Forgive me, guys. I am at best a novice when it comes to Ruby. I'm just curious to know the explanation for what seems like pretty odd behavior to me.
I'm using the Savon library to interact with a SOAP service in my Ruby app. What I noticed is that the following code (in a class I've written to handle this interaction) seems to pass empty values where I expect the values of member fields to go:
create_session_response = client.request "createSession" do
soap.body = {
:user => #user, # This ends up being empty in the SOAP request,
:pass => #pass # as does this.
}
end
This is despite the fact that both #user and #pass have been initialized as non-empty strings.
When I change the code to use locals instead, it works the way I expect:
user = #user
pass = #pass
create_session_response = client.request "createSession" do
soap.body = {
:user => user, # Now this has the value I expect in the SOAP request,
:pass => pass # and this does too.
}
end
I'm guessing this strange (to me) behavior must have something to do with the fact that I'm inside a block; but really, I have no clue. Could someone enlighten me on this one?
First off, #user is not a "private variable" in Ruby; it is an instance variable. Instance variables are available within the the scope of the current object (what self refers to). I have edited the title of your question to more accurately reflect your question.
A block is like a function, a set of code to be executed at a later date. Often that block will be executed in the scope where the block was defined, but it is also possible to evaluate the block in another context:
class Foo
def initialize( bar )
# Save the value as an instance variable
#bar = bar
end
def unchanged1
yield if block_given? # call the block with its original scope
end
def unchanged2( &block )
block.call # another way to do it
end
def changeself( &block )
# run the block in the scope of self
self.instance_eval &block
end
end
#bar = 17
f = Foo.new( 42 )
f.unchanged1{ p #bar } #=> 17
f.unchanged2{ p #bar } #=> 17
f.changeself{ p #bar } #=> 42
So either you are defining the block outside the scope where #user is set, or else the implementation of client.request causes the block to be evaluated in another scope later on. You could find out by writing:
client.request("createSession"){ p [self.class,self] }
to gain some insight into what sort of object is the current self in your block.
The reason they "disappear" in your case—instead of throwing an error—is that Ruby permissively allows you to ask for the value of any instance variable, even if the value has never been set for the current object. If the variable has never been set, you'll just get back nil (and a warning, if you have them enabled):
$ ruby -e "p #foo"
nil
$ ruby -we "p #foo"
-e:1: warning: instance variable #foo not initialized
nil
As you found, blocks are also closures. This means that when they run they have access to local variables defined in the same scope as the block is defined. This is why your second set of code worked as desired. Closures are one excellent way to latch onto a value for use later on, for example in a callback.
Continuing the code example above, you can see that the local variable is available regardless of the scope in which the block is evaluated, and takes precedence over same-named methods in that scope (unless you provide an explicit receiver):
class Foo
def x
123
end
end
x = 99
f.changeself{ p x } #=> 99
f.unchanged1{ p x } #=> 99
f.changeself{ p self.x } #=> 123
f.unchanged1{ p self.x } #=> Error: undefined method `x' for main:Object
From the documentation:
Savon::Client.new accepts a block inside which you can access local variables and even public methods from your own class, but instance variables won’t work. If you want to know why that is, I’d recommend reading about instance_eval with delegation.
Possibly not as well documented when this question was asked.
In the first case, self evaluates to client.request('createSession'), which doesn't have these instance variables.
In the second, the variables are brought into the block as part of the closure.
Another way to fix the issue would be to carry a reference to your object into the block rather than enumerating each needed attribute more than once:
o = self
create_session_response = client.request "createSession" do
soap.body = {
:user => o.user,
:pass => o.pass
}
end
But now you need attribute accessors.
Related
I'm writing a DSL for a project and I've run into an issue involving local variables bleeding into nested procs when I don't want them to. It seems like no matter what I try, I can't overwrite a local variable's value with something from another scope once it's been set into a proc's binding.
an example of what I'm trying to get to work:
class Obj
def foo
:foo
end
end
def run_proc(context, &block)
context.instance_exec(&block)
end
def run_example(context)
# This `foo` the local var that ends up in the binding of
# the proc on the next line that I can't seem to overwrite
foo = :bar
run_proc(context) { foo }
# ^ I want to be able to eval `foo` based on the passed context obj
end
obj = Obj.new
# I want all three of these calls to return `:foo`
obj.foo #=> :foo # works as expected
obj.instance_exec { foo } #=> :foo # works as expected
run_example(obj) #=> :bar # doesn't work, since the `run_example`
# method's local `foo` var takes precedence
# over the passed object's `foo` method
I did some digging around and found answers with a similar approach to what I've been trying: Change the binding of a Proc in Ruby. I've also looked into the possibility of undefining the local variables in a proc's binding but this answer claims doing so is impossible: Undefine variable in Ruby.
