Use Semigroup's (<>) for Doc/SDocwip/semigroup-sdoc

Before this patch, Outputable.hs defined its own (<>) which caused
conflicts with (Data.Semigroup.<>) and thus led to inconvenience.

However, replacing it is not trivial due to a different fixity:

  http://www.haskell.org/pipermail/libraries/2011-November/017066.html

Nevertheless, it is possible to update the pretty-printing code to work
with (<>) of a different fixitiy, and that's what this patch implements.

Now Doc and SDoc are instances of Semigroup.

1 files changed, 3 insertions, 8 deletions

diff --git a/compiler/utils/Pretty.hs b/compiler/utils/Pretty.hs
index 5adfdd7699..6e8686a62d 100644
--- a/compiler/utils/Pretty.hs
+++ b/compiler/utils/Pretty.hs
@@ -201,8 +201,7 @@ But it doesn't work, for if x=empty, we would have
 -- ---------------------------------------------------------------------------
 -- Operator fixity
 
-infixl 6 <>
-infixl 6 <+>
+infixr 6 <+>  -- matches that of (Semigroup.<>)
 infixl 5 $$, $+$
 
 
@@ -659,14 +658,10 @@ nilAboveNest g k q           | not g && k > 0      -- No newline if no overlap
 -- ---------------------------------------------------------------------------
 -- Horizontal composition @<>@
 
--- We intentionally avoid Data.Monoid.(<>) here due to interactions of
--- Data.Monoid.(<>) and (<+>).  See
--- http://www.haskell.org/pipermail/libraries/2011-November/017066.html
-
 -- | Beside.
 -- '<>' is associative, with identity 'empty'.
-(<>) :: Doc -> Doc -> Doc
-p <>  q = beside_ p False q
+instance Semigroup Doc where
+  p <> q = beside_ p False q
 
 -- | Beside, separated by space, unless one of the arguments is 'empty'.
 -- '<+>' is associative, with identity 'empty'.