I stumbled across a problem with IO and strictness that I could fix, but I can't understand why the
fix works. I've compressed it down into a program which simply computes the number of lines in a
file. Here is a version that doesn't work:
module Main where
import System.IO
import System.Environment
process_file :: FilePath -> IO ()
process_file filename =
do h <- openFile filename ReadMode
c <- hGetContents h
let cs = unlines $ lines c
hClose h
putStrLn $ show $ length cs
main :: IO ()
main = do args <- getArgs
process_file (args !! 0)
This will return a length of 0 lines for any input file. Obviously, the "let" is not being
evaluated strictly (nor would we expect it to be), so that when the evaluation is requested, the
file is already closed and the length of the list of lines is 0 (though I might have expected an
error). I then tried this:
process_file :: FilePath -> IO ()
process_file filename =
do h <- openFile filename ReadMode
c <- hGetContents h
let cs = id $! lines c -- try to strictly evaluate the let binding
hClose h
putStrLn $ show $ length cs
which also failed exactly as the previous version did (i.e. always returning 0). Then I gave up on
"let" and did this:
process_file :: FilePath -> IO ()
process_file filename =
do h <- openFile filename ReadMode
c <- hGetContents h
cs <- return $! lines c
hClose h
putStrLn $ show $ length cs
This works. However, I don't understand why this version works and the previous version doesn't.
Can anyone walk me through the evaluation? Also, is there a way to make "let" strict?
TIA,
Mike
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