build failures when hiding non-visible imports

[Simon Peyton-Jones]

OK we're doing this for 7.6.  See http://hackage.haskell.org/trac/ghc/ticket/7167

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-
| haskell-users-bounces at haskell.org] On Behalf Of Conrad Parker
| Sent: 21 August 2012 01:02
| To: John Lato
| Cc: glasgow-haskell-users at haskell.org
| Subject: Re: build failures when hiding non-visible imports
| 
| On 21 August 2012 07:36, John Lato <jwlato at gmail.com> wrote:
| >> From: Brandon Allbery <allbery.b at gmail.com>
| >>
| >> On Sat, Aug 18, 2012 at 9:10 PM, Carter Schonwald <
| >> carter.schonwald at gmail.com> wrote:
| >>
| >>> meaning: flags for treating it as a warning vs as an error?
| >>> (pardon, i'm over thinking ambiguity in phrasing).
| >>> if thats the desired difference, that sounds good to me!
| >>>
| >>
| >> I would expect it means that, having demoted it to a warning, we
| >> would have -fwarn-hiding-no-target / -fno-warn-hiding-no-target (or
| >> whatever we call
| >> it) as with all other warnings.
| >>
| >> For warning vs. error, it seems to me that should be more general:
| >> perhaps taking any of the -f[no-]warn-* options and replacing "warn"
| with "err".
| >
| > Yes.  To be concrete, this is what I would like to see.
| >
| > In a statement of the form:
| >
| >   import Module hiding (x)
| > where Module doesn't export x, ghc should report a warning instead of
| > an error
| >
| > This warning would be enabled/disabled by the usual flags (I like
| > -fwarn-unused-import-hiding, but -fwarn-hiding-no-target is good
| too).
| >
| > The warning would be on by default.
| >
| > If a user wants this to be an error, I think -Werror should be
| > sufficient.  I am unable to think of any case where hiding a
| > non-visible symbol would lead to errors on its own, and any errors
| > likely to occur in tandem with this issue already have their own,
| more
| > helpful, error conditions (e.g. symbols not in scope, symbols in a
| > qualified import list not visible).
| >
| > I agree with Ganesh's point that it would be beneficial to have this
| > available for ghc-7.6.1 if possible.
| 
| +1
| 
| and it must be a warning (not error) by default, or else we will need
| to tell everyone to use "cabal install --ghc-option=-fwarn-unused-
| import-hiding" to install lots of packages on hackage (currently
| including things like HTTP and gtk2hs-buildtools, which are pulled in
| by many packages).
| 
| Also, if this remains an error by default then it will become a
| sensible coding style to simply avoid using import hiding, to avoid
| build errors that will occur when some other library removes an
| interface (that you have explicitly marked as unused ...)
| 
| Conrad.
| 
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