It would be specifically handled by the return type from validateData and the argument to ProcessData. It sounds clunky, but with an algebraic data type it would be fairly clean - just another pattern to match in ProcessData.

For a simple example like this, that might be the cleanest solution. For anything more complicated (like recursive descent parsing), a processing chain like this is an ideal application for a monad, where the binding between functions would be customized to avoid calling ProcessData altogether.

Two ways: (1) (avoid) something similar to exceptions; (2) (much better) your datatype has an 'invalid' value. See Maybe in haskell, or Option in Scala (I think that last one's right...).

With error handling in Haskell your code would look like

   getData = fetchDataFromSource =<< validateData =<< ProcessData

By the way, the original getData with (.) is backwards. It should be:

  getData = ProcessData . validateData . fetchDataFromSource