Ruby DSL define_method with arguments

Question

This is what I'm looking to do.

# DSL Commands
command :foo, :name, :age
command :bar, :name

# Defines methods
def foo(name, age)
  # Do something
end

def bar(name)
  # Do something
end

Basically, I need a way to handle arguments through define_method, but I want a defined number of arguments instead of an arg array (i.e. *args)

This is what I have so far

def command(method, *args)
  define_method(method) do |*args|
    # Do something
  end
end

# Which would produce
def foo(*args)
  # Do something
end

def bar(*args)
  # Do something
end

Thoughts?

I don't understand but I want a defined number of arguments instead of an arg array (i.e. *args) - any example for this... — Arup Rakshit
– Arup Rakshit, Commented Nov 12, 2013 at 20:15
I would prefer to limit the number of arguments so I know exactly how many to call. — Dan Grahn
– Dan Grahn, Commented Nov 12, 2013 at 20:26

John Ledbetter · Accepted Answer · 2013-11-12 20:33:51Z

3

I think the best workaround for this would be do to something like the following:

def command(method, *names)
  count = names.length
  define_method(method) do |*args|

    raise ArgumentError.new(
      "wrong number of arguments (#{args.length} for #{count})"
    ) unless args.length == count

    # Do something
  end
end

answered Nov 12, 2013 at 20:33

John Ledbetter

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1 Comment

Dan Grahn Over a year ago

I half like this, half don't. Let me think about it. :)

dredozubov · Accepted Answer · 2013-11-13 12:06:10Z

3

It's a little weird, but you can use some type of eval. instance_eval, module_eval or class_eval could be used for that purpose, depending on context. Something like that:

def command(method, *args)
  instance_eval <<-EOS, __FILE__, __LINE__ + 1
    def #{method}(#{args.join(', ')})
      # method body
    end
  EOS
end

This way you'll get exact number of arguments for each method. And yes, it may be a bit weirder than 'a little'.

edited Nov 13, 2013 at 12:06

answered Nov 13, 2013 at 0:41

dredozubov

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1 Comment

jiehanzheng Over a year ago

Beautiful hack :)

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