Is this DFW line a run on sentence? [closed]

I don't think it's exactly a run-on sentence by the standard that our 6th-grade teachers taught us - but it is exactly the sort of sentence we're generally warned not to write, and it's actually a pretty good example of WHY we are warned not to write them. You can get away with this when you get your million-dollar publishing contract, but for the rest of us it's best to follow the rules.

Why did DFW write it this way? Well, he's writing in the voice of a long-term substance abuser - and if you've ever dealt with long-term substance abusers, you'll know that many of them are conversationally challenged in just this way: they ramble. They may have something to say that's worth hearing (depending on how intelligent they were to begin with, and just how many brain cells they've killed along the way), but you are definitely going to have to invest some time and effort into listening and figuring out what the point of it all was. Like DFW's novels, some people will find the process worthwhile, and many others won't.

In the passage you cited, Gately thinks Joelle's problem with AA is atheism, but it's actually grammar: the addicts telling their stories say "I'm here but for the grace of God" when they should be saying "I'm here by the grace of God." It's a very small, slight joke - and DFW tricked me into re-reading that monster of a sentence until I got it. Nice one!

In order to be exactly what's usually classified as a run on sentence, it should be possible to place a period somewhere and thereby obtain two complete sentences. As far as I can tell (though the sentence is hard to parse), there's no such location, just a long string of pieces with various connectives linking them together.

Sentences like that would, however, be terrible writing under most circumstances, even if grammatically correct. As always, good writing and formally correct grammar are correlated, but one can often have one without the other.

In this case, however, the author has written the sentence this way intentionally to capture the experience of talking to the speaker, who speaks in a similar fashion, endlessly stringing new clauses on without giving the listener/reader a chance to pause and make sense of what's been said.