It depends on what you mean by "with Hack". You cannot do this in Hack code -- we disallow the dynamic features of the language you need to do this; they are im

It depends on what you mean by "with Hack". You cannot do this in Hack code -- we disallow the dynamic features of the language you need to do this; they are impossible to statically typecheck and verify. However, Hack code iteroperates seamlessly with PHP code, so there's nothing stopping you from having some PHP files in your codebase that do anything that standard PHP can do.