will take the case of the two computers in the previous figure.

• listentcp: this primitive listentcp is the basis for all network communication. It doesn't need an argument. When you execute this primitive on a computer, the computer will listen for instructions sent from other computers on the network.
  
• executetcp: this primitive allows execution of instructions by a computer on the network.
  Syntax: executetcp word list $\longrightarrow$ The word is the called IP address or computer name, the list contains instructions to execute.
  
  Example: I'm on computer hare, I want to draw a square with a side of 100 on the other computer. Thus, on the computer turtle, I have to launch the command listentcp. Then, on the computer hare, I write:
  

   executetcp "192.168.1.2 [repeat 4[fd 100 rt 90]]
  or 
   executetcp "turtle [repeat 4[fd 100 rt 90]]
  

• chattcp: Allows chat between two computers on a network. On each computer, it displays a chat window.
  Syntax: chattcp word list $\longrightarrow$ The word is the called IP address or computer name, the list contains the sentence to display.
  
  Example: hare wants to talk with turtle.
  First turtle executes listentcp so it is waiting for instructions from network computers. Then hare writes: chattcp "192.168.1.2 [hello turtle].
  Chat windows will open on both computers, allowing them to talk with each other.
  
  
• sendtcp: Send data towards a computer on the network and return his answer.
  
  Syntax: sendtcp word list $\longrightarrow$ The word is the called IP address or computer name, the list contains the data to send. When Xlogo is launched on the other computer, il will answer OK. It is possible with this primitive to communicate with a robot through its network interface. Then, the answer of the robot could be different.
  
  Example: turtle wants to send to hare the sentence "3.14159 is quite pi".
  First hare executes listentcp so it is waiting for the other computer to communicate. Then, turtle writes: print sendtcp "hare [3.14159 is quite pi].
  

A little hint: Launch two instances of XLogo on the same computer.
- In the first window, execute listentcp.
- In the second one, write executetcp "127.0.0.1 [fd 100 rt 90]

You can move the turtle in the other window! (heh, heh, it's possible because 127.0.0.1 designates your local address, so it's your own computer...)