What if it's nil? What if it's method is nil?

Chris Wanstrath wrote a nice little post about a method he created called try(), I thought this was pretty cool, but I really want to be able to specify the return value if the object is nil. Plus, what if I want to use this sweetness on a method? So I wrote two methods to do just that:

class Object
  def if_nil out = nil
    return out if nil?
    self
  end

  def if_method_nil method, out = nil
    return out if nil?
    return send(method) if out.nil?
    return out if respond_to?(method) && send(method).nil?
    send method
  end
end

And here are some tests for them, which illustrate their usage:

  def test_if_nil1
    n = nil
    assert_equal nil, n.if_nil
  end

  def test_if_nil2
    n = 1
    assert_equal 1, n.if_nil
  end

  def test_if_nil3
    n = :yo
    assert_equal :yo, n.if_nil
  end

  def test_if_nil4
    n = nil
    assert_equal 'blah', n.if_nil('blah')
  end


  def test_if_method_nil1
    n = nil
    assert_equal nil, n.if_method_nil(:to_s)
  end

  def test_if_method_nil2
    n = 1
    assert_raise NoMethodError do
      n.if_method_nil :yo
    end
  end

  def test_if_method_nil3
    n = 1
    assert_nothing_raised do
      assert_equal '1', n.if_method_nil( :to_s)
    end
  end

  def test_if_method_nil4
    n = 1
    assert_nothing_raised do
      assert_equal '1', n.if_method_nil( :to_s, 'blah')
    end
  end

  def test_if_method_nil5
    n = nil
    assert_nothing_raised do
      assert_equal 'blah', n.if_method_nil( :to_s, 'blah')
    end
  end

2 Comments

Robert Fischer said on March 12, 2008

Y’know, Real Programming Languages don’t have this problem. In Ocaml (for instance), you never get a NullPointerException analog, because the type system knows when it can be null/nil, and the coder has to explicitly handle the case.

And if you don’t want nil, you just don’t handle the case. Then the compiler won’t let you pass nil in. :)

Steven Bristol said on March 12, 2008

Wow, I always heard ocaml was cool, now it sounds kinda like C#/Java (which is to say sucky).

This is simply a nice one liner way to handle nil checking. It’s not about passing in params to a method. Instead of doing:

val = blah.nil? ? ‘value’ : blah
you can do
val = blah.if_nil ‘value’

It’s just a bit of syntactic sugar.