So my question now is this: Should I just give up trying to nest procs and/or find a workaround that doesn't have local var/method name conflict issues, or is there actually a way to get around this?
You can use self.foo explicitely
Using it will call the bindings of the object self currently refers to rather then the closure of the state when proc/block is created.
Looking at this instance_eval example:
class KlassWithSecret
def initialize
#secret = 99
end
def get
#secret
end
end
k = KlassWithSecret.new
k.instance_eval { #secret }
print k.get
I added a get method to KlassWithSecret.
Here's the results of running the program:
>ruby InstanceEvalTest.rb
99
So, does instance_eval here somehow call the initialize method?
I think that I understand this method a bit from reading this helpful post. But I'm still in the dark.
The initialize method is automatically called by Ruby after the new method is called. instance_eval runs the block you supply in the context of the object. This means it has access to anything a normal line of code in the KlassWithSecret class would have.
#secret is an instance variable, meaning that it belongs to an instance of KlassWithSecret. Because we're evaluating { #secret } in the context of a KlassWithSecret instance, we can access #secret.
k.instance_eval gives you access to all of the instance variables (here just #secret) and all private methods (if there were any). It executes the code in the block, which in this case returns 99, the value of #secret. Then print k.get prints that value and returns nil.
If the block had been { #secret = 'cat' }, k.instance_val would have changed the value of #secret (and returned the new value).
When using instance_eval, class_eval, class < self and other metaprogramming constructs, you mind find it helpful to track the value of self using puts statements. For example:
k = KlassWithSecret.new #=> #<KlassWithSecret:0x00000101897810 #secret=99>
self #=> main
k.instance_eval { puts "self=#{self}"; #secret }
"self=#<KlassWithSecret:0x00000101897810>"
#=> 99
class Foo
def self.run(n,code)
foo = self.new(n)
#env = foo.instance_eval{ binding }
#env.eval(code)
end
def initialize(n)
#n = n
end
end
Foo.run( 42, "p #n, defined? foo" )
#=> 42
#=> "local-variable"
The sample program above is intended to evaluate arbitrary code within the scope of a Foo instance. It does that, but the binding is "polluted" with the local variables from the code method. I don't want foo, n, or code to be visible to the eval'd code. The desired output is:
#=> 42
#=> nil
How can I create a binding that is (a) in the scope of the object instance, but (b) devoid of any local variables?
The reason that I am creating a binding instead of just using instance_eval(code) is that in the real usage I need to keep the binding around for later usage, to preserve the local variables created in it.
so like this? or did i miss something important here?
class Foo
attr_reader :b
def initialize(n)
#n = n
#b = binding
end
def self.run(n, code)
foo = self.new(n)
foo.b.eval(code)
end
end
Foo.run(42, "p #n, defined?(foo)")
# 42
# nil
or move it further down to have even less context
class Foo
def initialize(n)
#n = n
end
def b
#b ||= binding
end
def self.run(n, code)
foo = self.new(n)
foo.b.eval(code)
end
end
Foo.run(42, "p #n, defined?(foo), defined?(n)")
# 42
# nil
# nil
Answer:
module BlankBinding
def self.for(object)
#object = object
create
end
def self.create
#object.instance_eval{ binding }
end
end
Description:
In order to get a binding with no local variables, you must call binding in a scope without any of them. Calling a method resets the local variables, so we need to do that. However, if we do something like this:
def blank_binding_for(obj)
obj.instance_eval{ binding }
end
…the resulting binding will have an obj local variable. You can hide this fact like so:
def blank_binding_for(_)
_.instance_eval{ binding }.tap{ |b| b.eval("_=nil") }
end
…but this only removes the value of the local variable. (There is no remove_local_variable method in Ruby currently.) This is sufficient if you are going to use the binding in a place like IRB or ripl where the _ variable is set after every evaluation, and thus will run over your shadow.
However, as shown in the answer at top, there's another way to pass a value to a method, and that's through an instance variable (or class variable, or global variable). Since we are using instance_eval to shift the self to our object, any instance variables we create in order to invoke the method will not be available in the binding.
I'm trying to keep a hash local to one function that remembers its state between calls to the function. But I don't know how to declare it without a closure (as some users suggested in a similar thread).
I know C++ more thoroughly than ruby, and in C++, I would have ordinarily used a static local variable, like in the first example here: http://msdn.microsoft.com/en-us/library/s1sb61xd.aspx
I managed to hack something together in ruby using the defined? function:
def func x
if not defined? #hash
#hash = Hash.new
end
if #hash[x]
puts 'spaghetti'
else
#hash[x] = true
puts x.to_s
end
end
func 1
func 1
This prints, the following, which is kind of what I want. The only problem is that #hash can be accessed outside of that function.
