[GHC] #16104: Plugin name lookup behavior change from GHC 8.4 series

28 Dec 2018

      #16104: Plugin name lookup behavior change from GHC 8.4 series
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           Reporter:  lerkok         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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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 I'm trying to port a core plugin to GHC 8.6.3, which was last working fine
 with GHC 8.4 series. Unfortunately, I'm running into issues. Wondering if
 pluging programming requirements have changed, or is this a regression in
 GHC itself. I boiled it down to the following example and would like some
 guidance on how to make this work:

 I have the following in file `TestPlugin.hs`:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}

 module TestPlugin (plugin) where

 import GhcPlugins
 import Data.Bits

 plugin :: Plugin
 plugin = defaultPlugin {installCoreToDos = install}
   where install _ todos = return (test : todos)

         test = CoreDoPluginPass "Test" check

         check :: ModGuts -> CoreM ModGuts
         check m = do mbN <- thNameToGhcName 'complement
                      case mbN of
                        Just _  -> liftIO $ putStrLn "Found complement!"
                        Nothing -> error "Failed to locate complement"

                      return m
 }}}

 And I have a very simple `Test.hs` file:

 {{{#!hs
 {-# OPTIONS_GHC -fplugin TestPlugin #-}

 main :: IO ()
 main = return ()
 }}}

 With GHC-8.4.2, I have:

 {{{
 $ ghc-8.4.2 --make -package ghc -c TestPlugin.hs
 [1 of 1] Compiling TestPlugin       ( TestPlugin.hs, TestPlugin.o )

 $ ghc-8.4.2 -package ghc -c Test.hs
 Found complement!
 }}}

 But with GHC 8.6.3, I get:

 {{{
 $ ghc-8.6.3 --make -package ghc -c TestPlugin.hs
 [1 of 1] Compiling TestPlugin       ( TestPlugin.hs, TestPlugin.o )

 $ ghc-8.6.3 -package ghc -c Test.hs
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.6.3 for x86_64-apple-darwin):
         Failed to locate complement
 }}}

 The problem goes away if I change `Test.hs` to:

 {{{#!hs
 {-# OPTIONS_GHC -fplugin TestPlugin #-}

 import Data.Bits  -- Should not be required in the client code!

 main :: IO ()
 main = return ()
 }}}

 That is, if I explicitly import `Data.Bits`. But this is quite
 undesirable, since `Test.hs` is client code and the users of the plugin
 have no reason to import all bunch of modules the plugin might need for
 its own purposes. (In practice, this would require clients to import a
 whole bunch of irrelevant modules; quite unworkable and not maintainable.)

 Should I be coding my plugin differently? Or is this a GHC regression?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16104
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