[GHC] #15745: Panicking typechecker plugins

13 Oct 2018

      #15745: Panicking typechecker plugins
-------------------------------------+-------------------------------------
           Reporter:  philderbeast   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If I use a typechecker plugin that fails then ghc panics and I'm asked to
 report a bug with GHC;

 {{{
 ghc: panic! (the 'impossible' happened)
       (GHC version 8.2.2 for x86_64-apple-darwin):
         Prelude.undefined
     CallStack (from HasCallStack):
       error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
       undefined, called at plugin/Undefined/Solve/Plugin.hs:14:39 in
       undefined-solve-
 plugin-1.0.0.0-56evBabJYBHHTUlrE3HO5m:Undefined.Solve.Plugin

     Please report this as a GHC bug:
 http://www.haskell.org/ghc/reportabug
 }}}

 Could we please say that it is the typechecker plugin that is panicking
 and ask for a bug report for the faulty typechecker, giving the issues URL
 for the plugin if we know it?

 The following [https://github.com/BlockScope/undefined-plugin undefined
 plugins] fail for init, solve and stop functions.

 {{{
 #!haskell
 module Undefined.Init.Plugin (plugin) where

 import Plugins (Plugin(..), tcPlugin, defaultPlugin)
 import TcRnTypes (TcPluginM, TcPluginResult(..), Ct, TcPlugin(..))
 import GHC.TcPluginM.Extra (tracePlugin)

 plugin :: Plugin
 plugin = defaultPlugin { tcPlugin = const $ Just undefinedPlugin }

 undefinedPlugin :: TcPlugin
 undefinedPlugin = tracePlugin "undefined-init-plugin" $
     TcPlugin
         { tcPluginInit  = undefined
         , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
         , tcPluginStop  = const $ return ()
         }
 }}}

 {{{
 #!haskell
 module Undefined.Solve.Plugin (plugin) where

 import Plugins (Plugin(..), tcPlugin, defaultPlugin)
 import TcRnTypes (TcPluginM, TcPluginResult, Ct, TcPlugin(..))
 import GHC.TcPluginM.Extra (tracePlugin)

 plugin :: Plugin
 plugin = defaultPlugin { tcPlugin = const $ Just undefinedPlugin }

 undefinedPlugin :: TcPlugin
 undefinedPlugin = tracePlugin "undefined-solve-plugin" $
     TcPlugin
         { tcPluginInit  = return ()
         , tcPluginSolve = \_ _ _ _ -> undefined
         , tcPluginStop  = const $ return ()
         }
 }}}

 {{{
 #!haskell
 module Undefined.Stop.Plugin (plugin) where

 import Plugins (Plugin(..), tcPlugin, defaultPlugin)
 import TcRnTypes (TcPluginM, TcPluginResult(..), Ct, TcPlugin(..))
 import GHC.TcPluginM.Extra (tracePlugin)

 plugin :: Plugin
 plugin = defaultPlugin { tcPlugin = const $ Just undefinedPlugin }

 undefinedPlugin :: TcPlugin
 undefinedPlugin = tracePlugin "undefined-stop-plugin" $
     TcPlugin
         { tcPluginInit  = return ()
         , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
         , tcPluginStop  = const $ undefined
         }
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15745
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler