It seems that part of Parsec's optimization is that it is actually limited to an LL(1) grammar (whatever that means) and it's <|> ('alt) will fail immediately if the first parser ever consumed anything at all. Not sure how that translates to treeparse.
LL(1) grammars only require one token of look-ahead to parse.
Parsec does not strictly require this, it can handle infinite look-ahead grammars. However, for good performance, it is best to use LL(1) grammars -- so there will be no backtracking required.
When using Parsec, I have often been surprised by the quick-failing behavior of <|> that you mentioned. Thus, I did not duplicate it in treeparse.
