if !

The ! operator returns a Bool that results from negating the truthiness of a value.

When used in an if in conjunction with a variable, is_a?, responds_to? or nil? the compiler will restrict the types accordingly:

a = some_condition ? nil : 3
if !a
  # here a is Nil because a is falsey in this branch
else
  # here a is Int32, because a is truthy in this branch
end

b = some_condition ? 1 : "x"
if !b.is_a?(Int32)
  # here b is String because it's not an Int32
end

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https://crystal-lang.org/reference/syntax_and_semantics/not.html