if var

If a variable is the condition of an if, inside the then branch the variable will be considered as not having the Nil type:

a = some_condition ? nil : 3
# a is Int32 or Nil

if a
  # Since the only way to get here is if a is truthy,
  # a can't be nil. So here a is Int32.

This also applies when a variable is assigned in an if's condition:

if a = some_expression
  # here a is not nil

This logic also applies if there are ands (&&) in the condition:

if a && b
  # here both a and b are guaranteed not to be Nil

Here, the right-hand side of the && expression is also guaranteed to have a as not Nil.

Of course, reassigning a variable inside the then branch makes that variable have a new type based on the expression assigned.

The above logic doesn’t work with instance variables or class variables:

if @a
  # here @a can be nil

This is because any method call could potentially affect that instance variable, rendering it nil. Another reason is that another thread could change that instance variable after checking the condition.

To do something with @a only when it is not nil you have two options:

# First option: assign it to a variable
if a = @a
  # here a can't be nil

# Second option: use `Object#try` found in the standard library
@a.try do |a|
  # here a can't be nil

That logic also doesn't work with proc and method calls, including getters and properties, because nilable (or, more generally, union-typed) procs and methods aren't guaranteed to return the same more-specific type on two successive calls.

if method # first call to a method that can return Int32 or Nil
          # here we know that the first call did not return Nil
  method  # second call can still return Int32 or Nil

The techniques described above for instance variables will also work for proc and method calls.

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