[GHC] #12074: RULE too complicated to desugar

16 May 2016


      #12074: RULE too complicated to desugar
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Another example of specialization failing. Unlike #12068, this example
 doesn't use constraint/type synonyms.

 {{{
 {-# LANGUAGE FlexibleContexts #-}

 data Bar a
 instance (Num a) => Num (Bar a)

 data Foo q
 instance (C1 q) => Num (Foo q)

 class C1 a

 class (Num r, Num (Bar r)) => C2 r
 instance (C1 q) => C2 (Foo q)
 instance (C2 r) => C2 (Bar r)

 {-# SPECIALIZE f :: (C1 q) => Foo q -> Foo q #-}
 f :: (C2 r, C2 (Bar r)) => r -> r
 f = undefined
 }}}

 Warning:
 {{{
 RULE left-hand side too complicated to desugar
       Optimised lhs: let {
                        $dNum_aFp :: Num (Foo q)
                        [LclId, Str=DmdType]
                        $dNum_aFp = Main.$fNumFoo @ q $dC1_aEj } in
                      let {
                        $dNum_aFq :: Num (Bar (Foo q))
                        [LclId, Str=DmdType]
                        $dNum_aFq = Main.$fNumBar @ (Foo q) $dNum_aFp } in
                      f @ (Foo q)
                        $dC2_aEl
                        (Main.$fC2Bar
                           @ (Foo q)
                           $dNum_aFq
                           (Main.$fNumBar @ (Bar (Foo q)) $dNum_aFq)
                           $dC2_aEl)
       Orig lhs: let {
                   $dNum_aFp :: Num (Foo q)
                   [LclId, Str=DmdType]
                   $dNum_aFp = Main.$fNumFoo @ q $dC1_aEj } in
                 let {
                   $dNum_aFq :: Num (Bar (Foo q))
                   [LclId, Str=DmdType]
                   $dNum_aFq = Main.$fNumBar @ (Foo q) $dNum_aFp } in
                 let {
                   $dNum_aFr :: Num (Bar (Bar (Foo q)))
                   [LclId, Str=DmdType]
                   $dNum_aFr = Main.$fNumBar @ (Bar (Foo q)) $dNum_aFq } in
                 let {
                   $dC2_aEl :: C2 (Foo q)
                   [LclId, Str=DmdType]
                   $dC2_aEl = Main.$fC2Foo @ q $dNum_aFp $dNum_aFq $dC1_aEj
 } in
                 let {
                   $dC2_aEm :: C2 (Bar (Foo q))
                   [LclId, Str=DmdType]
                   $dC2_aEm = Main.$fC2Bar @ (Foo q) $dNum_aFq $dNum_aFr
 $dC2_aEl } in
                 f @ (Foo q) $dC2_aEl $dC2_aEm
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12074
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