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The example problems have gone missing from the Io language website, so here’s a backup.
#Sample code
#Hello world
"Hello world!" print
factorial := method(n, if(n == 1, 1, n * factorial(n - 1)))
99 bottles of beer
bottle := method(i,
if(i == 0, return "no more bottles")
if(i == 1, return "1 bottle")
return "" .. i .. " bottles"
for(i, 99, 1,
writeln(current, " of beer on the wall, ", current, " of beer,\n",
"take one down, pass it around,\n",
bottle(i - 1), " of beer on the wall.\n")
#Create a simple object and show how to use it
Account := Object clone
Account balance := 0
Account deposit := method(v, balance = balance + v)
Account withdraw := method(v, balance = balance - v)
Account show := method(
writeln("Account balance: $", balance)
myAccount := Account clone
myAccount show
"Depositing $10\n" print
myAccount deposit(10)
myAccount show
#Concurrency example - the following will print "112233"
o1 := Object clone
o1 test := method(for(n, 1, 3, n print; yield))
o2 := o1 clone
// @ means send an asynchronous message
o1 @test; o2 @test
// wait for the messages to get processed
while(Scheduler activeActorCount > 1, yield)
#Look up a class/prototype given its name as a string
Lobby getSlot("PrototypeName")
#Execute a method given its name as a string
anObject perform("SomeMethodName", arg1, arg2)
#Find out if a method with a given name (again a string) exists
anObject hasSlot("SomeMethodName")
#Given an arbitrary reference to a class/prototype, instantiate it
newInstance := objectRef clone
#Add a method to a primitive
Number double := method(self * 2)
1 double
==> 2
#Subclass a primitive
MyList := List clone // Same as creating an instance
#Iterate through an inheritance hierarchy
Object printAllSlots := method(
self slotNames foreach(i, slotName,
if(self hasSlot("proto"), self proto printAllSlots
Object printAllSlots
#Implement a generic proxy
Proxy = Object clone
Proxy forward := method(
methodName := thisMessage name
args := thisMessage argsEvaluatedIn(sender)
// When an unrecognized message is received,
// this method will be invoked.
// Do what you like with the message here -
// such as send it over a network, etc.
#Implement a singleton
Foo := Object clone
Foo clone := Foo
#Print the source code of a method
myMethod := method(return testing(1+2+3))
getSlot("myMethod") code print
==> "return(testing(1+(2)+(3)))"
#Add an operator method to an object
MyObject := Object clone
MyObject setSlot("+", method(n, write("adding ", n, "\n")))
MyObject + 123
==> "adding 123"
#Mixins and Aspects
a := Object clone
Mix := Object clone
Mix logData := Buffer clone
Mix init := method(logData = logData clone)
Mix log := method(s, logData append(s))
a parent := Mix clone // Now a also inherits the state and behavior of Mix
b := Object clone
b parent := Mix clone // Now multiple objects use it so it's an "aspect"
#An auto source file importer
Object searchPaths = List clone add(launchPath)
Object forward := method(
methodName := thisMessage name
if (methodName characterAt(0) isUpperCase,
searchPaths foreach(i, searchPath,
fileName := searchPath appendPath(methodName .. ".io")
sourceFile := File clone setPath(fileName)
if (sourceFile exists) then (
Lobby doFile(path)
return Lobby getSlot(methodName)
raiseException("Importer", "Could not find slot for " .. methodName)
#A simple whois client
whois := method(host,
socket := Socket clone setHostName("") setPort(43)
socket connect write(host, "\n")
while(socket readNextChunk, nil) // socket read auto yields to other coroutines
return socket readBuffer
#An echo server
Echo := Object clone
Echo handleSocket := method(socket,
while(socket isOpen,
if(socket read, socket write(socket readBuffer))
socket readBuffer empty
EchoServer := Server clone setPort(8456)
EchoServer handleSocket := method(socket,
Echo clone @handleSocket(socket)
// async message so it's handled in separate coroutine
EchoServer start
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