[GHC] #12778: Expose variables bound in quotations to reify

28 Oct 2016

      #12778: Expose variables bound in quotations to reify
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:
  facundo.dominguez                  |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider the following program:
 {{{
 {-# LANGUAGE TemplateHaskell #-}
 module A where
 import Language.Haskell.TH as TH
 import Language.Haskell.TH.Syntax as TH

 foo :: IO ()
 foo = $([| let x = True
             in $(do addModFinalizer $ do
                       Just name <- TH.lookupValueName "x"
                       TH.reify name >>= runIO . print
                     [| return () |]
                 )
         |])
 }}}
 When compiled, {{{TH.lookupValueName}}} fails to find {{{x}}}.
 {{{
 $ inplace/bin/ghc-stage2 A.hs -fforce-recomp
 [1 of 1] Compiling A                ( A.hs, A.o )

 A.hs:7:9: error:
     • Pattern match failure in do expression at A.hs:9:23-31
     • In the expression: (let x_a3Jy = True in return ())
       In an equation for ‘foo’: foo = (let x_a3Jy = True in return ())
 }}}

 It would make producing bindings in {{{inline-java}}} better if the type
 of {{{x}}} could be found in the finalizer.

 According to comments in ghc,
 {{{[| \x -> $(f [| x |]) |]}}}
 desugars to
 {{{
 gensym (unpackString "x"#) `bindQ` \ x1::String ->
   lam (pvar x1) (f (var x1))
 }}}
 which erases any hint that a splice point existed at all. This information
 is necessary to know which variables were in scope.

 How about we add a some new methods to the `Q` monad for the sake of
 marking inner splices:
 {{{
 class Q m where
   ...
   qSpliceE :: m Exp -> m Exp
   qSpliceP :: m Pat -> m Pat
   qSpliceT :: m Type -> m Type
   ...
 }}}
 Now
 {{{[| \x -> $(f [| x |]) |]}}}
 would desugar to
 {{{
 gensym (unpackString "x"#) `bindQ` \ x1::String ->
   lam (pvar x1) (qSpliceE (f (var x1)))
 }}}

 When the renamer executes these primitives, it would be aware of the inner
 splices and could treat them similarly to top-level splices.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12778
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