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

Duncan Coutts duncan.coutts at
Sun Nov 6 22:46:21 CET 2011

On Sun, 2011-11-06 at 15:52 -0500, Edward Kmett wrote:
> Whichever way you go on this someone might get (slightly) annoyed, since:
> Text.PrettyPrint.Leijen
> infixr 6 <>,<+>

That turns out to be ok because in that module the property is:
empty <+> text "foo" = text " foo"

whereas for Text.PrettyPrint.HughesPJ, empty is a unit of <+>.

> Text.PrettyPrint.HughesPJ
> infixl 6 <>,<+>

Actually currently all the fixity declarations have been removed, but
yes that's what they were in previous released versions.

David: presumably this is a mistake? You didn't really mean to remove:

infixl 6 <>
infixl 6 <+>
infixl 5 $$, $+$

when merging with ghc's variant of the pretty print module.

> Ultimately no code breaks, after all it is changing the associativity of an
> operator that is by definition *associative*. ;)

That's what I thought too, but I already gave an example:

a <> empty <+> b

This means different things depending on whether we use infixl or
infixr. And as I noted, this doesn't bite Text.PrettyPrint.Leijen
because it lacks the <+> unit law.

So my tentative suggestion is:

infixr 6 <>
infixr 5 <+>
infixr 4 $$, $+$

So <> binds tighter than <+> and $$ and $+$ are still lower precedence
than <> and <+>, but switched round to being right associative.


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