closing file handles
Mon, 10 Sep 2001 08:44:08 +1200
Hal Daume writes:
| I'm sorry, I had a small error in my code. It should have been:
| >go filelist = mapM go' filelist -- no "do return"
| > where go' f = do h <- openFile f ReadMode
| > text <- hGetContents h
| > let c = head $! text
| > return (c `seq` (hClose h `seq` c))
I don't have a handy installation of ghc to check this on, but here's
a speculative attempt.
= do h <- openFile f ReadMode
text <- hGetContents h
case text of
(c:_) -> do hClose h
This way, the pattern match on `text' happens before the innermost
`do', which in turn closes the handle before letting the rest of the
You may be able to get a similar effect with one more strictness
return $! (c `seq` hClose h `seq` c)
but that's not quite as readable. BTW I'm not sure that the
strictness operator in `head $! text' makes any difference. It
ensures that when the expression's evaluated, `text' is reduced to
WHNF before `head' is applied to it. But then the very next thing
which happens is a pattern match (head (x:_) = x) which would force
that reduction anyway.
Hope that (isn't mistake-ridden and) helps.