if !obj.nil?

#crystal #todayilearned #ruby #javascript

When writing code that needs to handle nilable objects the resulting code can be sometimes verbose and difficult to read/follow.

In this post we will start with an example (presenting the problem), followed by a (not ideal) solution and finally present different ways that different languages (especially Crystal) use to handle nilable objects.

Let's use the following Crystal code to illustrate:

class IntWrapper
  getter inner_value : Int32?

  def initialize(@inner_value = nil)
  end
end

# returns an IntWrapper only if parameter is positive else it returns `nil`
def create_if_positive(n : Int32): IntWrapper?
  IntWrapper.new(n) if n > 0
  # else it will return `nil`
end

number = create_if_positive(40)

puts number.inner_value + 2

Notes:

The method create_if_positive does not make much sense but for the purpose of the example.
This is not an example of good design (although maybe it's an example of bad design) 🙃

The compiler will return:

$ Error: undefined method 'inner_value' for Nil (compile-time type is (IntWrapper | Nil))

And the compiler is right: create_if_positive may return nil as we specified in the return type IntWrapper?

So we need to check if the returned object is nil:

...

if number
  puts number.inner_value + 2
else
  puts "nil branch"
end

And that's it! ... wait ... what? ... the compiler is saying:

$ Error: undefined method '+' for Nil (compile-time type is (Int32 | Nil))

oooh right! Now number.inner_value can be also nil (remember getter inner_value : Int32?)
Let's fix it:

...

if !number.nil? && !number.inner_value.nil?
  puts number.inner_value + 2
else
  puts "nil branch"
end

Now it's fixed ... wait ...

Error: undefined method '+' for Nil (compile-time type is (Int32 | Nil))

And also, we need to tell the compiler that number.inner_value cannot be nil inside the if branch because we already check on that. For that we use the Object#not_nil! method:

...

if !number.inner_value? && !number.inner_value.nil?
  puts number.inner_value.not_nil! + 2
else
  puts "nil branch"
end

Well, it's working but I would really want to write the same thing in a more concise and clear way.
For example, I like the following idiom when dealing with nil and if condition:

if a = obj # define `a` only if `obj` is not `nil`
  puts a.inspect # => the compiler knows that `a` is not `nil`!
end

So let's try to go in that direction. Maybe something like this:

if number != nil && (value = number.not_nil!.inner_value)
  puts value + 2
else
  puts "nil branch"
end

Again, it's working but I think we can do better (I still don't like telling the compiler that number is not nil).

What can we do? 🤔

Safe Navigation ⛵️

At this point Ruby's Lonely Operator (aka Safe Navigation Operator) came to my mind:

class IntWrapper
  @inner_value = nil

  def initialize(inner_value = nil)
    @inner_value = inner_value
  end

  def inner_value
    @inner_value
  end
end

# 1. `number` is `nil` (using if)
number = nil

if number && number.inner_value # using if
  puts number.inner_value + 2
else
  puts "nil branch"
end

# 2. `number` is `nil`
number = nil

value = number&.inner_value
puts value + 2 unless value.nil?  # nothing is printed

# 3. `number` is not `nil`. `inner_value` is `nil`
number = IntWrapper.new()

value = number&.inner_value
puts value + 2 unless value.nil? # nothing is printed 

# 4. `number` is not `nil`. `inner_value` is not `nil`
number = IntWrapper.new(40)

value = number&.inner_value
puts value + 2  unless value.nil? # => "42"

Also JavaScript's Optional chaining:

// 0. Error
let number = null;
let value = number.inner_value; // Error: Cannot read properties of null (reading 'inner_value')

// 1. number is null
let number = null
let value = number?.inner_value;
console.log(value ? value + 2 : "value is null"); // > "value is null"

// 2. `number` is not `null`. `inner_value` is `null` 
let number = {
  inner_value: null
}
let value = number?.inner_value;
console.log(value ? value + 2 : "value is null"); // > "value is null"

// 3. `number` is not `null`. `inner_value` is not `null` 
let number = {
  inner_value: 40
}
let value = number?.inner_value;
console.log(value ? value + 2 : "value is null"); // > 42

Do we have some special syntax in Crystal?

The answer is no 😅
But don't despair! There is something really cool. It's not syntax but a method: Object#try

So we don't need to learn some new syntax but just know how this method works. It's super simple:

Yields self. Nil overrides this method and doesn't yield.

This means that:

nil.try { |obj| 
  # this block does not get called!
  puts obj.size 
}

and a "not-nil" object will yield self meaning:

"Hello!!".try { |obj|
  # the block gets called with the object itself as the parameter.
  puts obj.size # => 7
}

or simpler using short one-parameter syntax (not to be confused with the previous seen Ruby's Lonely operator!😉):

puts nil.try &.size # => nil

puts "Hello!!".try &.size # => 7

So in our example we can write:

if value = number.try &.inner_value
  puts value + 2
else
  puts "nil branch"
end

Great! It's easy to read, right? number is trying to number.inner_value and if number is not nil then value will be assigned with the value of inner_value (furthermore, in the case of inner_value being nil then the if-guard fails 🤓🎉)

The complete example (3 in 1):

number is nil
number is not nil and number.inner_value is nil
number is not nil and number.inner_value is not nil

class IntWrapper
  getter inner_value : Int32?

  def initialize(@inner_value = nil)
  end
end

def create_if_positive(n : Int32): IntWrapper?
  IntWrapper.new(n) if n > 0
  # else it will return `nil`
end

# 1. `number` is nil

number = create_if_positive(-1)

if value = number.try &.inner_value # the condition fails
  puts value + 2
else
  puts "nil branch" # => "nil branch"
end

# 2. `number` is not `nil` and `number.inner_value` is `nil`

number = IntWrapper.new # `inner_value` will be `nil` 

if value = number.try &.inner_value # the condition fails
  puts value + 2
else
  puts "nil branch" # => "nil branch"
end

# 3. `number` is not `nil` and `number.inner_value` is not `nil`

number = create_if_positive(40)

if value = number.try &.inner_value
  puts value + 2 # => 42
else
  puts "nil branch"
end