Code Blocks, Closures, and Continuations

Code Blocks, Closures, and Continuations Presented by: Bassem ELkarablieh

Outline • Getting Started with ruby • Some ruby syntax • Ruby classes • More ruby syntax • New concepts in ruby

Getting Started • Ruby is a dynamic, fully object-oriented scripting language. • Installing ruby: • http://www.ruby-lang.org • One click windows installation (takes 5 min to setup ruby) • Running ruby • Interactive ruby (irb) • Batch ruby (run program files)

Getting Started • Ruby documentation • http://www.ruby-doc.org • Using the ri tool • What did we learn

Some ruby Syntax • Ruby is fully OO • Every thing is an object (even literals) • “hello”.length • “bassem”.index(“e”) • -88.abs • Variable definition • Number = 5 • Number = “5” • Number = “five” • Delimiters: no ending ”;” is required

Some ruby Syntax • Simple I/O • Puts: write the value to the output with new line character appended • Gets: reads the value from standard input • A = gets • puts “Hello I am “ + A • Conditions • If else blocks • If A>8 • puts “greater” • Else • Puts “smaller” • end

Some ruby Syntax • Looping • While block • While line = gets • Puts line.uppercase • End • For … In blocks • For I in 1..7 • Print I, ““ • Control structures • Break, redo, next , retry

Some ruby Syntax • Assignment • a = b = 1+2+3 • Instrument = “piano” • Instrument [“ano”] = “ccolo” • Parallel Assignment • a,b = 6,7 • Method definition • def my_method(arg1,arg2,arg3) • #this is a comment and code • Def • What did we learn

Classes in Ruby • Initializer • Instance variable • Class variables • Instance methods • Class methods • Attributes • operators • Access controls

Classes in ruby • Demo of a class

More ruby syntax • Containers • Code blocks • Iterators

Containers • Arrays • A= [56,”hello”,99.66,”ll”] • B=Array.new • Un-initialized members of an array are set to nil • Indexing an array • A[0] ->56 , A[-1] =“ll” • A[1,1] ->[“hello”] • A[0..2]-> [56,”hello”,99.66]

Containers • Hashes • H ={‘dog’=>’canine’, ‘cat’ => ’feline’, ’bird’ => ‘duck’} • H.length => 3, h[‘dog’] =>’canine’ • H[12] = ‘hello’, H[‘cat’] =99

Code Blocks • Chucks of code that can be associated with method invocations • Great methods for implementing callbacks • Great tool for implementing iterators • Code blocks are flexible , and can be passed parameters • Great tool for implementing closures and continuations

Code Blocks • Code block can be stored between{} or between do…end blocks Def my_method puts “hello” yield yield End my_method{puts “world”} • Code block parameters differ from method parameters Def say_goodnight(name) puts “good night, #{name}” Yield (“sweet dreams”) End Say_goodnight(“bassem”){|i| print i}

Iterators • Built in iterators in ruby uses code block to support general plug-in architecture • Examples • 5.times{print “*”} • 3.upto(6) {|i| print i} • (‘a’..’z’).each{|char| print char} • [‘cat’,’dog’,’horse’].each{|name| print name} • [1,3,5,7,9].find{|v| v*v>3} • [“H”,”A”,”L”].collect{|x| x.succ} • [1,3,5,7].inject(0){|sum,element| sum+element}

New concepts in ruby • Closures • Continuations • Dynamic manipulations

Closures • A closure is a function created by a program at run time. This idea is written as a function that appears entirely within the body of another function. The nested, inner function may refer to local variables of the outer function. • A closure can be used to represent a delayed user defined behavior that can be passed around in methods and objects

Closures • Closure simulations • C: function pointers, void* parameters • C++: Functors • C# and java: Anonymous classes and methods • Ruby have a special built-in Proc class that simplifies implementing closures

Proc Class • Proc objects are blocks of code that have been bound to a set of local variables, once bound the code can be called from different context. • Example: Def gen_times(factor) return Proc.new{|n| n*factor} End Times3 = gen_times(3) Times5 = gen_times(5) Times3.call(12) Times5.call(3) Times3.call(times5.call(4))

Proc Objects • Proc objects can be created by associating a code block with it • Example Def proc_from Proc.new End Proc = proc_from{“hello”} Proc.call

Code blocks as closures • Example1 Def n_times (thing) Return lambda {|n| thing*n} End # here lambda returns a Proc object associated with the code block P1 = n_times(23) P1.call(3) P2 = n_times(“hello”) P2.call(4)

Code blocks as closures • Example2 Songlist = SongList.new Class JukeboxButton<Button Def initialize(label,&action) Super(label) @action = action End Def button_pressd @action.call(self) End End Start_button = JukeboxButton.new(“start”){songlist.start} pause_button = JukeboxButton.new(“pause”){songlist.pause}

Continuations • Continuations are objects that lets you save system state, and then return to that state on command • Code blocks defines the universe in a continuation

Continuations • A continuation object saves the execution state( mainly the execution stack) • A continuation is created using a call to a Kernel method “callcc” which associates a continuation to a code block • Similar to Proc object , continuations are triggered using a “call” method

Callcc method • Generates a continuation object , which it passes to the associated block • Performing a cont.call will use the callcc to return • The value returned by callcc is the value of the block or the value passed to cont.call

Continuations • Example1 Calloc do|cont| For i in 0..4 print “\n#{i}:” for j in i*5 … (i+1)*5 cont.call() if (j==7) print j end End End

Continuation Objects • Callcc return values implicitly Callcc{|cont| cont.call} Callcc{|cont| cont.call 1} Callcc{|cont| cont.call 1,2,3} • Callcc return values Explicitly • Callcc{|cont| return cont}

Continuations • Example2 Def strange calloc {|cont| return cont} print “back to method” End Print “before method” Temp = strange() Print “after method” If( Temp) Temp.call

Continuations • Example3 • Def loop • for i in 1..5 • puts I • callcc{|cont| return cont} if i==2 • end • return nil • End • Puts “before loop” • Temp = loop() • Puts “after loop call” • If( Temp) Temp.call • Puts “after continuation call”

Done • Final demo Any Questions

What's next in ruby • Ruby types • Ruby methods • Ruby Expressions

Code Blocks, Closures, and Continuations