This method:
def format_stations_and_date
from_station.titelize! if from_station.respond_to?(:titleize!)
to_station.titleize! if to_station.respond_to?(:titleize!)
if date.respond_to?(:to_date)
date = date.to_date
end
end
Fails with this error when date is nil:
NoMethodError (You have a nil object when you didn't expect it!
The error occurred while evaluating nil.to_date):
app/models/schedule.rb:87:in `format_stations_and_date'
app/controllers/schedules_controller.rb:15:in `show'
However, if I change date = date.to_date to self.date = self.date.to_date, the method works correctly.
What's going on? In general, when do I have to write self?
Edit: It's not related to the question, but please note that there is no "titleize!" method.
Whenever you want to invoke a setter method on self, you have to write self.foo = bar. If you just write foo = bar, the ruby parser recognizes that as a variable assignment and thinks of foo as a local variable from now on. For the parser to realize, that you want to invoke a setter method, and not assign a local variable, you have to write obj.foo = bar, so if the object is self, self.foo = bar
You disambiguiate between the instance method name and a local variable using self (it is allowed to have both with the same name in the same scope). In other words, there will be a method name resolution only if there is no local or block variable of the same name in scope. Behold:
class Foo
attr_accessor :boo
def do_boo
boo = 123
puts "Locvar: #{boo} Method: #{self.boo}"
end
end
Foo.new.do_boo
Here's why: imagine you have a module which implements a method. This method assigns something to it's internal local variable
"foo" which is used for some computation. If you skip the "self" part, the method will make a "foo=" method call on the object
whose class includes the module, which was not the intention of the author and can be downright disastrous.
class Foo
def bar=(new_value_of_bar)
set_off_nukes(new_value_of_bar / 3)
end
end
module InnocentModule # written by a different author elsewhere
def do_useful_stuff
...
bar = Math.sin(something) # we're dead
end
end
Foo.send(:include, InnocentModule)
Another crucial part where you have to use self is when invoking the Object#class method, because simply saying "class" means a class keyword for Ruby.
Related
From what I understand about self, it refers to the current instance of the class.
Isn't this the default behaviour at all times anyways? For example, isn't
self.var_one = method(args)
equivalent to
var_one = method(args)
If so, what is the use of self?
There are several important uses, most of which are basically to disambiguate between instance methods, class methods, and variables.
First, this is the best way to define class methods:
class Foo
def self.bar
"class method bar"
end
def bar
"instance method bar"
end
end
Foo.bar #returns "class method bar"
foo = Foo.new
foo.bar #returns "instance method bar"
Also, within instance methods self refers to the instance, within class methods it refers to the class, and it can always be used to distinguish from local variables.
class Bar
def self.foo
"foo!"
end
def baz
"baz!"
end
def self.success
foo #looks for variable foo, doesn't find one, looks for class method foo, finds it, returns "foo!"
end
def self.fail
baz #looks for variable baz, doesn't find one, looks for class method baz, doesn't find one, raises exception
end
def instance_success
baz #looks for variable baz, doesn't find one, looks for instance method baz, finds it, returns "baz!"
end
def instance_fail
foo #looks for variable foo, doesn't find one, looks for instance method foo, doesn't find one, raises exception
end
def local_variable
baz = "is my favorite method"
baz #looks for variable baz, finds it, returns "is my favorite method"
end
def disambiguate
baz = " is my favorite method"
self.baz + baz #looks for instance method baz, finds it, looks for local variable baz, finds it, returns "baz! is my favorite method"
end
end
So, in the end, you can avoid using self in many cases, but it's often helpful to use it to make sure that you don't inadvertently create naming conflicts later on. Sometimes those can create bugs that are very hard to find. In the end it's often a matter of personal style.
As noted in the comments, one more really important thing:
In a class, if you have a method like this:
def bar=(string)
...
end
And in another method you call:
def other_method
bar = "abcd"
end
It isn't going to call your bar= method, it's going to create a local variable bar. So, in this case you use self to tell Ruby not to create a local variable:
def other_method
self.bar = "abcd"
end
The same thing applies if you want to take an argument with the name of a method:
def example
...
end
def other_thing(example)
self.example(example)
end
If you left off self Ruby would assume you meant the local variable with the same name.
So, in general, self in method names is used to distinguish between class and instance variables, and everywhere else you use it when Ruby needs help distinguishing between method calls and local variables or local variable assignment.
I hope that makes sense.
In most cases self.foo is indeed redundant because you can just write foo for the same effect, but in this case it is not and the self is required.
var_one = method(args) will create a local variable called var_one, it will not call any method or do anything else to self.
self.var_one = method(args) will call the method var_one= on self with the argument method(args).
