I'm trying to do something like this in Ruby:
class A
def b(c)
Class.new do
def d
c
end
end
end
end
class B < A.b('whoa'); end
#
# What I want:
#
B.new.d # => 'whoa'
#
# What I get:
#
# NameError: undefined local variable or method `d' for B:0xfdfyadayada
Is there any way to do this?
The context is I'm trying to get some code reuse by constructing classes that are mostly similar except for some minor config.
I'm looking for an API similar to Sequel::Model where you can do something like:
class Order < Sequel::Model(:order)
The contents of a method definition using the def keyword are not lexically scoped the way blocks are. In other words you can’t do something like this:
foo = 7
def bar
# Code here inside the method definition can't
# "see" foo, so this doesn't work.
foo
end
However you can use the define_method method to dynamically define a method using a block, and the block will be able to refer to local variables in its outer scope. So in your case you could do this (I’ve also changed def b to def self.b, which I think is what you meant):
class A
def self.b(c)
Class.new do
define_method(:d) do # N.B. the d here is a symbol
c
end
end
end
end
class B < A.b('whoa'); end
B.new.d # => 'whoa'
This would also work if you created other classes:
class C < A.b('dude'); end
C.new.d # => 'dude'
# B still works
B.new.d # => 'whoa'
Assuming that b is a class method (i.e def self.b instead of def b), you could store c in a class variable.
class A
def self.b(c)
Class.new do
##c = c
def d
##c
end
end
end
end
class B < A.b('whoa'); end
puts B.new.d # => whoa
There could be a better solution, I haven't looked at the source code of Sequel. This is just the first thing that came to my mind.
Related
My understanding of instance_eval was that if I have module M then the following were equivalent:
module M
def foo
:foo
end
end
class C
class << self
include M
end
end
puts C.foo
equivalent to:
module M
def foo
:foo
end
end
class C
end
C.instance_eval do
include M
end
puts C.foo
However, the first example prints :foo and the second throws a NoMethodError? (Ruby 2.3.0)
In both cases above, if I had replaced:
include M
with:
def foo
:foo
end
ie directly defining the method rather than including a module then both cases would have resulted in a C.foo method being defined. Should I be surprised at this difference between include and defining the method directly?
Or does it ever even make sense to call include within the context of instance_eval? Should it only ever be called within a class_eval?
In each of these cases, what object are you calling include on? In your first example, you're calling include on C's singleton class:
class C
class << self
p self == C.singleton_class
include M
end
end
# => true
p C.foo
# => :foo
...so your include line is equivalent to C.singleton_class.include(M).
In your second example, however, you're calling include on C itself:
class C
end
C.instance_eval do
p self == C
include M
end
# => true
p C.foo
# => NoMethodError: undefined method `foo' for C:Class
p C.new.foo
# => :foo
...so you're doing the equivalent of C.include(M), which is the same as:
class C
p self == C
include M
end
# => true
p C.new.foo
# => :foo
What would work like you want would be to call instance_eval on C's singleton class:
class D
end
D.singleton_class.instance_eval do
p self == D.singleton_class
include M
end
# => true
p D.foo
# => :foo
Module#class_eval() is very different from Object#instance_eval(). The instance_eval() only changes self, while class_eval() changes both self and the current class.
Unlike in your example, you can alter class_instance vars using instance_eval though, because they are in the object scope as MyClass is a singleton instance of class Class.
class MyClass
#class_instance_var = 100
##class_var = 100
def self.disp
#class_instance_var
end
def self.class_var
##class_var
end
def some_inst_method
12
end
end
MyClass.instance_eval do
#class_instance_var = 500
def self.cls_method
##class_var = 200
'Class method added'
end
def inst_method
:inst
end
end
MyClass.disp
#=> 500
MyClass.cls_method
#=> 'Class method added'
MyClass.class_var
#=> 100
MyClass.new.inst_method
# undefined method `inst_method' for #<MyClass:0x0055d8e4baf320>
In simple language.
If you have a look in the upper class defn code as an interpreter, you notice that there are two scopes class scope and object scope. class vars and instance methods are accessible from object scope and does not fall under jurisdiction of instance_eval() so it skips such codes.
Why? because, as the name suggests, its supposed to alter the Class's instance(MyClass)'s properties not other object's properties like MyClass's any object's properties. Also, class variables don’t really belong to classes—they belong to class hierarchies.
If you want to open an object that is not a class, then you can
safely use instance_eval(). But, if you want to open a class definition and define methods with def or include some module, then class_eval() should be your pick.
By changing the current class, class_eval() effectively reopens the class, just like the class keyword does. And, this is what you are trying to achieve in this question.
