[GHC] #15835: Internal error when splicing value constructed using typed template haskell

30 Oct 2018

      #15835: Internal error when splicing value constructed using typed template haskell
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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:
-------------------------------------+-------------------------------------
 Related to #15833

 Compiling Test.hs leads to an internal compiler error.

 https://gist.github.com/f04a613bb5e20c241c5b91c2f38b8f06

 {{{

 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Test where

 import qualified Compiler as C

 main :: IO ()
 main = do
 $$(C.runQuery)
 }}}

 {{{
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# OPTIONS_GHC -Wall #-}
 module Compiler where

 import Language.Haskell.TH

 type QTExp a = Q (TExp a)

 fix :: (a -> a) -> a
 fix f = let x = f x in x

 while ::
   forall m . Monoid m =>
   QTExp (IO m -> IO m) -> QTExp (IO m)
 while b = [|| fix (\r -> whenM @(IO m) ($$b r)) ||]

 whenM :: Monoid m => m ->  m
 whenM _ = mempty

 execOp :: forall m . Monoid m => QTExp (IO m)
 execOp = while [|| \r -> $$(while @m [|| id ||]) >> r ||]

 runQuery :: QTExp (IO ())
 runQuery = execOp
 }}}

 Leads to the following internal errors even though `Compiler` type
 checked.

 {{{
 Prelude> :r
 [1 of 2] Compiling Compiler         ( Compiler.hs, interpreted )
 [2 of 2] Compiling Test             ( Test.hs, interpreted )
 Test.hs:9:6-15: Splicing expression
     C.runQuery
   ======>
     C.fix
       (\ r_a7K7
          -> (C.whenM @(IO m_a7Gp))
               ((\ r_a7K8
                   -> ((C.fix (\ r_a7K9 -> (C.whenM @(IO m_a7Gp)) (id
 r_a7K9)))
                         >> r_a7K8))
                  r_a7K7))

 Test.hs:9:6: error:
     • The exact Name ‘m’ is not in scope
         Probable cause: you used a unique Template Haskell name (NameU),
         perhaps via newName, but did not bind it
         If that's it, then -ddump-splices might be useful
     • In the result of the splice:
         $C.runQuery
       To see what the splice expanded to, use -ddump-splices
       In the Template Haskell splice $$(C.runQuery)
       In a stmt of a 'do' block: $$(C.runQuery)
   |
 9 |   $$(C.runQuery)
   |      ^^^^^^^^^^

 Test.hs:9:6: error:
     • The exact Name ‘m’ is not in scope
         Probable cause: you used a unique Template Haskell name (NameU),
         perhaps via newName, but did not bind it
         If that's it, then -ddump-splices might be useful
     • In the result of the splice:
         $C.runQuery
       To see what the splice expanded to, use -ddump-splices
       In the Template Haskell splice $$(C.runQuery)
       In a stmt of a 'do' block: $$(C.runQuery)
   |
 9 |   $$(C.runQuery)
   |      ^^^^^^^^^^

 Test.hs:9:6: error:
     • GHC internal error: ‘m’ is not in scope during type checking, but it
 passed the renamer
       tcl_env of environment: [a7K7 :-> Identifier[r_a7K7::a0,
 NotLetBound],
                                r5Fg :-> Identifier[main::IO (),
 TopLevelLet [] True]]
     • In the first argument of ‘IO’, namely ‘m’
       In the type ‘(IO m)’
       In the expression:
         (C.whenM @(IO m))
           ((\ r_a7K8
               -> ((C.fix (\ r_a7K9 -> (C.whenM @(IO m)) (id r_a7K9))) >>
 r_a7K8))
              r_a7K7)
   |
 9 |   $$(C.runQuery)
   |
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15835
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