Naildrivin' ❺

➠ Website of David Bryant Copeland

❺➠ a World Without Nil

Previously, we saw how just using functions in Ruby, we could create a lot of powerful code. Let’s continue the theme of “programming with constraints” and try to solve an actual problem. nil.

Is nil a problem?

nil creates problems in code clarity and revealing programmer intent. nil means “no value” sometimes, but other times it means false. Other times it means “the developer didn’t think of the proper default for a value”. Rails migrations, by default, allow database columns to be nullable. This is often not correct, and by making it the default, you cannot tell the difference between “the business requires that this field be nullable” and “the developer forgot to consider the nullability of this column”.

The problems manifest when you see a test failure or production error where something is nil that you weren’t expecting. Now you have to figure out if that value could be nil (and the original developer missed the edge case) or if it should never be nil, and you have a more serious problem in either your data, business logic, or worse.

The reason this becomes complex isn’t necessarily the concept of “no such value” (though this is a bit of a problem), but the way in which nil is treated by the language. In Ruby, nil is the only instance of NilClass and has the following magical properties that cannot be bestowed on any other object:

  • it is “falsey” (a trait shared with only one other value, false, the sole value of FalseClass)
  • it is the default value of every variable

Because of these two things, we use it all over the place to represent “no value”, and our code is littered with:

  • do_something if value.nil?
  • foo ||= {}
  • Array(some_list).each

And so forth. Avdi Grimm gave a talk at Ruby Midwest called “Confident Ruby” that deals, in part, with nil and how to avoid it. Things like Array(), String(), and null objects are good techniques.

But let’s take a different approach. What if there were no such thing as nil, and the language didn’t support it?

Can you imagine? A world without nil?

Suppose there were no such thing as nil in Ruby. Every variable would require that a value be assigned to it at declare time, and the runtime would raise an exception if you tried to use a variable/parameter/etc. without a value.

How would that change the way we code?

Of course, we could re-invent it:

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class NilClass
  @@nil = NilClass.new
  def self.new
    @@nil
  end

  def nil?
    true
  end
end

class BasicObject
  def nil?
    false
  end
end

$nil = NilClass.new

This gives us a value that means “no value”, but without the magic provided by the language, what good is it?

Let’s return to our domain from the previous post, where we want to make a system that manages users in a database. Since we now have our fully armed and operational object-oriented programming language, we might be inclined to make a Person class:

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class Person
  attr_reader :name, :birthdate, :gender, :title, :id
  def initialize(name,birthdate,gender,title,id)
    @name = name
    @birthdate = birthdate
    @gender = gender
    @title = title
    @id = id
  end
end

Remember, we don’t have nil, so we don’t have a default value for any variable - we must assign one explicitly or we’ll get runtime errors. Given the code above, this shouldn’t be a problem, since we assign values to our ivars when they are declared.

If you recall, however, we have two optional values in our Person: title, and id. title is simply optional - a person might not have a title - while id will only be populated if the person has been stored in the database. How can we model this?

Generic optional values?

Scala (a statically-typed functional/OO language that runs on the JVM), “solves” this by creating an Option type that makes explicit the concept of an optional value1. In Ruby, it would look like this:

An optional type
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# The base class that also serves as a factory for instances
class Optional
  # Optional value that has a value
  def self.some(value)
    Some.new(value)
  end

  # Optional value with NO value
  def self.none
    None
  end
end

class Some < Optional
  attr_reader :value
  def initialize(value)
    @value = value
  end

  def exists?
    true
  end
end

class None < Optional
  def self.exists?
    false
  end
end

We can use it like so:

Using our optional type
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dave = Person.new("Dave","1972-01-01",:male,Optional.none,Optional.none)
rudy = Person.new("Rudy","2001-01-01",:male,Optional.some("cat"),Optional.some(42))

class Person
  def salutation
    if title.exists?
      title.value + ' ' + name
    else
      name
    end
  end
end

So, we’ve replaced what would be a call to .nil? in regular Ruby with a call to .exists? in our nil-less Ruby. Is this really any better? We could wrap the logic of “do one thing if there’s a value, do another if there isn’t” into a method on Optional:

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class Some
  def with_value(&block,&_)
    block.call(self.value)
  end
end

class None
  def self.with_value(&_,&block)
    block.call
  end
end

We could then implement salutation like so:

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class Person
  def salutation
    self.title.with_value(
      ->(title) { title + ' ' + name },
      ->        { name }
    )
  end
end

Yech. We might be able to play some syntax games and clean this up, but this is not an improvement. if/else statements are easy to understand and with the magic of nil, the logic is pretty straightforward:

using nil’s falsiness
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class Person
  def salutation
    if self.title
      self.title + ' ' + self.name
    else
      self.title
    end
  end
end

Optional, as we’ve defined it, is just a degenerate implementation of nil that has a terrible API2. It does have the advantage of not being magic - we are required to provide a value for every variable, which is nice - but can we do better?

