CODEWITHSUNDEEP
rubylambdametaprogramming
sandric
sandric

Reputation: 2470

when calling instance_eval(&lambda) to pass current context got error 'wrong number of arguments'

To be clear - this code is running perfectly - code with proc

but if instead I change Proc.new to lambda, I'm getting an error

ArgumentError: wrong number of arguments (1 for 0)

May be this is because instance_eval wants to pass self as a param, and lambda treats as a method and do not accept unknown params?

There is two examples - first is working:

class Rule
  def get_rule
    Proc.new { puts name }
  end
end

class Person
  attr_accessor :name

  def init_rule 
    @name = "ruby"
    instance_eval(&Rule.new.get_rule)
  end
end

second is not:

class Rule
  def get_rule
    lambda { puts name }
  end
end

class Person
   attr_accessor :name

   def init_rule 
     @name = "ruby"
     instance_eval(&Rule.new.get_rule)
   end
end

Thanks

Upvotes: 6

Views: 1766

Answers (1)

Michael Papile
Michael Papile

Reputation: 6856

You are actually correct in your assumption. Self is being passed to the Proc and to the lambda as it is being instance_eval'ed. A major difference between Procs and lambdas is that lambdas check the arity of the block being being passed to them.

So: 

 class Rule
   def get_rule
     lambda { |s| puts s.inspect; puts name; }
   end
 end


class Person
   attr_accessor :name

   def init_rule 
     @name = "ruby"
     instance_eval(&Rule.new.get_rule)
   end
end

 p = Person.new
 p.init_rule

 #<Person:0x007fd1099f53d0 @name="ruby">
 ruby

Here I told the lambda to expect a block with arity 1 and as you see in the argument inspection, the argument is indeed the self instance of Person class.

Upvotes: 4

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