while

A while executes its body as long as its condition is truthy.

while some_condition
  do_this
end

The condition is first tested and, if truthy, the body is executed. That is, the body might never be executed.

Similar to an if, if a while's condition is a variable, the variable is guaranteed to not be nil inside the body. If the condition is an var.is_a?(Type) test, var is guaranteed to be of type Type inside the body. And if the condition is a var.responds_to?(:method), var is guaranteed to be of a type that responds to that method.

The type of a variable after a while depends on the type it had before the while and the type it had before leaving the while's body:

a = 1
while some_condition
  # a : Int32 | String
  a = "hello"
  # a : String
  a.size
end
# a : Int32 | String

Checking the condition at the end of a loop

If you need to execute the body at least once and then check for a breaking condition, you can do this:

while true
  do_something
  break if some_condition
end

Or use loop, found in the standard library:

loop do
  do_something
  break if some_condition
end

As an expression

The value of a while is the value of the break expression that exits the while's body:

a = 0
x = while a < 5
  a += 1
  break "four" if a == 4
  break "three" if a == 3
end
x # => "three"

If the while loop ends normally (because its condition became false), the value is nil:

x = while 1 > 2
  break 3
end
x # => nil

break expressions with no arguments also return nil:

x = while 2 > 1
  break
end
x # => nil

break expressions with multiple arguments are packed into Tuple instances:

x = while 2 > 1
  break 3, 4
end
x         # => {3, 4}
typeof(x) # => Tuple(Int32, Int32)

The type of a while is the union of the types of all break expressions in the body, plus Nil because the condition can fail:

x = while 1 > 2
  if rand < 0.5
    break 3
  else
    break '4'
  end
end
typeof(x) # => (Char | Int32 | Nil)

However, if the condition is exactly the true literal, then its effect is excluded from the return value and return type:

x = while true
  break 1
end
x         # => 1
typeof(x) # => Int32

In particular, a while true expression with no breaks has the NoReturn type, since the loop can never be exited in the same scope:

x = while true
  puts "yes"
end
x         # unreachable
typeof(x) # => NoReturn