Networking in Logo
Ľubomír Salanci
Department of Informatics Education
Faculty of Mathematics, Physics and Informatics
Comenius University, 842 48 Bratislava, Slovak Republic
phone: +421 7 602 95 284
e-mail: salanci@fmph.uniba.sk
web: www.edi.fmph.uniba.sk/salanci
Abstract
Similar to most modern programming environments, Imagine – new version of Logo – exploits
communication and co-operation technologies. Imagine makes it possible to establish an on-line
communication between two or several users located anywhere on the network. Imagine hides all
technical problems concerning network connections and sending data thought the network. In a
clear and intuitive way it is possible to send and receive any kind of Imagine data objects between
connected users. It is possible to send Logo commands as well. Thus, for example turtle in one
computer can be controlled from Imagine running in another remote computer. Utilizing this
support, we can create simple yet rather effective network projects. In our paper we explore cooperation tools of Imagine and present several network projects. We also show how they can be
extended into exciting network games.
Keywords: Imagine, network, Internet, cooperation, programming, remote execution
1 Introduction
Similar to most modern programming environments, Imagine allows users to exploit
Internet network in several ways. A network support has been built into Imagine at several
logical levels:



Presentation level (tools for publishing Imagine projects on the Web)
Direct manipulation level (tools for direct manipulating Web objects)
Logo programming level (commands in Logo language to connect, control
connection and transmit data)
More information about about Imagine environment and programming language can be
found in [1, 2, 4].
2 The Presentation Level
The presentation level is designed for wide range of Internet users. A project written in
Imagine can be easily published as a Web page on the Internet by selecting File | Save
as Web Project… in the main menu. Thus, anybody can see, try and play with published
projects; only the Internet browser and installed Imagine plug-in are necessary.
When a user saves his/her Imagine project as Web project, an applet and a simple html file
that reads and displays the applet is generated by Imagine. At present, the plug-ins for
Internet Explorer and Netscape Navigator are available free of charge. If you have not
installed the plug-in yet, you are automatically offered to download it when displaying such
a Web page for the first time. Some examples of published project are available at
www.logo.com.
Thanks to this technology, it is easy enough already for rather young children to present
their projects on Web and allow anybody with appropriate Web browser and installed
Imagine plug-in to explore and play with these projects.
3 Direct Manipulation Level
The components developed to access Internet are of great help when creating and
programming one’s own projects. The following components are accessible via Imagine’s
main toolbar:
 Web Browser – displays the Web page inside your project
 Web Link – behaves like a button. When clicked, it opens browser window and
displays the Web page the address of which is defined by the Web link
Both components are very easy to use. The basic settings they offer help us to create simple
and interesting projects without any programming.
This is an example of a project which contains a Web Brower component and three Web Link
components. Web Browser displays picture – a map of the actual weather of Slovakia from www.shmu.sk.
Web Link buttons open window with detailed weather forecast for selected parts of Slovakia.
4 Logo Programming Level
This is the level on which projects requiring a network communication are developed. The
message passing via TCP/IP network protocol is supported by Imagine. It is an engine
behind the Imagine that takes care about connecting, keeping connection up, sending and
receiving packets etc. A programmer of Imagine projects is protected from these low level
and technical details. Instead, he/she is offered a smart object class Net that makes it
possible to:





Establish client–server connections
Recognize status of the connection and the list of actually connected users
Send messages, data and objects easily
Send programs and run their remote execution
Process events and received data
We will illustrate network object features of Imagine in the following examples:




Establishing Communication among Several Computers
Sending Text
Sending Objects
Remote Program Execution
We will also explain language primitives on simple examples.
4.1 Establishing Communication Among Several Computers
Let us have three computers and three users Alice, Bob and Jane sitting behind them.
To recognize whose computer we are watching and what commands are being run there, we
will write the commands into the named boxes (see bellow).
First we will create the server object in the following way (let the Alice’s computer be a
server):
Alice
? new "Net [name a nickName Alice style server]
? a'connect

