Henning Thielemann
I don't think that it is in general possible to use the same parser for lazy and strict parsing, just because of the handling of parser failure.
Polyparse demonstrates that you can mix-and-match lazy parsers with strict parsers in the different parts of a grammar (by choosing whether to use applicative or monadic style). You can also switch between lazy or strict interpretations of the applicative parts of your grammar (by changing the import that decides which version of the parser primitives is in scope).
I also used polyparse for lazy parsing, but I found it unintuitive how to make a parser lazy.
It can certainly be tricky, and requires a certain amount of experimentation. I think the difficulties are mainly due to the mix of lazy (applicative) and strict (monadic) styles in different non-terminals. A parser that you intend to be lazy, may turn out to be stricter than you hope, because of the strictness of another parser that it depends upon. Regards, Malcolm