Ruby dynamic arguments in dynamically created methods

Question

I have the following kind of method definition:

method_name = :foo
method_arguments = [:bar, :baz]
method_mandatory_arguments = {:quux => true}
method_body = ->{ quux ? bar + baz : bar - baz }

So I want to get a real method. But define_method has no any possibility to define method arguments dynamically. I know another way to use class_eval but I know than defining methods with class_eval is much slower than define_method. How I can effectively archive this?

I did some benchmarks in rails console:

class Foo; end
n = 100_000

Benchmark.bm do |x|
  x.report('define_method') do
    n.times { |i| Foo.send(:define_method, "method1#{i}", Proc.new { |a, b, c| a + b + c }) }
  end
  x.report('class_eval') do
    n.times { |i| Foo.class_eval %Q{ def method2#{i}(a, b, c); a + b + c; end } }
  end
end

So I've got the following results:

                user       system     total      real
define_method  0.750000   0.040000   0.790000 (  0.782988)
class_eval     9.510000   0.070000   9.580000 (  9.580577)

class_eval makes sense to me. Is performance really an issue here? It's hard to imagine a use case where it would be. — Darshan Rivka Whittle
– Darshan Rivka Whittle, Commented Apr 7, 2013 at 22:21
You could have your define_method block accept varargs, then interpret the args array based on your argument meta data captured in the closure. — dbenhur
– dbenhur, Commented Apr 7, 2013 at 22:41
@dbenhur, ok but in this case in fact I have one method that implements multiply behaviour so I must implement basic argument behaviour by hand (argument presence, default arguments etc). This is what i trying to avoid. — freemanoid
– freemanoid, Commented Apr 8, 2013 at 7:11
Are you sure class_eval is slower for this sort of stuff? My recollection was that because define_method creates a closure it can be slower (or cause leaks) — Frederick Cheung
– Frederick Cheung, Commented Apr 8, 2013 at 8:09
Can you show us your implementation using class_eval given that method_body is a lambda with no parameters? — Simon Perepelitsa
– Simon Perepelitsa, Commented Apr 8, 2013 at 8:54

Simon Perepelitsa · Accepted Answer · 2013-04-08 11:41:32Z

3

There is no simple way to implement what you are asking using either class_eval or define_method. Since method_body is a lambda it can only access local variables defined right before it.

method_body = ->{ quux ? bar + baz : bar - baz }
quux = true
method_body.call # undefined local variable or method ‘quux’

quux = true
method_body = ->{ quux ? bar + baz : bar - baz }
method_body.call # undefined local variable or method ‘bar’

I would suggest you to revise your requirements. If your method body should be a lambda, define all arguments for it. Then this is a natural fit for define_method:

method_body = ->(quux, bar = 0, baz = 0){ quux ? bar + baz : bar - baz }
define_method(method_name, &method_body)

If your method body can be a string, eval is the only option:

method_body = "quux ? bar + baz : bar - baz"
eval <<RUBY
def #{method_name} #{arguments}
  #{method_body}
end
RUBY

answered Apr 8, 2013 at 11:41

Simon Perepelitsa

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3 Comments

freemanoid Over a year ago

From your point of view I want to create a proc (or lambda that doesn't matter in my case) with the set of arguments that I store in a specific form. So I have an array method_arguments = [:bar, :baz] and I want to use it like this: method_body = ->(*method_arguments){ quux ? bar + baz : bar - baz }

Simon Perepelitsa Over a year ago

This is not possible with procs/lambdas. Can you store your method body as a string? It would help if you give a more specific real-world example.

freemanoid Over a year ago

It seems that the only way is to use class_eval.

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