By new "Net [Settings] new object of the class Net is created on Alice’s
computer,
 Settings specifies that the name of the new object will be a, while nickname of
this user will be Alice and the fact that her computer will be a server,
 By command a'connect the network connection of object a is created.
After the server Alice is created, new clients can connect to Alice. To establish the
connection, the server has to be identified either by its name or by its IP number (let the
Alice’s name be alice.edi.sk):
Bob
? new "Net [name b nickName Bob style client server alice.edi.sk]
? b'connect
Jane
? new "Net [name x nickName Jane style client server alice.edi.sk]
? x'connect
Now, the communication is established between all of the participants.
Once the connection has been established, the computers are being identified only via their
nicknames. It isn’t necessary any more to distinguish client and server when manipulating
objects of the Net class.
4.2 Sending Text
When Alice wants to send a simple text
message to Bob, she will write:
Alice
? a'send "Bob "|Hello Bob!|
Bob
can display the contents of the received
message using his Net object b:
Bob
? show b'message
Hello Bob!
Bob
can recognize that he has received a message
through the OnReceive event. It will be run
automatically by Imagine immediately after any
data has been received.
Based on this knowledge it is easy to create
simple chat program.
4.3 Sending Objects
Similar to text, other data structures such as
numbers and lists can also be sent via net.
Moreover, we are also allowed to send the whole
objects. Let us have the following situation: Alice
has drawn interesting picture and has set it as a
shape of a turtle t1. She wants to share her picture
with her friends. That is why she sends the whole
object to Bob.
Alice
? a'sendObject "Bob "t1
Bob
receives the object in a similar way as he
received the text. Receiving the object means that
the turtle with Alice’s drawing (for example, the
shape of a flower) appears on Bob’s computer and
is immediately displayed on his screen.
While receiving data through a Net connection
automatically generates the OnRecevie event,
receiving object will runs the OnRecevieObject
Bob
? b'receiveObject []
event. The user is allowed to react and control the process of receiving an object.
Mechanism of sending objects is useful for building projects that develop cooperation between children.
Running the project Building Bricks several users cooperate in building a structure out of several coloured
bricks. The screen is divided into two parts – bottom one displays palette of colours and bricks, the upper part
displays the structure. The user can select a brick and its colour and drag it into the upper part of the screen.
This change is immediately sent to all other users.
4.4 Remote Program Execution
Imagine makes it also possible to send a list of Logo instructions to another connected
computer (or several connected computers) and make it be run there. Alice, for example,
can send to Bob commands that will be executed on his computer only. Due to security,
however, remote program execution has to be accepted by Bob first:
Bob
? b'setRunEnabled "True
Now, Alice is allowed to send commands to be executed on Bob’s computer:
Alice
? a'sendRun "Bob [ask "t1 [repeat 200 [fd repc lt 91]]]
Commands ask "t1 [repeat 200 [fd repc lt 91]] are executed on Bob’s computer
and create well known spiral on his screen:
Bob
It is easy to see that the remote execution of Logo programs allows us:
 To control remote computers,
 To run independent processes on remote computers.
5 Additional Activities Illustrating Network Support
Network capabilities of Imagine can be used for developing different kinds of activities,
applications and projects. In this part we briefly present following illustrative activities:
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Object teleporting
Distributed Computing
Data Collecting
Game Programming
5.1 Object Teleporting
In previous chapters we described how objects can be sent via network in Imagine. In fact,
it is a copy of the object, which is always being sent. Now we want to implement
teleporting of objects. In object teleporting we will send a copy of an object to another
computer and then erase the original one. Thus, only a copy of the original object will
continue to exist. In this way we will “pretend” the effect that the object has been
transferred from one computer to another.
Teleporting is well known and rather popular process in science fiction and computer
games as a way how to transport things, people and other objects from one place to another.
Usually this is done through a special device called Teleport. It is easy to create a Teleport
metaphor in Imagine. Whenever a user places an object into a Teleport, it will be
transported to another (connected) computer.
Alice
Bob
Jane
The space ship is moving (in the bounce rangeStyle) along the screen. When it arrives at the teleport, it is
transported to another computer. From now on the space ship exists and continues moving in the other
computer. Here you can see a process of teleporting the space ship from the Alice’s computer to the computer
of Jane captured in a series of three pictures (see the marked sequence of space ship's positions: 1, 2, … 6).
Teleporting objects using the network of Imagine in the way we have just described is
intuitively clear. It can be used in a lot of various activities based on sharing objects by
connected multi users.
5.2 Distributed Computing
This illustrates another group of interesting tasks suitable for experiments. The distributed
computing takes place in the situations when the sequential algorithm is too much time
consuming. If possible, the divide and conquer principle is applied then. The task is split
into several independent parts that are solved on different computers. Then, the result is
composed from all partial solutions.
One such time consuming task is drawing a fractal. The possible distributed solution
follows:
 Alice sends a program that solves the part of a task to Bob and other friends,
 The program computes the values of colours for specified line of a picture; these are
sent back to Alice,
 It is Alice who is responsible for distributing of tasks and then handling the
results.
The figure illustrates the process of drawing the picture in detail. Some computers have completed their tasks
more quickly, thus their results are received immediately. Some computers work slowly – therefore there are
still some empty lines in the picture.
5.3 Data Collecting
Another group of task we present here to demonstrate new feature of Imagine is oriented to
data collection. Let us focus on statistical research – we are going to collect information
about children's moods of today:

We are using a Web page containing an Imagine applet. It displays several smilies
with different faces and actual voting status. Each visitor of the Web page is
allowed to vote by clicking some of the smilies.
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
The actual data about voting are being sent to the server.
The server is an Imagine project – a simple program that receives voting data and
builds the statistics. Server will notify all active clients about results of voting after
it will have processed the data.
Left picture is a Web page with Imagine applet for voting. Picture to the right is a server – the Imagine project
itself.
5.4 Game Programming
Creating our own network games in Imagine is definitely one of the most challenging
topics where network support can be exploited. It is easy to find different kinds of games
where network can be utilized in several ways:

In desktop games the game state and also information about which player’s turn is it
must be sent there and back,
 In action games information about each object's movement must be sent,
 Games often have to share certain common information like game settings, high
score table etc.
Picture to the left shows very simple game for two players – TicTacToe. The network is utilised here to send
information about which player's turn is it in the game. Picture to the right illustrates very simple action game
– Tennis for two players. The network is utilised here for sending positions of objects in real-time, namely
positions of both paddle and the ball.
6 Conclusion
We presented network features built in new programming environment Imagine. Imagine
allows us to send data, objects and Logo instructions via network. We illustrated these
features in several activities and projects like object teleporting, distributed computing, data
collecting and game programming. The Imagine network support is easy to use and
interesting to explore new Logo programming techniques. It stimulates users to
communicate and cooperate together.
Comaparing network support in Imagine and MSW Logo:
Imagine
MSWLogo
web support
primitive component
no
e-mail
yes
yes
other internet services
no
yes
number of connections
any
up to 2
sending/receiving texts and data
yes
yes
sending/receiving objects
yes
no
remote program execution
yes (using primitive
command)
yes (requires additional
programming)
ease of use
simple (1 to 2 command
line instructions)
more advanced
programming
References
[1] Blaho A and Kalas I (2001) Object Metaphor Helps Create Simple Logo Projects. To
appear in Proc. of EuroLogo 2001
[2] Kalas I and Blaho A (2000) Imagine… a new Generation of Logo: Programmable
Pictures Proc. of WCC2000, Educational Uses of Information and Communication
Technologies, Beijing 2000, pp. 427 – 430
[3] Zaveršnik M and Batagelj V (1999) Networking in MSWLogo. Proc. of EuroLogo
1999, Sofia, pp. 127 – 134
[4] www.logo.com