Template Haskell pattern quotations

Brad Larsen brad.larsen at gmail.com
Thu Feb 4 16:58:49 EST 2010

On Thu, Feb 4, 2010 at 5:47 AM, Sebastian Fischer
<sebf at informatik.uni-kiel.de> wrote:
> On Feb 4, 2010, at 8:58 AM, Simon Peyton-Jones wrote:
>> Unless I have a sudden revelation I don't expect to implement pattern
>> splices anytime soon.
>> On the other hand, pattern *quasiquotes* are fine; they are run by the
>> renamer before scope analysis is done.  So you can certainly say
>>        f [qq| ...blah.. |] = ...g...
> If I understand Brad correctly, then what he needs is what he called pattern
> quotation rather than splicing.
> He can write
>    [e|True|]  instead of  conE (mkName "True")
> to implement the  Exp  parser of his quasi quoter but he cannot write
>    [p|True|]  instead of  conP (mkName "True")
> to implement his  Pat  parser because GHC tells him that "Tempate Haskell
> pattern brackets are not supported yet".
> My impression is that the problems with pattern splicing are not affected by
> support for pattern brackets. We can define a quasi quoter  qq  to implement
>    id :: a -> a
>    id [$qq|x|] = x
> independently of whether we use pattern brackets in its definition or not.
> Or am I missing something?
> Is there a problem with adding support for pattern brackets on the
> right-hand side of function definitions in order to simplify the definition
> of quasi quoters?
> Sebastian
> --
> Underestimating the novelty of the future is a time-honored tradition.
> (D.G.)

Yes, I think you have nailed my problem, and described it better than
I did!  It would be convenient to be able to use the pattern quotation
in the right hand side of a definition, when implementing a
quasiquoter without relying on the Data.Generics technique.


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