Confusion with the assignment operation inside a falsy `if` block [duplicate]

问题

This question already has an answer here:

I don't understand ruby local scope 5 answers

I was playing with the assignment operation within if blocks, and discovered the below result, which surprised me:

C:\\>irb --simple-prompt
if false
x = 10
end
#=> nil
p x
nil
x.object_id
#=> 4
#=> nil
p y
NameError: undefined local variable or method `y\' for main:Object
        from (irb):5
        from C:/Ruby193/bin/irb:12:in `<main>\'

In the above code you can see that the x local variable has been created even though it was only assigned to in the falsy if block. I tried to to see the content of x with p x which forced me to believe that assignment was not done, but the x variable exists. x.object_id also proved that is the case.

Now my question is how that x local variable was created even though the if block entry point is closed forever intentionally?

I expected the output of p x to be similar to the output from p y. But instead I got a surprising answer from p x.

Could someone explain to me how this concept works?

EDIT

No, here is another test. This is not the case with only local variables. The same happened with instance and class variables also. See the below:

class Foo
  def show
    @X = 10 if false
    p @X,\"hi\",@X.object_id
  end
end
#=> nil
Foo.new.show
nil
\"hi\"
4
#=> [nil, \"hi\", 4]

class Foo
  def self.show
    @@X = 10 if false
    p @@X,\"hi\",@@X.object_id
  end
end
#=> nil
Foo.show
nil
\"hi\"
4
#=> [nil, \"hi\", 4]

Successful case :

class Foo
  def self.show
    @@X = 10 if true
    p @@X,\"hi\",@@X.object_id
  end
end
#=> nil
Foo.show
10
\"hi\"
4
#=> [10, \"hi\", 4]

回答1:

In Ruby, local variables are defined by the parser when it first encounters an assignment, and are then in scope from that point on.

Here's a little demonstration:

foo # NameError: undefined local variable or method `foo' for main:Object

if false
  foo = 42
end

foo # => nil

As you can see, the local variable does exist on line 7 even though the assignment on line 4 was never executed. It was, however, parsed and that's why the local variable foo exists. But because the assignment was never executed, the variable is uninitialized and thus evaluates to nil and not 42.

In Ruby, most uninitialized or even non-existing variables evaluate to nil. This is true for local variables, instance variables and global variables:

defined? foo       #=> nil
local_variables    #=> []
if false
  foo = 42
end
defined? foo       #=> 'local-variable'
local_variables    #=> [:foo]
foo                #=> nil
foo.nil?           #=> true

defined? @bar      #=> nil
instance_variables #=> []
@bar               #=> nil
@bar.nil?          #=> true
# warning: instance variable @bar not initialized

defined? $baz      #=> nil
$baz               #=> nil
# warning: global variable `$baz' not initialized
$baz.nil?          #=> true
# warning: global variable `$baz' not initialized

It is, however, not true for class hierarchy variables and constants:

defined? @@wah     #=> nil
@@wah
# NameError: uninitialized class variable @@wah in Object

defined? QUUX      #=> nil
QUUX
# NameError: uninitialized constant Object::QUUX

This is a red herring:

defined? fnord     #=> nil
local_variables    #=> []
fnord
# NameError: undefined local variable or method `fnord' for main:Object

The reason why you get an error here is not that unitialized local variables don't evaluate to nil, it is that fnord is ambiguous: it could be either an argument-less message send to the default receiver (i.e. equivalent to self.fnord()) or an access to the local variable fnord.

In order to disambiguate that, you need to add a receiver or an argument list (even if empty) to tell Ruby that it is a message send:

self.fnord
# NoMethodError: undefined method `fnord' for main:Object
fnord()
# NoMethodError: undefined method `fnord' for main:Object

or make sure that the parser (not the evaluator) parses (not executes) an assignment before the usage, to tell Ruby that it is a local variable:

if false
  fnord = 42
end
fnord              #=> nil

And, of course, nil is an object (it is the only instance of class NilClass) and thus has an object_id method.

回答2:

Ruby always parses all of your code. It doesn't look at false as a sign to not parse what's inside, it evaluates it and sees that the code inside shouldn't be executed

回答3:

Ruby has local variable "hoisting". If you have an assignment to a local variable anywhere within a method, then that variable exists everywhere within the method, even before the assignment, and even if the assignment is never actually executed. Before the variable is assigned, it has a value of nil.

Edit:

The above is not quite correct. Ruby does have a form of variable hoisting in that it will define a local variable when a local variable assignment is present, but not executed. The variable will not be found to be defined at points in the method above where the assignment occurs, however.

来源：https://stackoverflow.com/questions/15183576/confusion-with-the-assignment-operation-inside-a-falsy-if-block

标签

ruby

if-statement

nameerror