Re: [GHC] #16178: Brackets and splices should be overloaded like the static keyword

14 Jan 2019

      #16178: Brackets and splices should be overloaded like the static keyword
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        Reporter:  mpickering        |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.6.3
      Resolution:                    |             Keywords:
                                     |  TypedTemplateHaskell
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 If you define a partially static monoid type such as the following which
 tries to evaluate as many concatenations statically as possible.

 {{{
 data PS a = Static a | Dynamic (Code a)

 instance Monoid a => Monoid (PS a) where
    mempty = Static mempty
    (Static a) <> (Static b) = Static (a <> b)
    -- Using the overloading here
    a <> c = [|| $$a <> $$c ||]

 lower :: Lift a => PS a -> Code a
 lower (Static a) = lift a
 lower (Dynamic c) = c

 instance IsSplice PS where
   toBracket = lower

 instance IsBracket PS where
   fromBracket = Dynamic

 sta :: a -> PS a
 sta = Static
 }}}

 Writing programs with this data type becomes much more pleasant as the
 additional lifting and lowering become transparent. You can combine
 together these layers multiple times to implicitly perform different
 rewritings but the client's splice will still end up looking the same.

 {{{
 prog = (sta () <> sta ()) <> [|| () ||]
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16178#comment:2
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