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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.

A while's type is always Nil.

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

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https://crystal-lang.org/docs/syntax_and_semantics/while.html