Andrew W. answered 01/16/24
Oxford Graduate - Math, Writing, and History Tutor - ACT/GRE Prep
'Purple Prose' refers to writing that is ornate, flowery, indulgent, to the point of being difficult to understand or masking the main idea. As Katerina explains in her comment, it is always bad.
However, it's true that many literary giants use long and complex sentences and flowery diction. Some authors even intentionally write sentences that are grammatically incorrect. What distinguishes this from purple prose?
The answer lies both in skill and intent. Some authors are particularly gifted at crafting sentences that are every bit as convoluted as purple prose but, through clever structuring, rhythm, or whatever other artful magic, remains perfectly clear. Perhaps this is what you mean by 'the novelistic version of monologues or poetry.' For the vast majority of writers, including myself, attempts to write ln this style will just result in purple prose. That said, many classic authors would indeed be criticized today for purple prose. There are plenty of authors whose works are known for reasons other than their beautiful writing. If you want to replicate this style, the best way to start is learning to differentiate the purple prose from the good flowery prose.
Other authors craft complex, drawn-out sentences to fulfill a specific artistic intent. For instance, stream-of-consciousness authors may use such sentences to suit a certain voice. Some authors deliberately use confusing sentences to highlight a certain emotion rather than paint a clear picture of a scene, just as Impressionist painters used rougher brushstrokes to create more emotive paintings. Again, I leave a warning to writing students here: trying to write like James Joyce is akin to trying to paint like Van Gogh. It may look easy or lazy, but try and you will see--it is not.