1
spaghetti
Is there any "cleaner", more preferred way to declare a variable with this behavior (without a factory)? I was going to create two or three variables like #hash, so I was looking for a better way to express this concisely.
What you're doing is pretty common in Ruby, but also so common you don't need to make a big fuss about it. All #-type instance variables are local to that instance only. Keep in mind "instance" generally refers to an instance of a class, but it can refer to the instance of the class as well.
You can use ## to refer to a class instance variable from the context of an instance, but this tends to get messy in practice.
What you want to do is one of the following.
A variable that persists between method calls, but only within the context of a single object instance:
def func(x)
# Instance variables are always "defined" in the sense that
# they evaluate as nil by default. You won't get an error
# for referencing one without declaring it first like you do
# with regular variables.
#hash ||= { }
if #hash[x]
puts 'spaghetti'
else
#hash[x] = true
puts x.to_s
end
end
A variable that persists between method calls, but only within the context of all object instances:
def func(x)
# Instance variables are always "defined" in the sense that
# they evaluate as nil by default. You won't get an error
# for referencing one without declaring it first like you do
# with regular variables.
##hash ||= { }
if ##hash[x]
puts 'spaghetti'
else
##hash[x] = true
puts x.to_s
end
end
This is usually made cleaner by wrapping the ##hash into a class method. This has the secondary effect of making testing easier:
def self.func_hash
#func_hash ||= { }
end
def func(x)
if self.class.func_hash[x]
puts 'spaghetti'
else
self.class.func_hash[x] = true
puts x.to_s
end
end
When I invoke a method that doesn't exist, method_missing will tell me the name of the method. When I attempt to access a variable that hasn't been set, the value is simply nil.
I'm attempting to dynamically intercept access to nil instance variables and return a value based on the name of the variable being accessed. The closest equivalent would be PHP's __get. Is there any equivalent functionality in Ruby?
I do not believe this is possible in Ruby. The recommended way would be to use a ''user'' method rather than a ''#user'' instance var in your templates.
This is consistent with the way you deal with Ruby objects externally (''obj.user'' is a method which refers to ''#user'', but is actually not ''#user'' itself). If you need any kind of special logic with an attribute, your best bet is to use a method (or method_missing), regardless if you're accessing it from inside or outside the object.
See my answer to another similar question. But just because you can do it doesn't mean that it's a good idea. Sensible design can generally overcome the need for this kind of thing and allow you to produce more readable and hence maintainable code.
instance_variable_get seems to be the closest equivalent of PHP's __get from what I can see (although I'm not a PHP user).
Looking at the relevant Ruby source code, the only 'missing' method for variables is const_missing for constants, nothing for instance variables.
there isn't an instance_variable_missing (at least that I know of)
But why are you accessing randomly named instance variables anyway?
If your thread all the access to the object state through method calls (as you should anyway) then you wouldn't need this.
If you are looking for a way to define magic stuff without messing up with the method lookup, you may want to use const_missing.
A bit late but, instance_variable_missing is the same as method_missing to a point... Take the following class:
class Test
def method_missing(*args)
puts args.inspect
end
end
t = Test.new
Now let's get some instance variables:
t.pineapples #=> [:pineapples]
t.pineapples = 5 #=> [:pineapples=,5]
Not sure why the method is nil for you...
EDIT:
By the sounds of it you want to accomplish:
t = SomeClass.new
t.property.child = 1
So let's try returning a Test object from our previous example:
class Test
def method_missing(*args)
puts args.inspect
return Test.new
end
end
So what happens when we call:
t = Test.new
t.property.child = 1
#=>[:property]
#=>[:child=,1]
So this goes to show that this is indeed possible to do. OpenStruct uses this same technique to set instance variables dynamically. In the below example, I create EternalStruct which does exactly what you wanted:
require 'ostruct'
class EternalStruct < OpenStruct
def method_missing(*args)
ret = super(*args)
if !ret
newES = EternalStruct.new
self.__send__((args[0].to_s + "=").to_sym, newES)
return newES
end
end
end
Usage of EternalStruct:
t = EternalStruct.new
t.foo.bar.baz = "Store me!"
t.foo.bar.baz #=> "Store me!"
t.foo #=> #<EternalStruct bar=#<EternalStruct baz="Store me!">>
t.a = 1
t.a #=> 1
t.b #=> #<EternalStruct:...>
t.b = {}
t.b #=> {}
def t.c(arg)
puts arg
end
t.c("hi there") #=> "hi there"