def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name#this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct way?
def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name #this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct way?
def starts a new local scope, so it won't see label. You need to get
into obj's singleton class, using code blocks rather than class and
def blocks:
def foo(obj, label)
(class << obj; self; end).class_eval do
define_method("name") { label }
end
end
You could also do something with eval but I prefer to use the code
block approach rather than the string-eval'ing approach.
(Still hoping for Object#singleton_class to make the access to the
singleton class a little less awkward
does get the behaviour that I want. Is this the best way to do this or is
there a better, cleaner way?
cheers
nilesh
···
On Thu, Sep 10, 2009 at 6:05 PM, Nilesh Trivedi <nilesh.tr@gmail.com> wrote:
How do I define a singleton method dynamically?
I have code similar to this:
def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name #this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct
way?
On Thu, Sep 10, 2009 at 6:05 PM, Nilesh Trivedi <nilesh.tr@gmail.com> wrote:
How do I define a singleton method dynamically?
I have code similar to this:
def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name #this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct
way?
Replacing line 2 by:
obj.instance_eval "def name; '#{label}'; end"
does get the behaviour that I want. Is this the best way to do this or is
there a better, cleaner way?
It may be overkill, but I pretty much always choose the block form of
*_eval rather than the string form, as per my response of a couple of
minutes ago. If you're going to eval strings, you could also do:
I agree, but it's needed a little less in Ruby 1.9 which adds defined_singleton_method(). Thus, in 1.9 your code could be:
def foo(obj, label)
obj.define_singleton_method(:name) { label }
end
As a side topic, the fact that this method was added seems to kill the argument about why we can't have singleton_class(). As I understand it, the method we want is never added because we aren't sure if the name is right yet. Seems like we're committed to the name at this point and might as well just get on with defining the methods that help us.
James Edward Gray II
···
On Sep 10, 2009, at 7:49 AM, David A. Black wrote:
Hi --
On Thu, 10 Sep 2009, Nilesh Trivedi wrote:
How do I define a singleton method dynamically?
I have code similar to this:
def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name #this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>" whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct way?
def starts a new local scope, so it won't see label. You need to get
into obj's singleton class, using code blocks rather than class and
def blocks:
def foo(obj, label)
(class << obj; self; end).class_eval do
define_method("name") { label }
end
end
You could also do something with eval but I prefer to use the code
block approach rather than the string-eval'ing approach.
(Still hoping for Object#singleton_class to make the access to the
singleton class a little less awkward
On Thu, Sep 10, 2009 at 6:26 PM, David A. Black <dblack@rubypal.com> wrote:
Hi --
On Thu, 10 Sep 2009, Nilesh Trivedi wrote:
On Thu, Sep 10, 2009 at 6:05 PM, Nilesh Trivedi <nilesh.tr@gmail.com> >> wrote:
How do I define a singleton method dynamically?
I have code similar to this:
def foo(obj,label)
def obj.name; label; end #we want to define a singleton method called
"name" on obj which returns the value in label
puts obj.name #this is an example. Actually I have some code here which
expects this singleton method in obj
end
When I try to run this as foo(Object.new,"xyz"), it throws a "NameError:
undefined local variable or method `label' for #<Object:0xb7b02018>"
whereas
I want it to print "xyz" to screen.
Of course, line 2 in method foo is not correct. But what is the correct
way?
Replacing line 2 by:
obj.instance_eval "def name; '#{label}'; end"
does get the behaviour that I want. Is this the best way to do this or is
there a better, cleaner way?
It may be overkill, but I pretty much always choose the block form of
*_eval rather than the string form, as per my response of a couple of
minutes ago. If you're going to eval strings, you could also do:
