New submission from Simon Marlow <[EMAIL PROTECTED]>: David Roundy wrote: > I'm cc'ing this to the bug tracker to create a bug for the feature request > described below, which came up from an idea of Juliusz... > > On Fri, Jan 12, 2007 at 01:19:36AM +0100, Josef Svenningsson wrote: > >>On 1/11/07, David Roundy <[EMAIL PROTECTED]> wrote: >> >>>I think we'd really like to apply it not to the method, but to the instance >>>itself. So that use of the Show instance of Control.Exception.Exception >>>would be deprecated. This'd be more severe, since there might be classes >>>that depend on a show instance, but don't actually use it. >>> >>>Another possibility for this particular problem would be to be able to >>>rewrite the show instance. If we could just remove the stupid "User >>>error:" text from the show instance, I don't think it'd be a bug to use it >>>anymore. >> >>I did a little testing and it turns out that you can actually achieve >>what you want. Just write another Show instance for C.E.Exception and >>import it wherever there is a possibility that a bad show might be >>lurking. GHC will only complain about duplicate instances if you >>actually use a method from the Show class with the particular type >>which has several instances. So GHC is doing exactly what you want! > > > Cool! (I would have expected it to cause trouble even when we don't use > show.) I think this'd be a great little cleanup. We could put the import > in impossible.h, which is #included almost everywhere. :) > > For the benefit of the bug tracker, the idea is that we want to avoid using > the Show instance of Control.Exception.Exception, which has the > disadvantage of sometimes displaying "User error: xxx" when we failed with > code such as error "xxx". This isn't the user's fault, and we should never > use this show instance (except if we've already checked that the exception > is not a UserException or whatever it's called). Adding this code to > impossible.h will probably also require fixing a few (bad) uses of show.
I suggest not using userError, use a more descriptive exception type with dynamic exceptions (throwDyn, catchDyn). This is what we do in GHC: http://darcs.haskell.org/ghc/compiler/utils/Panic.lhs To avoid the use of IO.bracket, there are a couple of options: - Use -hide-package haskell98. This probably works in 6.4.x. No doubt you'll have to change a lot of code to import the base-package version of a lot of haskell98 modules, but you get to avoid a few of these legacy issues in one go. - the other way is to have a local 'IO' module that will override the haskell98 one. This only works with GHC 6.6, however. - or just search for 'import IO' and replace them all with 'import System.IO'. Cheers, Simon ---------- messages: 1407 nosy: EricKow, beschmi, droundy, simonmarhaskell, tommy status: unread title: GHC and flagging use of a given constant ____________________________________ Darcs issue tracker <[EMAIL PROTECTED]> <http://bugs.darcs.net/issue391> ____________________________________ _______________________________________________ darcs-devel mailing list darcs-devel@darcs.net http://www.abridgegame.org/cgi-bin/mailman/listinfo/darcs-devel
