To be clear, if you do !ismissing(foo), this just calls ismissing(foo) and then negates the result with !
If you use !ismissing with no function call (...), then Julia uses the higher-order function @jling mentioned, which yields a function that is the antonym of ismissing. This is useful for passing to other functions, e.g. for calling filter(!ismissing, somearray) to extract the non-missing elements.