MyClass.class_eval do
def inst_method
:inst
end
end
MyClass.new.inst_method
#=> :inst
How can I programmatically get the class on which the currently executing code is defined? I need to find the class when control flow runs through multiple method definitions due to super():
class A
def foo
puts(get_current_class)
end
end
class B < A
def foo
puts(get_current_class)
super
end
end
class C < B
def foo
puts(get_current_class)
super
end
end
C.new.foo
# => C
# => B
# => A
I know how to get the method name (using __callee__, caller_locations or __method__); but what about the class?
Since classes in ruby are also modules, this could be achieved with Module#nesting:
class A
def foo
puts(Module.nesting.first)
end
end
class B < A
def foo
puts(Module.nesting.first)
super
end
end
class C < B
def foo
puts(Module.nesting.first)
super
end
end
C.new.foo
# => C
# => B
# => A
Alternatively, if the goal is to construct a list of which methods may be called by the object's ancestor chain, then you could use Method#owner and Method#super_method (available since ruby version 2.2.0):
c = C.new
c.method(:foo).owner # => C
c.method(:foo).super_method.owner # => B
c.method(:foo).super_method.super_method.owner # => A
c.method(:foo).super_method.super_method.super_method # => nil
As a quick off-the-cuff implementation to programatically print all classes then, how about:
c = C.new
method = c.method(:foo)
while(method)
puts m.owner
method = method.super_method
end
# => C
# => B
# => A
(However, there is no guarantee that all of these methods will actually be invoked - as this is determined at runtime via super!)
I want to call instance_eval on this class:
class A
attr_reader :att
end
passing this method b:
class B
def b(*args)
att
end
end
but this is happening:
a = A.new
bb = B.new
a.instance_eval(&bb.method(:b)) # NameError: undefined local variable or method `att' for #<B:0x007fb39ad0d568>
When b is a block it works, but b as a method isn't working. How can I make it work?
It's not clear exactly what you goal is. You can easily share methods between classes by defining them in a module and including the module in each class
module ABCommon
def a
'a'
end
end
class A
include ABCommon
end
Anything = Hash
class B < Anything
include ABCommon
def b(*args)
a
end
def run
puts b
end
end
This answer does not use a real method as asked, but I didn't need to return a Proc or change A. This is a DSL, def_b should have a meaningful name to the domain, like configure, and it is more likely to be defined in a module or base class.
class B
class << self
def def_b(&block)
(#b_blocks ||= []) << block
end
def run
return if #b_blocks.nil?
a = A.new
#b_blocks.each { |block| a.instance_eval(&block) }
end
end
def_b do
a
end
end
And it accepts multiple definitions. It could be made accept only a single definition like this:
class B
class << self
def def_b(&block)
raise "b defined twice!" unless #b_block.nil?
#b_block = block
end
def run
A.new.instance_eval(&#b_block) unless #b_block.nil?
end
end
def_b do
a
end
end
Suppose I have a class A as follows:
class A
class B
end
class C < B
end
...
end
And I want to create a class D that has the same nested class structure as A
class D
Replicate nested class structure of A here
end
I want D to look like:
class D
class B
end
class C < B
end
...
end
So that I can do A::B and D::B with different results
How can I achieve this? Thanks in advance.
class Module
def replicate m
m.constants.each do |sym|
case mm = m.const_get(sym)
when Class then const_set(sym, Class.new(mm.superclass)).replicate(mm)
when Module then const_set(sym, Module.new(mm.superclass)).replicate(mm)
end
end
end
end
class D
replicate A
end
But the superclass part may not be correct with this code.
class A
end
class D
end
[A, D].each do |c|
c.class_eval %Q(
class B
def bar; puts "B#bar in #{c} is not shared" end # <--- new
end
class C < B
def foo; puts "C#foo in #{c}" end
end
)
end
p A.constants
p A::C.instance_methods(false)
p D.constants
p D::C.instance_methods(false)
A::C.new.foo
D::C.new.foo
New
A::B.new.bar
D::B.new.bar
=begin
class B # creates a new class B
def quux; puts "A::B#quux in #{self}" end
end
A::B.new.quux #=> undefined method `quux' for #<A::B:0x101358a98> (NoMethodError)
=end
class A::B # reopens B in A
def quux; puts "A::B#quux in #{self}" end
end
A::B.new.quux
Execution :
$ ruby -w t.rb
["B", "C"]
["foo"]
["B", "C"]
["foo"]
C#foo in A
C#foo in D
New
B#bar in A is not shared
B#bar in D is not shared
A::B#quux in #<A::B:0x10402da28>
It's more duplicating than replicating the whole internal structure, including the methods and possible variables. For this you need reflection, or maybe marshall out and in.
