Trifecta is best used via the parsers library, which is fairly well documented. This also (in theory) allows you to switch which parser you use between trifecta, parsec, attoparsec and even ReadP. I say in theory because the the semantics of these libraries differ.
I recommend the much smaller project by Merijn, lambda-except [1]. I learned Trifecta from it, and it’s really not all that difficult, just embarassingly poorly documented. I then wrote the parser for my STGi project based on Trifecta, so that may also be worth a look.