Solving the problem in front of us

Let’s step back and just try solving the problem in front of us, instead of adding the general concept of optional values. What if we used the type system more explicitly?

Suppose we define Person to be only the required values, i.e. the bare essence of a person in our system, and then create mixins for the optional values. We could make a mixin like Stored act as both a “tag” for an object that is stored in the database, and as the location for related code.

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class Person
  # Every person must have a name, birthdate, and gender.
  attr_reader :name, :birthdate, :gender
  def initialize(name,birthdate,gender)
    @name = name
    @birthdate = birthdate
    @gender = gender
  end
end

module Stored
  attr_reader :id
end

module Titled
  attr_reader :title
end

Note that we don’t make id or title mutable; they are still read-only fields. So, how do they get set? We tightly couple Person with these new modules and set the fields there.

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class Person
  def title=(title)
    @title = title
    self.extend(Titled) # THIS object is now a Titled, but other Person instances are not
  end

  def id=(id)
    @id = id
    self.extend(Stored)
  end
end

Note that if we wanted to maintain total immutability, we would need to jump through a few hoops:

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class Person
  def with_title(title)
    self.dup.tap { |person|
      person.extend(Titled)
      person.instance_variable_set("@title",title)
    }
  end

In either case, we end up with an instance that has mixed in Titled and absolutey has a value for title.

How would this affect our salutation method?

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class Person
  def salutation
    if self.kind_of?(Titled)
      "#{title} #{name}"
    else
      name
    end
  end
end

We’ve replaced a generic check - for nil - with a specific check - for being Titled. This may not seem like an improvement, but I’d argue that it makes our domain a bit richer and more intention-revelaing. It turns an implementation decsion (treating nil as not having a title) into something explicit. And, at the end of the day, if we need logic based on the existence of a value, well, we’re going to need to use if statements.

Or are we?

Before we answer that, it’s worth noting that although Titled is specific to our Person class, Stored is a more generic concept that could be broadly used to explicitly call out records not stored in the database. Imagine an update method like so:

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def update(record)
  raise NotStoredError unless record.kind_of?(Stored)
end

That reads a lot better to me than a nil check3. It also abstracts away the way in which we know that a record is stored, but without requiring a common superclass.

Back to our if statement. We have a business rule based on the existence of a value, so it seems we just have to live with the conditional logic, right? Not exactly. What if both Person and Titled implemented salutation?

Person would use the “default if no title” version, because a raw Person has no title:

Person’s default implementation of salutation
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class Person
  def salutation
    name
  end
end

Once Titled is mixed in, we know that we absolutely have a title, so we override it with the correct logic given a title:

Titled overrides it, since it knows it has a value
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module Titled
  def salutation
    title + ' ' + super
  end
end

Now, this is interesting. We’re using polymorphism and inheritance as a way to avoid if statements. If we’d used nil to represent “no title”, we’d be stuck with conditional logic. The added constraint of programming without nil has forced us to get creative and resulted in a cleaner solution.

We’ve now used the type system to create an explicit description of our domain, and we didn’t need nil. Of course, a type that has a lot of optional values will require a lot of these sorts of modules, and it could get ugly. This might be a good thing.

Now that we can handle optional values in our data structures, what about containers?

Optional values in container classes

I see a lot of code using first or last on an array as a shortcut for checking if the array is empty and, if not, getting the first or last element respectively. Obviously, this would have to stop, but what about so-called “sparse arrays” where some indeces contain nil values? Dealing with this cleanly is not simple given the currently API of Array. Of course, if the language never had nil, you could imagine that Array would have some facility for dealing with this. On idea would be that each accessor method would accept an optional block that would be run if there were no value, so that the caller could provide a default:

Imaginary Array API when we don’t have nil
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list = []
list[0]                               # => raises IndexError
list[0] { |index| "default#{index}" } # => default0
list.first { "default" }              # => default

When we’re talking about containers, however, we’d need to be able to model “there is no value at this location” more explicitly. Since this actually is a generic problem, we can bring back our Optional class to handle it. We could assume that the Array class bakes in the use of Optional, but a) the API would be somewhat inconvienient and b) it doesn’t help us in the real world. What if we created a mix-in that we could use for Array instances that contained optional values?