Another case where the use of self is non-optional would be if you want to pass it as an argument to a method, i.e. some_method(self) - you can't do that without the self keyword.
One other use of self is to declare class methods (similar to static methods in Java).
class foo
def self.bar
#do class related stuff here
end
end
That being said, you could also have used def foo.bar instead for the method signature.
Here's an example:
def run miles
self.miles = miles
end
In this case self will help. In most cases self is redundant.
Is there a way to bind an existing method to an existing instance of an object if both the method and the instance are passed as symbols into a method that does that if the instance is not a symbol?
For example:
def some_method
#do something
end
some_instance = Klass.new(something)
def method_that_binds(:some_method, to: :some_instance)
#how do I do that?
end
Your requirements are a little unusual, but it is possible to do this mostly as you say:
class Person; end
harry = Person.new
barry = Person.new
def test
puts 'It works!'
end
define_method :method_that_binds do |a_method, to|
eval(to[:to].to_s).singleton_class.send(:define_method, a_method, &Object.new.method(a_method))
end
method_that_binds :test, to: :harry
harry.test
# It works! will be sent to STDOUT
barry.test
# undefined method 'test'
This doesn't actually use a named parameter, but accepts a hash with a to key, but you can see you can call it in the way you want. It also assumes that the methods you are defining are defined globally on Object.
The API you want doesn't easily work, because you have to know from which scope you want to access the local variable. It's not quite clear to me why you want to pass the name of the local variable instead of passing the content of the local variable … after all, the local variable is present at the call site.
Anyway, if you pass in the scope in addition to the name, this can be accomplished rather easily:
def some_method(*args)
puts args
puts "I can access some_instance's ivar: ##private_instance_var"
end
class Foo; def initialize; #private_instance_var = :foo end end
some_instance = Foo.new
def method_that_binds(meth, to:, within:, with: [])
self.class.instance_method(meth).bind(within.local_variable_get(to)).(*with)
end
method_that_binds(:some_method, to: :some_instance, within: binding, with: ['arg1', 'arg2'])
# arg1
# arg2
# I can access some_instance's ivar: foo
As you can see, I also added a way to pass arguments to the method. Without that extension, it becomes even simpler:
def method_that_binds(meth, to:, within:)
self.class.instance_method(meth).bind(within.local_variable_get(to)).()
end
But you have to pass the scope (Binding) into the method.
If you'd like to add a method just to some_instance i.e. it's not available on other instances of Klass then this can be done using define_singleton_method (documentation here.)
some_instance.define_singleton_method(:some_method, method(:some_method))
Here the first use of the symbol :some_method is the name you'd like the method to have on some_instance and the second use as a parameter to method is creating a Method object from your existing method.
If you'd like to use the same name as the existing method you could wrap this in your own method like:
def add_method(obj, name)
obj.define_singleton_method(name, method(name))
end
Let's say we have a class A with a method a and a local variable c.
class A
def a; 10 end
end
c = '5'
And we want to add the method A#a to c.
This is how it can be done
c.singleton_class.send :define_method, :b, &A.new.method(:a)
p c.b # => 10
Explanations.
One way to add a method to an object instance and not to its class is to define it in its singleton class (which every ruby object has).
We can get the c's singleton class by calling the corresponding method c.signleton_class.
Next we need to dynamically define a method in its class and this can usually be accomplished by using the define_method which takes a method name as its first argument (in our case :b) and a block. Now, converting the method into a block might look a bit tricky but the idea is relatively simple: we first transform the method into a Method instance by calling the Object#method and then by putting the & before A.new.method(:a) we tell the interpreter to call the to_proc method on our object (as our returned object is an instance of the Method, the Method#to_proc will be called) and after that the returned proc will be translated into a block that the define_method expects as its second argument.
I've been writing a DSL, and I'm trying to get a dynamically defined method to be accessible from a lambda. This works fine unless you try to do a setter something= in which case the lambda invocation just sets a local variable instead.
A simplified example:
class Caller
attr_accessor :cmd
def callme
self.class.send(:define_method, "something") { puts "Retrieve Something" }
self.class.send(:define_method, "something=") {|val| puts "Set Something = #{val}" }
instance_exec &cmd
end
end
c = Caller.new
c.cmd = lambda { something = 1 }
c.callme
This also works fine if I use self.something=. However that's less than ideal in the case of a DSL.
Is it possible to get this to work without self. in front of the method?
This has absolutely nothing to do with blocks or dynamically defined methods. It's just simple basic Ruby syntax:
foo = bar
is local variable assignment. Always.
self.foo = bar
is a method call.
Is it possible to get this to work without self. in front of the method?
No.
