Perhaps Heraclitus lived before Parmenides, perhaps he lived after, perhaps he lived at the same time. Whichever way, his sayings cry out to be read in their own right, as a radically anti-materialist project unlike anything previously known. They bitterly resist the attempt to package them along with the pre-Parmenidean thinkers; they flourish in a situation in which we are able to juxtapose them with alternatives, such as Parmenides's world view, for which they may indeed have been a foil.
Presocratic Philosophy: A Very Short Introduction (2004) - Ch. 5 : Heraclitus