• Vladislav Zavialov's avatar
    Remove Ord SrcLoc, Ord SrcSpan · 26e8fff3
    Vladislav Zavialov authored
    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.
    26e8fff3
GHC.hs 63.8 KB