Remove Ord SrcLoc, Ord SrcSpan

Before this patch, GHC relied on Ord SrcSpan to identify source elements, by
using SrcSpan as Map keys:

	blackList :: Map SrcSpan ()      -- compiler/GHC/HsToCore/Coverage.hs
	instanceMap :: Map SrcSpan Name  -- compiler/GHC/HsToCore/Docs.hs

Firstly, this design is not valid in presence of UnhelpfulSpan, as it
distinguishes between  UnhelpfulSpan "X"  and  UnhelpfulSpan "Y", but those
strings are messages for the user, unfit to serve as identifiers for source
elements.

Secondly, this design made it hard to extend SrcSpan with additional data.
Recall that the definition of SrcSpan is:

	data SrcSpan =
	    RealSrcSpan !RealSrcSpan
	  | UnhelpfulSpan !FastString

Say we want to extend the RealSrcSpan constructor with additional information:

	data SrcSpan =
	    RealSrcSpan !RealSrcSpan !AdditionalInformation
	  | UnhelpfulSpan !FastString

	getAdditionalInformation :: SrcSpan -> AdditionalInformation
	getAdditionalInformation (RealSrcSpan _ a) = a

Now, in order for  Map SrcSpan  to keep working correctly, we must *ignore* additional
information when comparing SrcSpan values:

	instance Ord SrcSpan where
	  compare (RealSrcSpan r1 _) (RealSrcSpan r2 _) = compare r1 r2
	  ...

However, this would violate an important law:

	a == b  therefore  f a == f b

Ignoring  AdditionalInformation  in comparisons would mean that with
f=getAdditionalInformation, the law above does not hold.

A more robust design is to avoid  Ord SrcSpan  altogether, which is what this patch implements.
The mappings are changed to use RealSrcSpan instead:

	blackList :: Set RealSrcSpan         -- compiler/GHC/HsToCore/Coverage.hs
	instanceMap :: Map RealSrcSpan Name  -- compiler/GHC/HsToCore/Docs.hs

All SrcSpan comparisons are now done with explicit comparison strategies:

	SrcLoc.leftmost_smallest
	SrcLoc.leftmost_largest
	SrcLoc.rightmost_smallest

These strategies are not subject to the law mentioned above and can easily
discard both the string stored in  UnhelpfulSpan  and  AdditionalInformation.

Updates haddock submodule.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Hs/Binds.hs b/compiler/GHC/Hs/Binds.hs
index 6796216c87..483a952e62 100644
--- a/compiler/GHC/Hs/Binds.hs
+++ b/compiler/GHC/Hs/Binds.hs
@@ -46,7 +46,7 @@ import BooleanFormula (LBooleanFormula)
 
 import Data.Data hiding ( Fixity )
 import Data.List hiding ( foldr )
-import Data.Ord
+import Data.Function
 
 {-
 ************************************************************************
@@ -667,7 +667,7 @@ pprLHsBindsForUser binds sigs
     decls = [(loc, ppr sig)  | L loc sig <- sigs] ++
             [(loc, ppr bind) | L loc bind <- bagToList binds]
 
-    sort_by_loc decls = sortBy (comparing fst) decls
+    sort_by_loc decls = sortBy (SrcLoc.leftmost_smallest `on` fst) decls
 
 pprDeclList :: [SDoc] -> SDoc   -- Braces with a space
 -- Print a bunch of declarations