Ruby Notes

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Dynamic languages such as Ruby can be very
productive in specific areas, such as
prototyping, web development or for gluing
various applications together.
Dynamic programming languages are often
referred to as 'weakly typed' which means
that variables are not bound to a particular
type until runtime.
It typically features "dynamic typing," which
gives the programmer more freedom to pass
parameters at runtime without having to
define them beforehand.
This dynamic typing is accomplished by using
an interpreted language, rather than a
compiled language.
A scripting language is a programming language
that allows control of one or more software
applications. "Scripts" are distinct from the core
code of the application, which is usually written in
a different language, and are often created or at
least modified by the end-user. Scripts are often
interpreted from source code whereas the
applications they control are traditionally
The name "script" is derived from the written
script of the performing arts, in which dialogue is
set down to be spoken by human actors.
interpreted (no linking or compilation)
less efficient to run
faster to develop (factor of 5 to 10)
high level – a single statement is more powerful
no type declarations, typeless
powerful built-in types
can generate and interpret source code at run time
(via eval)
interface to underlying operating system – can call
operating system commands
plays well with others – glue systems together
rarely used for complex algorithms or data
Examples: javascript, perl, php, smalltalk, ruby,
vbscript, groovy, emacslisp, awk, python, tcl, unix
Japanese Design Aesthetics Shine Through
 Focus on human factors
 Principle of Least Surprise: when two
elements of an interface conflict, or are
ambiguous, the behavior should be that
which will least surprise the human user or
programmer at the time the conflict arises.
Principle of Succinctness
The supreme design goal of Ruby
Makes programmers happy and makes Ruby easy to learn
What class does an object belong to ? – can ask
o.class (returns the class object)
Is it Array#size or Array#length?
Either one - same method – they’re aliased
Is this good or bad design?
Array operators? – if it makes sense, they are defined.
diff = ary1 – ary2 (removes elements of ary2 from ary1)
union = ary1 + ary2 (concatenate arrays)
intersection = ary1 & ary2 (elements common to both)
also known as the Principle of Least Effort
We don’t like to waste time
The quicker we program, the more we
Sounds reasonable enough, right?
Less code means less bugs
Common needs are built-in.
All classes derived from Object including Class (like
Java) but there are no primitives (not like Java at
all). EVERYTHING is an Object.
 Ruby uses single-inheritance
What are the dangers of multiple inheritance?
 Mixins give you the power of multiple inheritance
without the headaches
 Modules allow addition of behaviors to a class
 Reflection is built in along with lots of other highly
dynamic metadata features
 Things like ‘=‘ and ‘+’ that you might think are
operators are actually methods (like Smalltalk)
a mixin is a class that is mixed with a
module. In other words the implementation
of the class and module are joined,
intertwined, combined, etc.
a module as a degenerate abstract class. A
module can’t be instantiated and no class can
directly extend it but a module can fully
implement methods.
# Convert a integer value to English.
02.module Stringify
03. # Requires an instance variable
04. def stringify
05. if @value == 1
07. elsif @value == 2
09. elsif @value == 3
11. end
12. end
makes use
of a @value instance
The class that will be
mixed with this
module needs to
define and set a
@value instance
a module could
invoke methods
defined not in the
module itself but in the
class that it will be
mixed with.
# A Math module akin to Java Math class.
2.module Math
3. # Could be called as a class, static, method
4. def add(val_one, val_two)
5. + val_two)
6. end
# Base Number class
02.class Number
03. def intValue
04. @value
05. end
08.# BigInteger extends Number
09.class BigInteger < Number
11. # Add instance methods from Stringify
12. include Stringify
// mix methods at the instance level
14. # Add class methods from Math
15. extend Math // mix methods at the class level
17. # Add a constructor with one parameter
18. def initialize(value)
19. @value = value
20. end
# Call class method extended from Math
bigint2 = BigInteger.add(-2, 4)
puts bigint2.intValue # --> 2
# Call a method included from Stringify
puts bigint2.stringify # --> 'Two'
# Format a numeric value as a currency
module CurrencyFormatter
def format
# Add the module methods to.# this object
instance, only!
bigint2.extend (CurrencyFormatter)
puts bigint2.format # --> '$2'
Method Chaining – applies methods from left to right
print array.uniq.sort.reverse
Method Names include ! and ?
? returns a true or false
◦ examples: defined? equal?
! changes the object that calls the method
(do it here!)
Iterators and Blocks vs. Loops
files.each { |file| process(file) }
Case usage:
◦ Class names begin with a Capital letter
◦ Constants are ALL_CAPS
◦ Everything else - method call or a local variable
Convention in naming: Under_score instead of
0.upto(9) do |x|
print x, " "
Produces: 0 1 2 3 4 5 6 7 8 9
[ 1, 1, 2, 3, 5 ].each {|val| print val, " " }
Produces: 1 1 2 3 5
songList.each do |aSong|
Duck Typing
Based on signatures, not class inheritance
In Ruby, we rely less on the type (or class) of an object and
more on its capabilities.
