[Haskell-cafe] Type checker for haskell-src-exts (was: Typechecking Using GHC API)

[Niklas Broberg]

On Thu, Dec 15, 2011 at 6:24 PM, Thomas Schilling
<nominolo at googlemail.com>wrote:

> What exactly are the hopes for such a type checker?  I can understand
> it being interesting as a research project, but as a realistic tools
> there are two huge issues:
>
>  1. It's going to take a LOT of time to reach feature parity with
> GHC's type checker.
>
>  2. Assuming that can be done, how is it going to be maintained and
> kept up to date with GHC?
>
> If it is going to be used as a development tool, both of these are a
> major requirement.  I haven't looked into the issues, but I'd expect
> it would be more realistic (although definitely not trivial) to
> translate from GHC's internal AST into an annotated haskell-src-exts
> AST.
>

With all due respect, the sentiments you give voice to here are a large
part of what drives me to do this project in the first place. Haskell is
not GHC, and I think that the very dominant position of GHC many times
leads to ill thought-through extensions. Since GHC has no competitor, often
extensions (meaning behavior as denoted and delimited by some -X flag) are
based on what is most convenient from the implementation point of view,
rather than what would give the most consistent, logical and modular user
experience (not to mention
third-party-tool-implementor-trying-to-support-GHC-extensions experience).

As such, I'm not primarily doing this project to get a development tool
out, even if that certainly is a very neat thing. I'm just as much doing it
to provide a Haskell (front-end) implementation that can serve as a better
reference than GHC, one that very explicitly does *not* choose the
convenient route to what constitutes an extension, and instead attempts to
apply rigid consistency and modularity between extensions. Also, just like
for haskell-src-exts I hope to build the type checker from the roots with
the user interface as a core design goal, not as a tacked-on afterthought.

Mind you, in no way do I intend any major criticism towards GHC or its
implementors. GHC is a truly amazing piece of software, indeed it's
probably my personal favorite piece of software of all times. That does not
mean it comes free of quirks and shady corners though, and it is my hope
that by doing what I do I can help shine a light on them.

Answering your specific issues:

1) Yes, it's a lot of work. Probably not half as much as you'd think
though, see my previous mail about walking in the footsteps of giants. But
beyond that, I think it's work that truly needs to be done, for the greater
good of Haskell.

2) Well, I think I've done a reasonably good job keeping haskell-src-exts
up to date so far, even if the last year has been pretty bad in that regard
(writing a PhD thesis will do that to you). I'll keep doing it for the type
checker as well. But I am but one man, so if anyone else feels like joining
the project then they are more than welcome to.

Sort-of-3) Yes, both implementation and maintenance are likely going to be
far more costly than the alternative to do a translation via the GHC API.
I'm not interested in that alternative though.

Cheers,

/Niklas
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