I have been updating the ghc-exactprint library for real world use cases on the about to be released GHC 9.2.1, and realised I need to be able to put an Anchor into every SrcSpan in the ParsedSource AST.
I prepared !6854 to sort it out in master and turned to the problem of GHC 9.2.1, where I had missed the boat.
And then I discovered that we have SrcSpan defined as
data SrcSpan =
RealSrcSpan !RealSrcSpan !(Maybe BufSpan)
| UnhelpfulSpan !UnhelpfulSpanReason
and the (Maybe BufSpan) is only used for attaching haddock comments after parsing.
This means there is an isomorphism between the RealSrcSpan variant and an Anchor, which I take advantage of with the code in [1], by using the Maybe to encode the AnchorOperation and the BufSpan to encode the DeltaPos.
And it struck me that perhaps we should make this a more official approach. The only problem is the detail of the BufSpan, to be able to play both roles cleanly.
Alan