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

On Sun, 2011-08-14 at 14:22 +0100, Johan Tibell wrote:

On Sun, Aug 14, 2011 at 1:52 PM, Paterson, Ross wrote:

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Johan Tibell writes:

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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