Arthur van Leeuwen
Ah, but this is exactly where lazyness wins bigtime: a smart implementation of readFile will lazily read the actual file for as far as needed. Thus, reading with readFile will not read the entire file into memory at once. What will happen is that writeFile starts writing, and upon discovery of needing the value of urls it will then start reading. Any value already written in this case obviously turns into garbage and will be garbage collected. I would be slightly surprised if this code uses more than constant memory.
so i need to read the file line by line but stunned by the loop in IO Monad: --- main = do h1 <- openFile "url1.txt" ReadMode h2 <- openFile "url2.txt" ReadMode line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2 : [] -- i don't howto do hClose h1 hClose h2 -- any ideas? thank you all.
Yes. You need to split the lines
line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2: []
into a separate function that will then recurse over the file.
ah, readFile is lazy,that's great! my hat's off to the haskell design/implement teams for their robust and elegant work. thank all you guys! BTW, sorry to Doei Arthur for my reply to you by mistake. -- Sun Yi Ming