How to convert method or lambda to non-lambda proc

Question

As shown in the ruby example below, I can't call a lambda with wrong number of arguments as Proc created from a Method because it is strict about the number of arguments:

# method with no args
def a; end

instance_eval(&method(:a))
# ArgumentError: wrong number of arguments (1 for 0)

method(:a).to_proc.call(1, 2, 3)
# ArgumentError: wrong number of arguments (3 for 0)

method(:a).to_proc.lambda?
# => true

How do I get a Proc that is not a lambda from either a Proc that is or from a Method?

Answer 1

There is no way to do this.

Besides the argument passing, I wonder what you would expect from a return in the method. It can only behave in lambda way...

If you really have to do this, you will need to build your own block, e.g.

Proc.new{ a }

For a more generic way, you'll have to check the arity of the method and pass only the required parameters.

Answer 2

Try wrapping it in a non-lambda Proc, like so:

l = lambda {|a,b| puts "a: #{a}, b: #{b}" }
p = proc {|a,b| l.call(a,b) }

l.lambda?
#=> true
l.arity
#=> 2
l.call("hai")
#=> ArgumentError: wrong number of arguments (1 for 2)
l.call("hai", "bai", "weee", "womp", "woo")
#=> ArgumentError: wrong number of arguments (5 for 2)

p.lambda?
#=> false
p.arity
#=> 2
p.call("hai")
#=> a: hai, b: 
p.call("hai", "bai", "weee", "womp", "woo")
#=> a: hai, b: bai

Answer 3

Convert Lambda to Proc

Here's a work-around that wraps a lambda or a method call in a Proc and uses splat to handle any number of arguments:

def lambda_to_proc(lambda)
  Proc.new do |*args|
    diff = lambda.arity - args.size
    diff = 0 if diff.negative?
    args = args.concat(Array.new(diff, nil)).take(lambda.arity)

    lambda.call(*args)
  end
end

This would always work no matter the number of arguments passed; Extra arguments will be dropped and nil will replace the missing arguments.

---

Example:

# lambda with two args
some_lambda = -> (a,b) { [a, b] }

# method with no args
def some_method; "hello!"; end

lambda_to_proc(some_lambda).call(5)
# => [5, nil]

lambda_to_proc(method(:some_method)).call(1,2,3)
# => "hello!"

---

^{Note: There is no direct way to convert a lambda or a method call to a proc. This is just a workaround and obviously slower than the real deal (because of wrapping one call in another).}