This is just basic Ruby syntax. The define_method metaprogramming, the instance_exec, the blocks in your code sample are just a red herring, the problem can be demonstrated with a much simpler example:
def foo=(*)
puts 'I was called!'
end
foo=('bar') # even removing spaces and adding parentheses won't help!
self.foo = 'bar'
# I was called!
Note also that foo= is private but was actually called with an explicit receiver (which is illegal for private methods). That's a special exception for setter methods in the rule for private methods, because they otherwise couldn't be called at all, precisely because they would always be interpreted as a local variable assignment.
I Just started learning ruby and I don't see the difference between an #instace_variable and an attribute declared using attr_accessor.
What is the difference between the following two classes:
class MyClass
#variable1
end
and
class MyClass
attr_accessor :variable1
end
I searched lot of tutorials online and everybody uses different notation, Does it have to do anything with the ruby version? I also searched few old threads in StackOverflow
What is attr_accessor in Ruby?
What's the Difference Between These Two Ruby Class Initialization Definitions?
But still I am not able to figure out what is the best way to use.
An instance variable is not visible outside the object it is in; but when you create an attr_accessor, it creates an instance variable and also makes it visible (and editable) outside the object.
Example with instance variable (not attr_accessor)
class MyClass
def initialize
#greeting = "hello"
end
end
m = MyClass.new
m.greeting #results in the following error:
#NoMethodError: undefined method `greeting' for #<MyClass:0x007f9e5109c058 #greeting="hello">
Example using attr_accessor:
class MyClass
attr_accessor :greeting
def initialize
#greeting = "hello"
end
end
m2 = MyClass.new
m2.greeting = "bonjour" # <-- set the #greeting variable from outside the object
m2.greeting #=> "bonjour" <-- didn't blow up as attr_accessor makes the variable accessible from outside the object
Hope that makes it clear.
Instance variables are not directly visible outside of the class.
class MyClass
def initialize
#message = "Hello"
end
end
msg = MyClass.new
#message
#==> nil # This #message belongs to the global object, not msg
msg.message
#==> NoMethodError: undefined method `message'
msg.#message
#==> SyntaxError: syntax error, unexpected tIVAR
Now, you can always do this:
msg.instance_eval { #message }
or ask for the variable directly like this:
msg.instance_variable_get :#message
But that's awkward and sort of cheating. Poking around someone else's class may be educational, but your client code shouldn't be required to do it to get reliable results. So if you want clients to be able to see those values, don't make them use the above techniques; instead, define a method to expose the value explicitly:
class MyClass
def message
return #message
end
end
msg.message
# ==> "Hello"
Because you so often want to do that, Ruby provides a shortcut to make it easier. The code below has exactly the same result as the code above:
class MyClass
attr_reader :message
end
That's not a new type of variable; it's just a shorthand way to define the method. You can look at msg.methods and see that it now has a message method.
Now, what if you want to allow outsiders to not only see the value of an instance variable, but change it, too? For that, you have to define a different method for assignment, with a = in the name:
class MyClass
def message=(new_value)
#message = new_value
end
end
msg.message = "Good-bye"
msg.message
# ==> "Good-bye"
Note that the assignment operators are semi-magical here; even though there's a space between msg.message and =, Ruby still knows to call the message= method. Combination operators like += and so on will trigger calls to the method as well.
Again, this is a common design, so Ruby provides a shortcut for it, too:
class MyClass
attr_writer :message
end
Now, if you use attr_writer by itself, you get an attribute that can be modified, but not seen. There are some odd use cases where that's what you want, but most of the time, if you are going to let outsiders modify the variable, you want them to be able to read it, too. Rather than having to declare both an attr_reader and an attr_writer, you can declare both at once like so:
class MyClass
attr_accessor :message
end
Again, this is just a shortcut for defining methods that let you get at the instance variable from outside of the class.
attr_accesor gives you methods to read and write the instance variables. Instance variables are deasigned to be hidden from outside world so to communicate with them we should have attr_ibute accesor methods.
In OOPS we have a concept called encapsulation which means, the internal representation of an object is generally hidden from view outside of the object's definition. Only the Object 'itself' can mess around with its own internal state. The outside world cannot.
Every object is usually defined by its state and behavior, in ruby the instance variables is called internal state or state of the object and according to OOPS the state should not be accessed by any other object and doing so we adhere to Encapsulation.
ex: class Foo
def initialize(bar)
#bar = bar
end
end
Above, we have defined a class Foo and in the initialize method we have initialized a instance variable (attribute) or (property). when we create a new ruby object using the new method, which in turn calls the initialize method internally, when the method is run, #bar instance variable is declared and initialized and it will be saved as state of the object.
