Am Sonntag 28 Februar 2010 02:08:18 schrieb zaxis:
Then can i change it to : case timeout of Just str -> do [(t, _)] <- reads str addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) return () _ -> return ()
Sincerely!
No. The "| [(t,_)] <- reads str" in case timeout of Just str | [(t,_)] <- reads str -> ... is a "pattern guard", not a monadic bind (and where "p <- reads str" is a monadic bind, it's in the list monad). You can change it to case timeout of Just str -> case reads str of [(t,_)] -> addtimeout (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) _ -> return () _ -> return () but why would you?
Brandon S. Allbery KF8NH wrote:
On Feb 27, 2010, at 04:07 , zaxis wrote:
xxxMain = do timeout <- getEnv "xxx_TIMEOUT" case timeout of Just str | [(t, _)] <- reads str -> do addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) return () _ -> return () .......
What does the `|` mean in "Just str | [(t, _)] <- reads str" ? Is it a logical `or` ?
It's a guard. Same as with function definitions (in fact, function definitions of that form are converted to case expressions).
-- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH