Monoid for SrcSpan and SrcSpanInfo?

[mgsloan]

There're a couple things in the way of having a proper Monoid for SrcSpan and SrcSpanInfo:

• A good representation for empty. The existence of noLoc = SrcLoc "" (-1) (-1) sets a precedent for this representing an empty location / interval.
• If mergeSrcSpan had a special case for empty, then it would be a proper definition for mappend. combSpanInfo / (<++>) throw away srcInfoPoints, which isn't so nice. You could still have a proper mappend, though, if they preserved srcInfoPoints when combined with mempty.

Maybe it would be better to have an explicit representation for empty SrcLoc and SrcSpan? Main issue is that it would be quite a breaking change.

[niklasbroberg]

I lean towards the breaking change. We're up for a rather big major release anyway next time, so it feels like a time to Do It Right (tm).

[UnkindPartition]

An explicit representation for an empty SrcSpan looks like a good thing to me.

I have serious doubts regarding SrcLoc, though. Is there such a thing as an empty location?

grep tells that noLoc isn't used anywhere HSE, so I'm not sure about its usefulness.

[pjonsson]

@mgsloan: What is the status for this ticket, is there code for this somewhere that should be merged? I agree with all of Roman's points.

[mgsloan]

I haven't done any work on this. Feel free to start work on it! I might get around to it sometime, though, if this becomes important for my usage of HSE. I also agree with Roman's points.

[pjonsson]

I will not fix this before the next release. It doesn't seem to be a blocker for your needs either.