In "tales from the tourbus" https://www.youtube.com/watch?v=jKWRZBjC95w Mike Judge states that Purple Rain was released during the 'heyday of straight funk, 1984'.
What does he mean by 'straight funk' in this context?
"Straight" is an adjective with many different meanings, but in a context like this, it means "unmixed with other things" (as in "straight whiskey"). Therefore, "straight funk" is funk as its own independent genre, as opposed to "jazz funk," "pop funk", "G-funk", psychedelic funk, or "funk rock."
Given the referenced era, he's probably thinking about artists/groups such as Zapp, Rick James or Cameo --although those are arguably just representatives of another yet another sub-genre ("electro-funk").