Parsing haskell preserving comments / formatting

Question

I want to do some source code transformation (automatic import list cleanup) and I'd like to preserve comments and formatting. I heard some stuff on and off about parsers that do this, I think for the ghc parser.

It looks like I might be able to do this with hs-src-exts Language.Haskell.Exts.Annotate and its SrcSpans by pulling things out of the file. I think the SrcsSpanInfo only covers the parsed parts, but I could theoretically figure out the comments by looking at what's in between. But it's not documented in much detail, and there are no helper functions I can find, and it looks like a hassle, e.g. there's no easy way to print out a parsed expression including formatting and comments. So I think it's not meant to be used in this way, it's just so you can highlight code in the file or something. My impression is that the author meant to use annotations to support this, but never got around to it.

It looks like neither yi nor leksah do this. I feel like HaRe might, but it's not super documented. Is there a haskell parser out there that does this?

Answer 1

The haskell-src-exts recently got support for preserving comments, and it already records src spans. I'm not sure if pretty printing is supported, but you could probably get that working.

The GHC parser also does similar things.