un-used record wildcards

[Neil Mitchell]

Hi Simon,

I've seen this issue with GHC 6.12.3 (and assumed it was by design).
It occurs with a slight modification of your example:

{-# LANGUAGE RecordWildCards #-}

module Test where

data T = MkT { f,g :: Int }

p x = let MkT{..} = x in f

This example warns about "Defined but not used: `g'" on the line
defining p. I've raised a GHC bug:
http://hackage.haskell.org/trac/ghc/ticket/4411 about this warning.

Thanks, Neil

> Which version of GHC are you using?  GHC 6.12 does not complain about unused variables bound by "..".  Try this, which complains about y, but not g.
>
> Simon
>
> {-# LANGUAGE RecordWildCards #-}
> module Test where
>
> data T = MkT { f,g :: Int }
>
> p (MkT { .. }) y = f
>
>
> |  -----Original Message-----
> |  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
> |  bounces at haskell.org] On Behalf Of Serge D. Mechveliani
> |  Sent: 14 October 2010 11:01
> |  To: Antoine Latter
> |  Cc: glasgow-haskell-users at haskell.org
> |  Subject: Re: un-used record wildcards
> |
> |  On Wed, Oct 13, 2010 at 01:47:11PM -0500, Antoine Latter wrote:
> |  > On Wed, Oct 13, 2010 at 1:02 PM, Serge D. Mechveliani <mechvel at botik.ru>
> |  wrote:
> |  > > Dear GHC developers,
> |  > >
> |  > > I use the language extension of RecordWildcards, for example,
> |  > >                                f (Foo {foo1 = n, foo2 = m, ..}) = ...
> |  > >
> |  > > But the complier warns about un-used values of  foo3, foo4,
> |  > > probably, due to the extension of
> |  > >                     Foo {foo1 = n, foo2 = m, foo3 = foo3, foo4 = foo4}.
> |  > >
> |  > > In such cases, these warnings look as unneeded.
> |  > > Is it possible to have an un-used binding warnings with exception for
> |  > > wildcards in records?
> |  > > If not, then has it sense to introduce an option?
> |  > >
> |  >
> |  > If you're not using foo3 and foo4, can you not put it the ellipsis?
> |  > that won't cover every case (such as where you're using foo3 but not
> |  > foo4).
> |  >
> |  > Antoine
> |  >
> |
> |  Indeed, thank you.
> |  It occurs that under RecordWildcards the compiler allows to skip some
> |  record fields in a pattern.
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