Duck Typing is a type of dynamic typing in which the object’s
current set of methods and properties determines the valid
semantics, rather than its inheritance from a particular class
or implementation of a specific interface.
The name of the concept refers to the duck test, attributed to
James Whitcomb Riley which may be phrased as follows:
"when I see a bird that walks like a duck and swims like a duck
and quacks like a duck, I call that bird a duck."
# Check whether the object defines the to_str method
puts ('A string'.respond_to? :to_str) # => true
puts ( :to_str) # => true
puts (4.respond_to? :to_str) # => false
The above example is the simplest example of Ruby's
philosophy of "duck typing:" if an object quacks like a
duck (or acts like a string), just go ahead and treat it
as a duck (or a string). Whenever possible, you should
treat objects according to the methods they define
rather than the classes from which they inherit or the
modules they include.
class Duck
def quack
def swim
'Paddle paddle paddle...'
class Goose
def honk
def swim
'Splash splash splash...'
class DuckRecording
def quack
def play
If you are calling
“quack”, Duck and
DuckRecording are
If you are calling
“swim”, Duck and
Goose are
Method isn’t
expecting a specific
type JUST one with
the needed
Dynamic Dispatch
A key concept of OOP: methods are actually messages that
are sent to an object instance
It is a runtime decision to decide which code to execute
Can’t be determined statically (method could have been
overridden dynamically)
foobar =
foobar.class # => Array
foobar.size # => 0
Conceptually: First we search for “size” in Array. If it isn’t there, we look to
Object to define size. If the method isn’t found at the top of the
hierarchy, we have a “method_missing” message.
The new search path
foobar =
def foobar.size // Adds a size method ONLY for the instance
“Infinity and beyond" ;
Languages with weak or no typing
systems often carry a dispatch table as
part of the object data for each object.
This allows instance behavior as each
instance may map a given message to a
separate method.
Languages with strong (static) typing use
a virtual table which defines method to
code mapping. Instances of the type
store a pointer to this table.
Suppose a program contains several classes in
an inheritance hierarchy: a superclass Cat, and
two subclasses, HouseCat and Lion. Class Cat
defines a virtual function named speak, so its
subclasses may provide an appropriate
implementation (e.g. either meow or roar).
When a Cat variable calls speak, we need to be
able to tell which method to call.
dispatch table contains addresses of methods
Use same layout for type-compatible methods
– so we look for address at a constant offset
Dynamic Behavior
Scope Reopening (Kind of like AOP)
Breakpoint debugger
One of the many advantages of dynamic
languages such as Ruby is the ability to
introspect---to examine aspects of the
program from within the program itself.
Some call this feature reflection.
We might discover:
what objects it contains,
the current class hierarchy,
the contents and behaviors of objects, and
information on methods.
Everything is an object EVEN the class.
#=> Fixnum
"hello".class #=> String
class Foo
Foo.class #=> Class
Foo is a constant known to the system as a
Have you ever craved the ability to traverse all the living objects
in your program? Ruby lets you perform this trick with
ObjectSpace::each_object .
For example, to iterate over all objects of type Numeric, you'd
write the following.
a = 102.7
b = 95
# Won't be returned
c = 12345678987654321 # Won't be returned
count = ObjectSpace.each_object(Numeric) {|x| p x }
puts "Total count: #{count}“
Total count: 6
Hey, where did those last numbers come from? We didn't define
them in our program. The Math module defines constants for e
and PI; since we are examining all living objects in the system,
these turn up as well.
click on Project name,
Select Properties
Click Run
Define Ruby Option
For instance, we can get a list of all the methods to
which an object will respond.
num = 14
list = r.methods
# 60
# [exclude_end?, to_a, include?, member?]
r.respond_to?("frozen?") #>> true
r.respond_to?("hasKey") #>> true
num.instance_of? Fixnum #>> true
one_class = Fixnum
print one_class
one_class = one_class .superclass
print " < " if one_class
end while one_class
puts p Fixnum.ancestors
puts self.class # scripts are automatically in Object
puts 3.class
# numbers are Fixnum
[Fixnum, Integer, Precision, Numeric, Comparable, Object, Kernel]
trane="John Coltrane".method(:length)
miles="Miles Davis".method("sub")
trane=%q{"John Coltrane".length}
miles=%q{"Miles Davis".sub(/iles/,'.')}
puts eval trane
puts eval miles
produces 14
M. Davis
M. Davis
class CoinSlot
At instance variable begins with @
def initialize(amt)
and its scope is confined to
whatever object self refers to
A global variable begins with $
It can be referred to from anywhere
in the program
eval ("puts @amt", $here) # 35
eval ("puts @amt” )
# nil
class CoinSlot
def initialize(amt)
We can even modify
variables in original scope
eval ("puts @amt", $here) # 35
eval ("puts @amt” )
# nil
eval "@amt=23.92", $here
eval "puts @amt", $here
# 23.92
One nice feature of globals, is that they can
be traced; you can specify a procedure which
is invoked whenever the value of the variable
is changed.
trace_var :$here, proc{print "$here is now",
$here, "\n"}
class Demo
def initialize(secret)
@secret = secret
def getBinding
return binding()
k1 =
b1 = k1.getBinding
k2 =
b2 = k2.getBinding
puts eval("@secret", b1) #=> 99
puts eval("@secret", b2) #=> -3
puts eval("@secret")
#=> nil
caller is a Ruby method of the Kernel class,
which all objects inherit.
