Singleton methods with method_missing

[Pat Maddox]

I don't have any practical use for this, but I was experimenting a bit.
I just want to create singleton methods on objects of this class.
There's more to come, but this is the part I'm stuck at

This code doesn't actually work, but I think it's close (I hope?)

class OneShot
def method_missing(m)
class << self
define_method(m) { puts m.to_s }
end
send(m)
end
end

When I do this I get "NameError: undefined local variable or method `m'
for #<Class:#<OneShot:0x32cf74>>" so I think it means that m just isn't
in scope for the singleton class. How can I define the singleton
method? Is there any way to use binding here?

Pat

 

[Ezra Zygmuntowicz]

I don't have any practical use for this, but I was experimenting a
bit.
I just want to create singleton methods on objects of this class.
There's more to come, but this is the part I'm stuck at

This code doesn't actually work, but I think it's close (I hope?)

class OneShot
def method_missing(m) def self.method_missing(m)
class << self
define_method(m) { puts m.to_s }
end
send(m)
end
end

When I do this I get "NameError: undefined local variable or method
`m'
for #<Class:#<OneShot:0x32cf74>>" so I think it means that m just
isn't
in scope for the singleton class. How can I define the singleton
method? Is there any way to use binding here?

Pat

That works unless I misunderstand your question.

-Ezra

 

[dblack]

Hi --

I don't have any practical use for this, but I was experimenting a bit.
I just want to create singleton methods on objects of this class.
There's more to come, but this is the part I'm stuck at

This code doesn't actually work, but I think it's close (I hope?)

class OneShot
def method_missing(m)
class << self
define_method(m) { puts m.to_s }
end
send(m)
end
end

When I do this I get "NameError: undefined local variable or method `m'
for #<Class:#<OneShot:0x32cf74>>" so I think it means that m just isn't
in scope for the singleton class. How can I define the singleton
method? Is there any way to use binding here?

Try this:

module Kernel
def singleton_class # RCR 231
class << self; self; end
end
end

class OneShot
def method_missing(m)
singleton_class.class_eval do
define_method(m) { puts m }
end
end
end

o = OneShot.new
o.x # method 'x' is created
o.x # 'x' is printed


David

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[Sander Land]

I don't have any practical use for this, but I was experimenting a bit.
I just want to create singleton methods on objects of this class.
There's more to come, but this is the part I'm stuck at

This code doesn't actually work, but I think it's close (I hope?)

When I do this I get "NameError: undefined local variable or method `m'
for #<Class:#<OneShot:0x32cf74>>" so I think it means that m just isn't
in scope for the singleton class. How can I define the singleton
method? Is there any way to use binding here?

It is close
class<<self .... end is not a closure (only blocks are), so m is not
defined there. A solution is to use blocks for everything, using *eval
functions.

class OneShot
def method_missing(m,*a)
puts "method #{m} missing!"
(class<<self;self;end).class_eval {
define_method(m) { puts m.to_s }
}
send(m)
end
end

a = OneShot.new
a.foo
a.foo
b = OneShot.new
b.foo
b.foo

output:
method foo missing!
foo
foo
method foo missing!
foo
foo

Which is what you want, right?

 

[Pat Maddox]

It is close
class<<self .... end is not a closure (only blocks are), so m is not
defined there. A solution is to use blocks for everything, using *eval
functions.

class OneShot
def method_missing(m,*a)
puts "method #{m} missing!"
(class<<self;self;end).class_eval {
define_method(m) { puts m.to_s }
}
send(m)
end
end

a = OneShot.new
a.foo
a.foo
b = OneShot.new
b.foo
b.foo

output:
method foo missing!
foo
foo
method foo missing!
foo
foo

Which is what you want, right?

Okay yeah, that works great. Why do you use class_eval there? fwiw, I
tried it with instance_eval and got the same behavior. I still don't
really understand the difference.

Alright, so now that that's out of the way, I want to be able to
undefine methods. I also want to keep track of any that get undefined
so that they can't be redefined at all:

class OneShot
def initialize
@methods = []
end

def method_missing(m, *args, &block)
puts @methods.inspect
raise "Sorry bud, you already had your shot" if @methods.include?(m)
puts "Defining #{m}"
(class << self; self; end).class_eval { define_method(m) { puts
m.to_s } }
send(m)
end

private
def remove_method(m)
@methods << m
(class << self; self; end).class_eval { remove_method(m) }
end
end

If I make remove_method public, I can just call o.remove_methodfoo)
and it works as expected. Apparently remove_method is a private method
though, so I was wondering if I could keep it that way. The code I saw
in Ruby Cookbook showed something like
class << o
remove_method :foo
end

That doesn't actually call my custom remove_method though. If I
understand correctly, it's because that syntax is actually calling
remove_method on the singleton class. Other than making remove_method
public, I'm not sure how to do what I want.

Pat