[GHC] #2041: Allow splicing in concrete syntax

[GHC]

#2041: Allow splicing in concrete syntax
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        Reporter:  igloo             |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  ⊥
       Component:  Template Haskell  |                 Version:  6.8.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by ezyang):

 The feature as stated here is rather conservatively stated, but in a more
 generalized form it could greatly improve the usability of Template
 Haskell. Basically, you are asking for proper quote/antiquote (antiquotes
 are nested splices.)

 There's a few ways this could be implemented:

 1. (Original String) As this ticket suggests, a new constructor is defined
 which contains bits of concrete syntax.  When, converting from the TH
 representation to GHC representation, the GHC parser is invoked to convert
 syntax into concrete representation.

 2. (Monadic String) Alternately, some new methods could be added to the
 Quasi monad, which is the "hook" interface that allows template-haskell to
 call into GHC's code. These methods would like in the Q monad. This has
 the benefit of letting us error out early if syntax is not well-formed.

 3. (ghc-api) We could make splices accept proper `ghc` AST returns, in
 which case we skip the TH conversion pass.  In this case, a user could
 just directly call out to the ghc-api to parse syntax, and then that could
 be inserted into the TH splcie.

 4. (Parser) Introduce a new syntactic form for quotation (something like
 quasi-quote syntax? quasi-quote with a special sigil?); contents of the
 quotation are parsed by the parser. Some tricky stuff happens.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2041#comment:8>
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