Point-free do block gives missing instance error

https://www.haskell.org/ghc/docs/latest/html/users_guide/compiler-plugins.html#manipulating-bindings

This API example won't compile for me.

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:

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.

Trac metadata

Trac field	Value
Version	7.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
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