On May 19, 2006, at 6:16 PM, Henning Thielemann wrote:
let ~(ts, strm') = idX strm ~(us, strm'') = idX strm'
I seem to have found a partial solution to the problem. It's rather ugly, however, and I think there should be a better way. The original definition for one of the clauses was like this: idX (StartEvent a : strm) = let (ts, strm') = idX strm (us, strm'') = idX strm' in (StartEvent a [] : ts ++ EndEvent a : us, strm'') The intention was to thread strm through the recursive calls and free the items in strm as soon as they are processed. However, with this definition I needed about 8Mb of heap for a 1Mb input file. Since I suspected that delayed evaluation of us and strm'' keeps the pointer to strm around for too long, the natural solution seems to be forcing their evaluation. So I tried: idX (StartEvent a : strm) = case idX strm of (ts, strm') -> case idX strm' of (us, strm'') -> (StartEvent a [] : ts ++ EndEvent a : us, strm'') which made things worse. The heap size increased a bit more and I get a typical "bell" shaped heap profile suggesting idX cannot output anything before processing the whole input. So I have to allow idX to output something while consuming the input. What eventually worked was this messy mixture of let and case: idX (StartEvent a : strm) = let (xs, rest) = case idX strm of (ts, strm') -> let (ws, rest) = case idX strm' of (us, strm'') -> (us, strm'') in (ts ++ EndEvent a : ws, rest) in (StartEvent a [] : xs, rest) The heap residency reduced from about 8MB to around 80Kb. This is rather tricky, however. The following "nicer looking" version has a heap profile as bad as the worse case. idX (StartEvent a : strm) = let (ts,us,strm'') = case idXsp strm of (ts, strm') -> case idXsp strm' of (us, strm'') -> (ts, us, strm'') in (StartEvent a [] : ts ++ EndEvent a : us, strm'') And I cannot quite explain why (Anyone?). Is there a more structured solution? Can I leave the hard work to the compiler? sincerely, Shin-Cheng Mu