Every instance variable has its own internal state and unique to the object itself, every method we define in the class will alter the internal state of the object according to the method definition and purpose. here initialize method does the same, such as creating a new instance variable.
var object = Foo.new(1)
#<Foo:0x00000001910cc0 #bar=1>
In the background, ruby has created an instance variable (#bar =1) and stored the value as state of the object inside the object 'object'. we can be able to check it with 'instance_variables' method and that methods returns an array containing all the instance variables of the object according to present state of the object.
object.instance_variables
#[
[0]: #bar
]
we can see '#bar' instance variable above. which is created when we called the initialize method on the object. this '#bar' variable should not be visible (hidden) by default and so it cannot be seen by others from outside of the object except the object, from inside. But, an object can mess around with its own internal state and this means it can show or change the values if we give it a way to do so, these two can be done by creating a new instance methods in the class.
when we want to see the #bar variable by calling it we get an error, as by default we cannot see the state of an object.
show = object.bar
#NoMethodError: undefined method `bar' for #<Foo:0x00000001910cc0 #bar=1>
#from (irb):24
#from /home/.rvm/rubies/ruby-2.0.0-p648/bin/irb:12:in `<main>'
But we can access the variables by two methods, these two are called setter and getter methods, which allow the object to show or change its internal state (instance variables/attributes/properties) respectively.
class Foo
def bar
#bar
end
def bar=(new_bar)
#bar = new_bar
end
end
We have defined a getter(bar) and setter(bar=) methods, we can name them any way but the instance variable inside must the same as instance variable to which we want to show or change the value. setters and getters are a violation to OOPS concepts in a way but they are also very powerful methods.
when we define the two methods by re-opening the class and defining them, when we call the object with the methods, we can be able to view the instance variables(here #foo) and change its value as well.
object.bar
1
object.bar=2
2
object.bar
2
Here we have called the bar method (getter) which returns the value of #bar and then we have called bar= method (setter) which we supplied a new_value as argument and it changes the value of instance variable (#bar) and we can look it again by calling bar method.
In ruby we have a method called attr_accessor , which combines the both setter and getter methods, we define it above the method definitions inside the class. attr_* methods are shortcut to create methods (setter and getter)
class Foo
attr_accessor :bar
end
we have to supply a symbol (:bar) as argument to the attr_accessor method which creates both setter and getter methods internally with the method names as supplied symbol name.
If we need only a getter method, we can call attr_reader :bar
If we need only a setter method, we can call attr_writer :bar
attr_accessor creates both attr_writer and attr_reader methods
we can supply as many instance variables as we want to the attr_* methods seperated by commas
class Foo
attr_writer :bar
attr_reader :bar
attr_accessor :bar, :baz
end
Because attr_accessor defines methods, you can call them from outside the class. A #variable is only accessible from inside the class.
And another answer more compact (for Java developers)
attr_accessor :x creates the getters and setters to #x
class MyClassA
attr_accessor :x
end
is the same as
class MyClassB
def x=(value) #java's typical setX(..)
#x=value
end
def x
#x
end
end
In Ruby, inside a class's instance method, we use a getter by
foo
and we use a setter by
self.foo = something
One doesn't need to have a self. and the other does, is there a way to make them look more similar, and not using something like self.foo as the getter, as it also looks verbose.
(update: note that getter and setter may simply get or set an instance variable, but they might also do a lot of work, such as going into the DB and check the existence of a record and if not, create it, etc)
Since local scope takes precedence, when you say foo = something, a local variable foo will be created and assigned the contents of something.
The reason you can write foo in order to use the getter is because Ruby will move up in scope when it can't find a variable with that name and it will eventually find the method.
If there is a local variable with the same name as the getter method, Ruby will use its value instead:
class Foo
attr_accessor :foo
def initialize
#foo = :one
end
def f
foo = :two
foo
end
end
Foo.new.f
# => :two
In order to make it clear that you want to access the setter, you must write self.foo = something. That will tell Ruby you want to execute the foo= method on the self object with something as parameter.
If you are willing to break the conventions, you can write your setters using jQuery style, using the same method for getter and setter, depending of whether it has arguments or not:
def foo *args
return #foo if args.empty?
#foo = args.first
end
# => nil
foo
# => nil
foo(:bar) # foo = :bar
# => :bar
foo
# => :bar
As far as I know, there isn't a way around this in Ruby. I'm pretty confident this is simply how Ruby evaluates expressions.
When given a value, Ruby will first check if there is a local variable within the context which matches the one being called. If there is (perhaps 'foo' in your case), that will be the value used. If there is no such value, then Ruby will try to look up the value as a method call (falling through to "self" as the caller). If no such method exists in the look up path, an error will be raised.
The need to use "self" in the setter is to avoid Ruby setting the value as a local variable, while the lack of the use of "self" only works in the getter instance when there is no local variable of the same name being used in that context. It is probably better and clearer, albeit slightly more verbose, to be explicit with your use of self as to avoid confusion about where values are coming from.