Ruby: convert proc to lambda?

Answer 1

This one was a bit tricky to track down. Looking at the docs for Proc#lambda? for 1.9, there's a fairly lengthy discussion about the difference between procs and lamdbas.

What it comes down to is that a lambda enforces the correct number of arguments, and a proc doesn't. And from that documentation, about the only way to convert a proc into a lambda is shown in this example:

define_method always defines a method without the tricks, even if a non-lambda Proc object is given. This is the only exception which the tricks are not preserved.

 class C
   define_method(:e, &proc {})
 end
 C.new.e(1,2)       => ArgumentError
 C.new.method(:e).to_proc.lambda?   => true

If you want to avoid polluting any class, you can just define a singleton method on an anonymous object in order to coerce a proc to a lambda:

def convert_to_lambda &block
  obj = Object.new
  obj.define_singleton_method(:_, &block)
  return obj.method(:_).to_proc
end

p = Proc.new {}
puts p.lambda? # false
puts(convert_to_lambda(&p).lambda?) # true

puts(convert_to_lambda(&(lambda {})).lambda?) # true