Re: [Haskell] Probably a trivial thing for people knowing Haskell

21 Oct 2008

      Udo Stenzel <u.stenzel@web.de> writes:
...
...
Friedrich wrote:
...
Ok to  be more concrete is the laziness "hidden" here?
check_line line sum count =  
   let match = matchRegex regexp line
       in case match of
              Just strs -> (sum + read (head strs) :: Integer, count + 1)
              Nothing -> (sum, count)
Yes, part of it.  To see why, put yourself into the role of an evaluator
for your program.  An application of check_line will not be evaluated
until necessary, and it becomes necessary only if the result is bound to
a pattern (and that binding is needed for some reason).  At that point,
enough has to be evaluated to determine whether the result is actually a
pair or bottom.
So what will you do?  The body of check_line is a case expression, so
you need to sufficiently evaluate its scrutinee.  You evaluate enough of
matchRegex to see whether the result is Nothing or Just.  Let's say it's
Just.  So you descent into the Just branch, and you see the result is a
pair (and not bottom).  The elements of the pair have not been
evaluated, there was no need to.  Also, the arguments to check_line have
not been evaluated, except for line.
You need to force the evaluation of the elements of the result pair
whenever the pair itself is demanded, for example:
...
...
check_line line sum count =  
   let match = matchRegex regexp line
       in case match of
              Just strs -> ((,) $! (sum + read (head strs) :: Integer)) $! count + 1
              Nothing -> ((,) $! sum) $! count)
(The associativity of ($!) is inconvenient here.  I want
left-associative ($!).  Actually, a strict pair type would be even more
convenient here.)
On recent GHC with bang-patterns, this short-cut works, too.  It's not
quite equivalent, because it will create unevaluated thunks, though they
won't pile up:
...
...
check_line line !sum !count =  
   let match = matchRegex regexp line
       in case match of
              Just strs -> (sum + read (head strs) :: Integer, count + 1)
              Nothing -> (sum, count)
Ok, I followed the suggestions. Now I have the following code:

module Main where
import System
import System.IO
import System.Directory
import System.IO.Error
import Text.Regex
import Control.Monad

regexp = mkRegex ("([0-9]+) Windows ex")

main = do
       files <- show_dir "[0-9].*"
       (sum,count) <- run_on_all_files (0,0) files
       let dd = (fromIntegral (sum::Integer))/ (fromIntegral (count::Int))
           in
            putStr("Download = " ++ show sum ++ " in " ++ show count ++ " days are " ++ show dd ++ " downloads/day\n") 

run_on_all_files (a,b) [] = return (a,b)
run_on_all_files (a,b) (x:xs) = do (s,c) <- run_on(a,b) x
                                   run_on_all_files (s,c) xs

run_on (a,b) file_name = do
    handle <- openFile file_name ReadMode
    (sum,count) <- for_each_line (a,b) handle
    hClose handle
    return ((sum,count))

for_each_line (sum, count) handle = do
                       l <- try (hGetLine handle)
                       case l of
                              Left err 
                                  | isEOFError err -> return(sum,count)
                                  | otherwise -> ioError err
                              Right line  -> do 
                                             let (nsum, ncount) = count_downloads line (sum, count) 
                                             for_each_line (nsum,ncount) handle 

count_downloads line (!sum, !count) =  
    let match = matchRegex regexp line
        in case match of
               Just strs -> (sum + read (head strs) :: Integer, count + 1)
               Nothing -> (sum, count)

show_dir regmatch = do   
                    files <- getDirectoryContents "."
                    let reg = mkRegex regmatch in
                              return(filter (\file_name -> let fm = matchRegex reg file_name
                                      in case fm of
                                      Just strs -> True
                                      Nothing -> False) files)

But it still  sucks  memor as wild and more or less crashes the
system. So why's that  than?

Regards
Friedrich