On Sat, 2 Feb 2008, Antoine Latter wrote:
To expand on this point, side-effect instances of Stream don't play nice with the backtracking in Text.Parsec.Prim.try:
import Text.Parsec import Text.Parsec.Prim import System.IO import Control.Monad
type Parser a = (Stream s m Char) => ParsecT s u m a
This particular instance was suggested by Derek.
<snip> We've been kicking this around on IRC a little, it took me a moment to remember the problem. Basically the problem isn't parsec/try playing badly with the stream - it's the stream playing badly with parsec/try. You need to keep track of where in the file you are and either cache it in an IORef or use seek to jump about appropriately. At the moment, Derek's instance doesn't know where it is in the stream so Parsec's backtracking is invisible to the underlying monad. That caching is the source of the potential leak I mentioned to you, because Parsec doesn't currently tell the stream when it's committed up to a given point and anything preceding it can be dropped safely. We'll tell the list a nice story about this over the coming week. For now, it's nearly 3am here and I should get to bed! -- flippa@flippac.org The task of the academic is not to scale great intellectual mountains, but to flatten them.