Mix-in to make it easier to work with Arrays that contain Optional values
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module OptionalValuesArray

  # Set a value directly
  def []=(index,value)
    super[index] = Optional.some(value)
  end

  # clear the value at this index
  def clear_value(index)
    super[index] = Optional.none
  end

  # get the value or, if it's not there, call the block
  def [](index,&block)
    super[index].with_value(
      ->(value) { value },
      ->()      { block.call(index) }
    )
  end

  # iterate over only the values that exist
  def each_value(&block)
    super.each(optional)
      optional.with_value(
        ->(value) { block.call(value) },
        ->()      { }
      )
    end
  end

  # Map only the values that exist
  def map_values(&block)
    [].tap { |new_array|
      self.each_value do |value|
        new_array << value
      end
    }
  end
end

optional_values_allowed = [].extend(OptionalValuesArray)
no_optional_values = []

This API might not be “right”, but we can see that, without nil, we have to be explicit about which arrays can be missing values and which cannot. That makes our code more intention-revealing. If Ruby really didn’t have nil, I would expect the Array class to better “bake-in” this concept so that the API was clean and easy.

Hash, on the other hand, comes built-in with everything we need to avoid nil, namely the fetch method:

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hash = { :foo => :bar, :baz => :quux }
hash.fetch(:foo)                   # => :bar
hash.fetch(:blah)                  # => raises IndexError
hash.fetch(:blah) { |key| "crud" } # => "crud"

By using fetch, we can be very clear about what we want to do. Without a block, we are getting the value for a key that must exist. With a block we indicate that we’re getting a value for a key that is optional…and we must specify the value to use if it’s missing.

An alternative is to specify a block that provides default values, and then use []:

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hash = Hash.new { |key,value| :default_value }
hash[:foo] = :bar
hash[:foo]  # => :bar
hash[:blah] # => :default_value

Another way in which we use nil in a Hash is in the “options hash” pattern where we can parameterize a method call, typically omitting keys where we want to use the default value provided by the API. In this case, we use nil to mean “don’t use the default, but omit the value entirely”.

For example, in Rails 3, we can use respond_with to send an object to the caller in the controller. By default, the HTTP location header is set by examining the type of the object and getting a URL for it. respond_with takes an options hash and, if we wish to avoid setting this header, we must set :location to nil:

Using nil to “unset” an option
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class SomeController
  respond_to :json

  def create
    record = create_record_somehow
    respond_with record, :location => nil
  end
end

Doing this without nil is trickier, and I think it requires a small change in how we design APIs using the options hash. The result, again, will be more intention-revelaing code. Instead of using nil for “don’t set the location header”, we would set an option that indicates that more clearly:

Imagined options for respond_with
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class SomeController
  respond_to :json

  def create
    record = create_record_somehow
    respond_with record, :set_location => false
  end
end

This would even improve the implementation of respond_with as well:

Imagined implementation of respond_with if nil were not an option
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def respond_with(record,options = {})
  options[:location] = options.fetch(:location) { default_location_for(record) }
  if options[:set_location]
    headers['Location'] = options[:location]
  end

  # and whatever else
end

Again, the absence of nil is making our code a bit longer, but much more intention-revealing and explicit.

In reality, though, nil exists and is used in many places. Can we take anything from this to the real world?

Back to reality

First and foremost, I would suggest that you design APIs in a way that nil is not required nor used. Methods that return collections should return an empty version instead of nil. Method parameters should not allow nil to be passed in for any value, and should use an options has for optional values.

Not every API is written this way, and to deal with them, there are a few handy methods provided by Ruby that can help:

  • String() - converts nil to the empty string, and converts any string to itself. Wrap a possibly-nil string in this and you avoid a nil check. (ActiveSupport’s #present? is a way to do this, too, but String() works everywhere in Ruby)
  • Array() - converts nil to an empty array and converts an array to itself. Perfect for dealing with pesky APIs that insist on returning nil instead of an empty array.
  • Hash[Array()] - by combining Hash#[] and Array(), we can convert nil to an empty hash and a hash to itself. Array() will turn a Hash into a two-dimensional array, and Hash#[] will turn a two-dimensional array back into a Hash. Since Array() turns nil into an empty array, Hash[Array(nil)] returns an empty has. Ruby really should include a method named Hash() that does this, but it doesn’t.

Beyond this, null objects are a useful pattern for encapsulating logic of the type “do this if some value is nil”, and the try method in Rails is also very useful.

It’s still interesting to think about a world without nil. Without it, we can still handle the absence of values in objects, as well as containers, and our code is more intentional-revelaing. Why do we need nil?


  1. The JVM still allows null and so does Scala, so Option only provides a way to express optional types more clearly; null is still there and is the default value of variables that aren’t given an initial value.
  2. It’s worth pointing out that in Scala, Option is a lot more useful, because null has no such magical properties on the JVM like it does in Ruby.
  3. I realize that Active Record encapsulates this concept in new_record?, but a) we’re in an imaginary domain without Active Record and b) that Active Record encapsulates the nil check gives more credence that doing so is a good idea in general.