This is not using haskell-src-exts, but the Haskell Refactorer has a structure to keep a parallel tree of tokens indexed by SrcSpan, which attempts to allocate comments to the appropriate point. 

See https://github.com/alanz/HaRe/blob/master/src/Language/Haskell/Refact/Utils/TokenUtils.hs. It does not make use of the AST itself, so may be usable with haskell-src-exts

Alan




On Tue, Aug 20, 2013 at 11:19 AM, Niklas Broberg <niklas.broberg@gmail.com> wrote:
Hi Niklas,

1) My most desired feature would be a syntax tree that does not pluck
pluck comments out and make me treat them separately. It looks much
easier to me to have a fully descriptive tree and (filter . concatMap) /
traverse them out in some way than getting a list of comments and having
to insert them back in the right places myself.
Is that possible?

Sadly not - it's theoretically impossible. The fact that you can put comments literally wherever, means that it's impossible to treat them as nodes of the AST. E.g.

  f {- WHERE -} x = -- WOULD
      -- THESE
      do -- COMMENTS
         a {- END -} <- g x -- UP
         return {- ? -} a

What would be theoretically possible is to define a restricted language that allows comments only in certain well-defined places (cf haddock), and ignores any others. That's a lot of work though, and it's not clear how big the gain is. :-\

A different solution could be to improve the support, through better helper functions, for handling a syntax tree and a list of comments together. That's something I think could be worthwhile.
  
2) Have you considered downloading the all-of-Hackage tarball and
running haskell-src-exts over it to get a benchmark of how much HSE can
already parse of the Haskell code out there?

Considered, yes. Done, no. Would love to see the results :-). The crew at OdHac (Roman, Erik, Simon) ensured that the current version handles all of 'base', which is a good start.

Cheers, Niklas

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