Extracting comments from the source code

[Facundo Domínguez]

Dear GHC devs,

I'm trying to port Liquid Haskell to ghc 9.2.5. And having much difficulty
using the GHC API for a specific task, so I come here to ask about it.

As you may know already, Liquid Haskell needs to read special comments from
Haskell source code which have the form {-@ ... @-}.

For example

> module Demo.Lib where
>
> {-@ type Pos = {v:Int | 0 < v} @-}
>
> {-@ incr :: Pos -> Pos @-}
> incr :: Int -> Int
> incr x = h x
>   where
>      {-@ h :: Pos -> Pos @-}
>      h x = x - 1

Since Liquid Haskell runs as a GHC plugin, it has access to the GHC API. It
used to be the case that one could find all of the comments together with
their source spans in a field

pm_annotations :: ParsedModule -> ApiAnns -- [1]

However this field has been removed in ghc-9.2, and now the comments are
all spread through the AST after parsing.

I managed to collect them all again with a generic traversal:

> go :: forall a. Data a => a -> [LEpaComment]
> go = gmapQr (++) [] go `extQ` (id @[LEpaComment])

but I'd expect this to be rather slow. So I'd like to ask here, is there a
better way to collect all of the comments?

The other obvious way would be to write the traversal manually, but it
would be some code to write, and it would likely need attention when
porting to newer GHCs onwards.

Thanks!
Facundo

[1]:
https://hackage.haskell.org/package/ghc-9.0.2/docs/GHC.html#t:ParsedModule
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