Immanuel Litzroth schrieb:
Anyway, there is one more problem I have related to exceptions that is about forcing strictness. It relates to option parsing. I have an option -t that says which track from a file of tracks to print. So I go
data Flags = TrackNumber !Int deriving(Read, Show, Eq)
makeTrackNumber :: String -> Flags makeTrackNumber str = TrackNumber $ read str
options = [GetOpt.Option ['t'] ["tracknumber"] (GetOpt.ReqArg makeTrackNumber "tracknumber") "number of track to show"]
Now my main goes main = do args <- getArgs opts <- evaluate $ GetOpt.getOpt GetOpt.RequireOrder options args print "done getting the opts" case opts of ...
which of course first prints "done getting opts" and then throws an exception if I give it a flag -t abc.
'evaluate' is strange here. Is it ok to put the print after the case ? But I have probably still not understood the problem. Let me point to two other issues: An elegant way to use GetOpt is: http://www.haskell.org/haskellwiki/High-level_option_handling_with_GetOpt I find it bad style to hide the exceptions, that can be raised. The type of an action should reflect what exceptions are to be expected. You can achieve this with Control.Monad.Exception.Synchronous from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/explicit-exceptio... or Control.Monad.Error from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/transformers For an application, using these techniques, see: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/equal-files