Haddock strings in .hi files

Edward Kmett ekmett at gmail.com
Thu Mar 20 16:41:47 UTC 2014

My knowledge of precisely how haddock works is somewhat fuzzy in that it
arises from a series of discussions a couple of years back.

My observation was mostly that I run 'cabal install' it goes through all
the modules building my .hi files, etc. Then I run cabal haddock and it
spends all that time redoing the same work, just to go through and get at
some information that we had right up until the moment we finished building.

I'm not wedded to bolting the information into the .hi files being the
solution, but the idea that we could avoid redoing that work is
tantalizing. I'm mostly trying to avoid redoing all the same work twice in
the build cycle of the average user.

If there is an alternative strategy, such as, oh, I don't know, making
haddock able to hook in plugin-style late as we're generating the .hi file
to spit out what it needs to something else and interrogate/rename/whatever
it needs the rest of the GHC API I'd be totally open that as well.


On Thu, Mar 20, 2014 at 12:18 PM, Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk
> wrote:

> On 20/03/14 16:08, Edward Kmett wrote:
> > One strong reason for considering at least including the haddocks in the
> > .hi files is build times.
> >
> > Currently if you have cabal configured to build and document every
> package
> > running hackage requires you to recompile your entire source tree a
> second
> > time to get information that we just dropped on the floor before spitting
> > out the .hi file.
> >
> > For most of the users of GHC this is a 50% difference in compile times if
> > they have cabal configured to generate haddocks.
> >
> > GHC doesn't have to understand the haddocks any more than it does now to
> > support it, just include the content.
> >
> > Haddock could then just go through and load the .hi files rather than
> > starting from scratch with parsing and typechecking the entire module,
> > running template-haskell, just to get at the documentation.
> >
> > Any pythonesque :doc command support to me would be gravy.
> >
> > The reason I care at all is the build times. I regularly lose minutes out
> > of each build just to regenerate docs and wind up skipping building them
> as
> > much as I can get away with to avoid he pain.
> >
> > -Edward
> >
> >
> As Simon M points out, we still have to run the renamer which seems to
> be tightly bound with the type-checker. Where do you suggest the
> sizeable performance increase would be coming from in this case? For all
> the existing packages, we already read the docs from .haddock files so
> there's no difference there. For new packages we have to type-check and
> generate .haddock anyway so there's no difference there either.
> It's not really about GHC having to know more about Haddock, it's about
> Haddock having to use GHC anyway, whether the strinsg are embedded or not.
> --
> Mateusz K.
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