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Hack runs in three different modes. You got strict, partial and decl. Partial is the default mode, which allows you to use PHP code. When you are using the strict mode you have to use type annotations on all your class variables, arguments and function return types. The decl mode is far more forgiving. You may do what ever you want and the type checker will not complain. It should not be used without discretion…

There is also an unsafe declaration that you may use when you want to disable the type checker. Read more about the unsafe and why decl mode exists on the blog.