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

[Duncan Coutts]

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?

Duncan