New : If you put something in the text inside %Q(), class_eval will evaluate it for each class, hence it is not shared. B is not independent, you have two different classes A::B and D::B.
If you want to add the same code to both classes, create a module and include it. Ruby creates a proxy which points to the module and inserts the proxy in the chain of pointers starting from the class of the object to its superclass, so that the search method mechanism will look for module's methods after methods of the class and before methods in the superclass.
class D
extend A
end
will define instance methods of A as class (singleton) methods of D. Sounds ugly. I think that you should experiment, display what happens with puts, p, instance_methods, singleton_methods and the like.
I'm looking for a way of making a method "personal" - note NOT PRIVATE to a class
here is an example - by "personal" I mean the behaviour of method "foo"
class A
def foo
"foo"
end
end
class B < A
def foo
"bar"
end
end
class C < B
end
a=A.new; b=B.new;c=C.new
I'm looking for a way of producing the following behaviour
a.foo #=> "foo"
b.foo #=> "bar"
c.foo #=> "foo" (ultimate base class method called)
Instead of creating 'personal' methods, change your inheritance structure.
It appears that you want the C class to have only some of the same functionality of the B class while not making changes to the A class.
class A
def foo
"foo"
end
end
class BnC < A
end
class B < BnC
def foo
"bar"
end
end
class C < BnC
end
a=A.new; b=B.new;c=C.new
There's no standard way of doing this. It circumvents how inheritance works. You could implement B's method to do the logic like this:
def foo
instance_of?(B) ? "bar" : super
end
And you could of course define a method on Class that would do this for you similar to public and private.
class Class
def personal(*syms)
special_class = self
syms.each do |sym|
orig = instance_method(sym)
define_method(sym) {|*args| instance_of?(special_class) ? orig.bind(self).call(*args) : super}
end
end
end
Then you can personal :foo in B just like you'd private :foo.
(This isn't at all optimized and I didn't implement the zero-argument behavior that public and private have because frankly it's a huge PITA to do right and even then it's a hack.)
Seems like it could be confusing, but here's one option:
class A
def foo
"foo"
end
end
class B < A
def initialize #when constructing, add the new foo method to each instance
def self.foo
"bar"
end
end
end
class C < B
def initialize #when constructing, do nothing
end
end
More generally, using a similar approach, you can always add a method to a given instance, which of course has no effect on inherited classes or indeed on other instances of the same class.
If you give us specifics of what you're ultimately trying to accomplish we can probably be more helpful.
Answering this is a bit tricky since I don't really see what you want to accomplish in practice, but you could try something like
class C < B
def foo
self.class.ancestors[-3].instance_method(:foo).bind(self).call
end
end
(The ancestors[-3] assumes that A inherits from Object and Kernel and your intent was to access the method from the topmost non-builtin class. Of course you could substitute self.class.ancestors[-3] with just A, or figure out the class from the Array ancestors yourself, etc.)
In practice it would be simpler to alias the original in class B if you can modify it (i.e. alias :foo_from_A :foo in class B < A before the new def foo, then you can call foo_from_A in C). Or just redefine what you want in C. Or design the whole class hierarchy differently.
You can write a shortcut function to handle personalizing methods.
def personalize(*methodNames)
old_init = instance_method(:initialize)
klass = self
modul = Module.new {
methodNames.each { |m|
define_method(m, klass.instance_method(m)) if klass.method_defined?(m)
}
}
methodNames.each { |m|
remove_method(m) if method_defined?(m)
}
define_method(:initialize) { |*args|
# I don't like having to check instance_of?, but this is the only way I
# could thing of preventing the extension of child classes. At least it only
# has to happen once, during initialization.
extend modul if instance_of?(klass)
old_init.bind(self).call(*args)
}
self
end
class A
def foo
"foo"
end
end
class B < A
def foo
"bar"
end
def bam
'bug-AWWK!'
end
personalize :foo, :bam, :nometh
end
class C < B
end
a=A.new; b=B.new; c=C.new
a.foo #=> "foo"
b.foo #=> "bar"
b.bam #=> "bug-AWWK!"
c.foo #=> "foo"
C.instance_method(:foo) # => #<UnboundMethod: C(A)#foo>
c.bam #throws NoMethodError
Sometimes you don't really want an "is a" (inheritance) relationship. Sometimes what you want is "quacks like a." Sharing code among "quacks like a" classes is easily done by using modules to "mix in" methods:
#!/usr/bin/ruby1.8
module BasicFoo
def foo
"foo"
end
end
class A
include BasicFoo
end
class B
def foo
"bar"
end
end
class C
include BasicFoo
end
p A.new.foo # => "foo"
p B.new.foo # => "bar"
p C.new.foo # => "foo"