[GHC] #9641: Point-free do block gives missing instance error

27 Sep 2014

      #9641: Point-free do block gives missing instance error
-------------------------------------+-------------------------------------
       Reporter:  rasfar             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Documentation      |                 Version:  7.8.1
       Keywords:  plugins            |        Operating System:  Linux
  HasDynFlags                        |         Type of failure:
   Architecture:  x86                |  Documentation bug
     Difficulty:  Unknown            |               Test Case:
     Blocked By:                     |                Blocking:
Related Tickets:                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 https://www.haskell.org/ghc/docs/latest/html/users_guide/compiler-
 plugins.html#manipulating-bindings

 This API example won't compile for me.

 {{{#!hs
 module SayNames.Plugin (plugin) where
 import GhcPlugins

 plugin :: Plugin
 plugin = defaultPlugin {
   installCoreToDos = install
   }

 install :: [CommandLineOption] -> [CoreToDo] -> CoreM [CoreToDo]
 install _ todo = do
   reinitializeGlobals
   return (CoreDoPluginPass "Say name" pass : todo)

 pass :: ModGuts -> CoreM ModGuts
 pass = do dflags <- getDynFlags
           bindsOnlyPass (mapM (printBind dflags))
   where printBind :: DynFlags -> CoreBind -> CoreM CoreBind
         printBind dflags bndr@(NonRec b _) = do
           putMsgS $ "Non-recursive binding named " ++ showSDoc dflags (ppr
 b)
           return bndr
         printBind _ bndr = return bndr
 }}}

 Compiling (GHC 7.8.1, or 7.6.3). I get:

 {{{
 SayNames/Plugin.hs:25:21:
     No instance for (HasDynFlags ((->) ModGuts))
       arising from a use of ‘getDynFlags’
     In a stmt of a 'do' block: dflags <- getDynFlags
     In the expression:
       do { dflags <- getDynFlags;
            bindsOnlyPass (mapM (printBind dflags)) }
     In an equation for ‘pass’:
         pass
           = do { dflags <- getDynFlags;
                  bindsOnlyPass (mapM (printBind dflags)) }
           where
               printBind :: DynFlags -> CoreBind -> CoreM CoreBind
               printBind dflags bndr@(NonRec b _)
                 = do { putMsgS
                        $ "Non-recursive binding named " ++ showSDoc dflags
 (ppr b);
                        .... }
               printBind _ bndr = return bndr
 }}}

 Changing to pointful style it compiles and works:

 {{{#!hs
 pass modguts = do dflags <- getDynFlags
                   bindsOnlyPass (mapM (printBind dflags)) modguts
 }}}

 Maybe this is a compiler bug, actually? Dunno, I never use point-free
 style with monadic functions.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9641
GHC http://www.haskell.org/ghc/
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