Meta-point: backward compatibility

Lennart Augustsson lennart at
Sat Apr 26 15:35:39 EDT 2008

I agree with Neil.  Translators are very difficult to do right, except for
the most trivial transformations.
Changing tabs to spaces is about as far as I would trust an automatic

  -- Lennart

On Thu, Apr 24, 2008 at 9:21 AM, Neil Mitchell <ndmitchell at> wrote:

> Hi
> I think Henrik's criteria are pretty close to perfect.
> >  As I have argued before on the committee list, I also think we should
> *not*
> > worry about backwards incompatible changes too much in cases where a
> simple
> > automatic translation from H98 to H' code is possible.  Even for a large
> > project, it is IMHO no big hardship to run a H98->H' translator over all
> > Haskell sources.
> Some questions:
> 1) Will I (the programmer) have to mode switch in painful ways between
> H98 and H'? Do I have to learn a pile of exceptions, or is there some
> common rule that works? (i.e. removing n+k is fine since I just don't
> learn it, changing the fixity of $ is not since I need to know both)
> 2) Will tool T (the translator) work on academic papers that have
> previously been published.
> 3) Will tool T handle all of GHC's extensions.
> 4) Will tool T deal with things like CPP, hsc, trhsx, happy, alex
> etc... - all these file formats which include embedded Haskell.
> 5) Will tool T ever exist.
> I think the answer to 2-5 is nearly certainly going to be "No". I
> don't think the relevance of a conversion tool should even be
> considered until some person steps forward and says without doubt that
> _they_ will write the converter. Even if they did there very best, it
> still won't be trivial to use in all cases.
> > As John Launchbury has said, given Haskell's current rise in popularity,
> anything that
> > we do not fix with H' will be much harder, if not impossible, to fix in
> the future.
> That is a very good point. Perhaps we're already a little too late.
> Thanks
> Neil
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