Ruby: How to determine how many variables are being assigned by method caller

Question

I would like to have my method change what it returns depending on how many variables are being assigned by the caller. Is there a way to query this in the method? Or is there a way to encode a 2-value return so that the second value will be ignored if an assignment destination is not supplied? Thanks! --Myles

def foo
  if <two-vars-being-assigned>
    return [:bar, :baz] 
  else
    return :bar
  end
end

x, y = foo

# I want x to get :bar, not [:bar, :baz]
x = foo

Answer 1

If you really wanted to, you could return an object which responds to to_ary. You could even monkey patch Symbol in your example with a to_ary method.

But … NO. Just NO.

class Symbol
  def to_ary
    [:bar, :baz]
  end
end

def foo
  :bar
end

a = foo
a #=> :bar

b, c = foo
b #=> :bar
c #=> :baz

Whatever kind of object you return, it will violate the Single Responsibility Principle, since it acts both as whatever the return value is supposed to act as and as a proxy for splitting itself into two return values.

Just … NO. Seriously. NO.

Answer 2

I would agrue that this is not possible. When you call x, y = foo then foo gets evaluated first and therefore runs and returns before the assignment is done. Furthermore, I see no easy and reasonable way to make a method aware of the code structure at the place from which it was called.

Why don't you just handle this at the place in which you call that method:

def foo
  [:bar, :baz] 
end

x, y = foo
x = foo.first