Dynamically creating accessors in Ruby module

Question

I have a Config module in Ruby that I want to be able to add arbitrary variables to. I have created it using method_missing and instance_variable_set as follows:

module Conf
  #add arbitrary methods to config array
  def self.method_missing(m, *args)
    args = args.pop if args.length==1
    instance_variable_set("@#{m}", args)     
  end
end

However, I'm having trouble with dynamically creating accessors. When I try to use attr_accessor as follows:

module Conf
  #add arbitrary methods to config array
  def self.method_missing(m, *args)
    args = args.pop if args.length==1
    instance_variable_set("@#{m}", args)     
    module_eval("attr_accessor :#{m}")
  end
end

I get the following:

Conf::s3_key('1234ABC') #Conf::s3_key=nil

And if I try to create the accessors separately:

module Conf
  #add arbitrary methods to config array
  def self.method_missing(m, *args)
    args = args.pop if args.length==1
    instance_variable_set("@#{m}", args)
    module_eval("def self.#{m};@#{m};end")
    module_eval("def self.#{m}=(val);@#{m}=val;end")
  end
end

The following happens:

Conf::s3_key('1234ABC') # Conf::s3_key='1234ABC' - correct

but if I try to overwrite the value I get an error

Conf::s3_key('1234ABC') # ok
Conf::s3_key('4567DEF') #(eval):1:in `s3_key': wrong number of arguments (1 for 0) (ArgumentError)

What am I doing wrong?

Setting the class instance variable works fine, so it appears to me that your main problem is the dynamic creation of a read-write accessor for that class instance variable, given that its name is one of missing_method's arguments. Is that correct? — Cary Swoveland
– Cary Swoveland, Commented Jul 25, 2014 at 4:45
@CodeGnome - I need other functionality besides just storing the hashes. — Iain
– Iain, Commented Jul 25, 2014 at 23:56

yohshiy · Accepted Answer · 2014-07-25 21:29:03Z

2

First, attr_accessor is unusable for Module, even if normally described.

module Conf
  attr_accessor :s3_key
end

Second, the error of overwriting is because method_missing is executed only once

  def self.method_missing(m, *args)
    #:
    instance_variable_set("@#{m}", args)
    module_eval("def self.#{m};@#{m};end") # <- method defined

the method is defined in first call. And the number of arguments is 0

Conf::s3_key('1234ABC') # call method_missing
Conf::s3_key('4567DEF') # call self.s3_key()

For example, how about like this:

module Conf
  def self.method_missing(m, *args)
    args = args.pop if args.length==1
    instance_variable_set("@#{m}", args)
    module_eval(<<EOS)
def self.#{m}(*args)
  if (args.empty?)
    @#{m}
  else
    @#{m} = (args.length==1) ? args.pop : args
  end
end
EOS
  end
end

Conf::s3_key('foo')
Conf::s3_key('bar')
p Conf::s3_key                   # "bar"

Or

module Conf
  def self.method_missing(m, *args)
    if (m.to_s =~ /^(.+)=$/)
      args = args.pop if args.length==1
      instance_variable_set("@#{$1}", args)
    else
      instance_variable_get("@#{m}")
    end
  end
end

Conf::s3_key = 'foo'
Conf::s3_key = 'bar'
p Conf::s3_key                   # "bar"

edited Jul 25, 2014 at 21:29

answered Jul 25, 2014 at 5:27

yohshiy

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4 Comments

Iain Over a year ago

That worked, thank you. What's the reason that calling module_eval twice in method_missing didn't work, as it looked to me like it should be functionally similar to this solution (obviously, it's not because yours works and mine doesn't!)

Cary Swoveland Over a year ago

yohshiy, actually attr_accessor can be used for modules. (See my answer.) Yesterday I would have agreed with you, but after thinking about it, and working on it, it's actually quite straightforward.

yohshiy Over a year ago

@lain In method_missing, s3_key() method is defined. the number of arguments of the method is 0. Conf::s3key('4567DEF') <- the number of arguments is 1. In my sample, s3_key(*args) is defined.

yohshiy Over a year ago

@CarySwoveland You are right. I didn't think of using eval on Module.

Cary Swoveland · Accepted Answer · 2014-07-26 18:34:00Z

1

You only need to change one line of your code.

Code

module Conf
  def self.method_missing(m, *args)
    args = args.pop if args.length==1
    instance_variable_set("@#{m}", args)     
    Module.instance_eval("attr_accessor :#{m}")
  end
end

Example

Conf.s3_key('1234ABC')
Conf.s3_key             #=> "1234ABC"
Conf.s3_key = '4567DEF'
Conf.s3_key             #=> "4567DEF"

(Or Conf::s3_key('1234ABC'), etc.)

Explanation

Accessors are defined for classes and apply to class instances. In this case the class instance is the module Conf, so attr_accessor must be defined for the class of which Conf is an instance:

Conf.class #=> Module

Note that

Module.is_a? Class       #=> true
Conf.instance_of? Module #=> true

We do this by invoking BasicObject#instance_eval on Module. We need to use instance_eval so that the variable m will be in scope when it is invoked.

One last observation. Suppose the module Conf were enclosed by another module M. Then the code still works:

   M::Conf.s3_key('1234ABC')
   M::Conf.s3_key             #=> "1234ABC"
   M::Conf.s3_key = '4567DEF'
   M::Conf.s3_key             #=> "4567DEF"

That's because all modules, including nested ones, are instances of the class Module.

edited Jul 26, 2014 at 18:34

answered Jul 26, 2014 at 4:39

Cary Swoveland

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3 Comments

Iain Over a year ago

That approach works well when I try it as published, but in my actual use-case, I get wrong number of arguments (1 for 0) (ArgumentError) Rather than calling Conf.s3_key directly, I am doing: config do s3_key "1234ABC" s3_key "4567DEF" end This then calls a function in my Base class: def self.config (&block) Conf.module_eval(&block) end Which invokes Conf as above

Cary Swoveland Over a year ago

Iain, try calling config like this: BaseClass.config { s3_key "1234ABC"; s3_key = "456DEF" }. I think your problem is that s3_key "456DEF" invokes the read method for s3_key that method_missing created when s3_key "1234ABC" was executed. That method takes zero arguments, but you passed one.

Cary Swoveland Over a year ago

Iain, note that you could use instance_eval rather than module_eval (aka class_eval) in your class method config (just as I could have used module_eval instead of instance_eval in method_missing.) That's because module_eval evaluates the string in the context of Conf, whereas instance_eval evaluates the string in the context of the instance Conf of the class Module. Same thing!

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