It returns an array of strings representing the
call stack.
class Demo
def initialize(secret)
@secret = secret
@other = 2*@secret
$here = binding
def aA
puts caller.join("\n")
def aB
def aC
def getBinding
return binding()
k1 =
k1.aC # prints call stack
Scoping.rb:25:in `aB'
Scoping.rb:28:in `aC'
Java features the ability to serialize objects,
letting you store them somewhere and
reconstitute them when needed. You might
use this facility, for instance, to save a tree of
objects that represent some portion of
application state, save it, and recreate is later.
Ruby calls this kind of serialization
Marshal: Def: To arrange or place (troops, for
example) in line for a parade, maneuver, or
Can be used to create a distributed object
class Note
attr :value
def initialize(val)
@value = val
def to_s
class Chord
def initialize(arr)
@arr = arr
def play
c = ["G"),"Bb"),"Db"),"E") ] )"posterity", "w+") do |f|
Marshal.dump(c, f)
end"posterity") do |f|
chord = Marshal.load(f)
puts #» G-Bb-Db-E
the format used by Marshal has changed a few
times in the past.
it's Ruby-only. AFAIK nothing else can read data
serialized with Marshal.
serializing objects with Marshal exposes
implementation details.
The basic problem with Marshal is that it
serializes an object by saving the name of its
class and all its instance variables
Makes interoperability harder – encoding may
change and implementation details are leaked by
Using Ruby's alias it is possible to reopen a class, override a method and still call the original.
class ClockRadio
def on!
@on = true
def on?
Clock Radio is complete,
Sometime later we can redefine methods and add new ones:
class ClockRadio
alias :old_on! :on!
def on!
old_on! # Calling original code
@display_time = true
def display_time?
The advantage you gain from being able to
reopen classes is that you can patch the
framework you’re using unobtrusively in order
to fix bugs or improve performance.
Violates OO principle by allowing anyone to add
members and methods to an existing class, even
outside of the original class definition.
The methods thus added have full access to all
other members and methods, including private
This is a “violation of the Open/Closed Principle,”
in that the original class is not kept closed
against modifications.
It certainly isn’t pure. There really isn’t a Grand
Unifying Idea. It clearly copies features from very
different languages and allows for a variety of
programming styles.
A closure is a nameless function.
 It has code to run (the executable)
 It has state around the code (the scope)
You capture the environment (the local variables)
in the closure.
Even after the function has returned, and its local
scope has been destroyed, the local variables
remain in existence as part of the closure object.
Local variables are shared between the closure and
the method. It's a real closure. It's not just a
copy. (This is a Perl feature that is copied.)
You can reconvert a closure back into a block, so
a closure can be used anywhere a block can be
Often, closures are used to store the status of a
block into an instance variable, because once you
convert a block into a closure, it is an object that
can be referenced by a variable.
And of course closures can be used like they are
used in other languages, such as passing around
the object to customize behavior of methods.
The important thing to remember about Ruby
is that there isn't a big difference between
``compile time'' and ``runtime.‘’ This
 adding code to a running process.
 redefining methods on the fly, change their
scope from public to private, and so on.
 altering basic types, such as Class and
eval "puts 2+2" # => 4
eval "'hello world!'.upcase “# => HELLO WORLD
 Tutoring – allows users to run their code in a
protected environment
 evaluating math expression (like in spreadsheet)
 more flexible than a parser as can call functions that
were just created.
 implemented with the same interpreter as normal
 input = # ... read from some form
eval input
What if they input 'rm -rf *‘ ?
Slow – string must be parsed
Side Effects: As an everyday example, the statement
x = gets
reads a line and sticks it into x just like you’d probably expect,
but it also has the side effect of sticking the line into the
global variable $_.
Ruby also blurs the line between names of variables,
classes, and methods. Some languages allow the
same identifier to be both a class and a variable, but
context distinguishes. Ruby does not.
Ruby delights in the bizarre usage of operator
overloading. ”<<” normally means “left shift”, but it
also means ”here document follows,” “append to
string,” and “extend array.” Other operators have
similar illogical overloadings; for example, the ”<"
operator not only means "less than" but also "is a
subclass of."

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