> Attoparsec and megaparsec don't backtrack.

There are two notion of backtracking at play here. One is backtracking out of a failing first branch of <|> even though it's consumed some input. The megaparsec authors consider this a form of backtracking (see https://hackage.haskell.org/package/megaparsec-9.0.1/docs/Te...).

The other is backtracking from a failure in the second argument to <> or <*>.

I agree that none of the libraries being considered do the latter. But the effect on memory use wouldn't just be slightly inefficient, it would mean keeping the entire input following a <|> in memory until the whole thing completes. Do you think this is the best way to do things in general, or just for specialized situations like parsing things with a fixed size such as config files?