I've been playing with Text.Parsers.Frisby to see how it stacks against other options and, while it's been great so far, I am finding that I can't encode a grammar where what's acceptable depends on what's already been parsed in some nontrivial way. To take a simple example, imagine a grammar where the only lower-case letters that are acceptable are those where their upper-case equivalent occurred earlier in the text. In Parsec I'd code this sort of thing as, nextChar = do allowed <- getState char <- oneOf $ ['A'..'Z'] ++ allowed updateState (union [toLower char]) return char test = runParser (many1 nextChar) [] "" Is this supposed to not be possible in Frisby, or (quite likely) am I missing something that allows me to? I've thought about things like trying to fmap further calls to apply runPeg to rest, but I've not figured out a trick that will actually work. -- Mark