Proposal #3339: Add (+>) as a synonym for mappend

[Johan Tibell]

On Sun, Nov 6, 2011 at 8:37 AM, Duncan Coutts
<duncan.coutts at googlemail.com>wrote:

> On Sun, 2011-08-14 at 14:22 +0100, Johan Tibell wrote:
> > On Sun, Aug 14, 2011 at 1:52 PM, Paterson, Ross <R.Paterson at city.ac.uk>
> wrote:
> > > Johan Tibell writes:
> > >> This is a call for consensus. Do we agree to add
> > >>
> > >>     infixr 6 <>
> > >>
> > >>     (<>) :: Monoid m => m -> m -> m
> > >>     (<>) = mappend
> > >
> > > As I recall, the only remaining issue is that this operator is
> > declared as infixl 6 in the pretty package.  Someone needs to
> > investigate the impact of changing its fixity there.
> >
> > Already done here:
> http://hackage.haskell.org/trac/ghc/ticket/3339#comment:22
> >
> > Seems to be a slight improvement actually.
>
> So I was preparing to commit this change in base and validating ghc when
> I discovered a more subtle issue in the pretty package:
>
> Consider
>
> a <> empty <+> b
>
> The fixity of <> and <+> is critical:
>
>  (a <> empty) <+> b
> = { empty is unit of <> }
>  (a         ) <+> b
>
>  a <> (empty <+> b)
> = { empty is unit of <+> }
>  a <> (          b)
>
> Currently Text.Pretty declares infixl 5 <>, <+>. If we change them to be
> infixr then we get the latter meaning of a <> empty <+> b. Existing code
> relies on the former meaning and produces different output with the
> latter (e.g. ghc producing error messages like "instancefor" when it
> should have been "instance for").
>
> Suggestions?


Don't use Monoid.(<>) in pretty for now, until someone has time to think
about how pretty fits into a world with Monoid exporting <>.

-- Johan
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