I like Cal Newport's work, but I'm not sure he's being fully charitable to flow here; I thought that some degree of challenge (but not so hard as to be making n

I like Cal Newport's work, but I'm not sure he's being fully charitable to flow here; I thought that some degree of challenge (but not so hard as to be making no progress) is generally considered a prerequisite to flow by its proponents. A more interesting question to my mind is: what does deliberate practice look like to programmers?