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When getting ready to launch, one of the steps is the plugs-out test. Can the spacecraft function on its own without power or fuel from all cables and umbilicals?

When debugging GHC plugins, I’ve added tracing and changed the wiring. Rather than throw those edits away, I’ve collected them in ghc-plugs-out, a package of tests that don’t supply any power or fuel for typechecking. This lightweight multiple library¹, multiple test-suite package shows how typechecker plugins interact with GHC depending on how they’re wired up, their purity and whether GHC needs help typechecking.

Wiring Diagram

Here’s a type checker plugin that doesn’t do any solving but instead writes its call count.

{-# LANGUAGE QuasiQuotes, NamedFieldPuns #-}
module CallCount.TcPlugin (callCount) where

import Language.Haskell.Printf (s)
import Data.IORef (IORef)
import IOEnv (newMutVar, readMutVar, writeMutVar)
import TcPluginM (tcPluginIO)
import TcRnTypes (TcPluginResult(..), TcPlugin(..), unsafeTcPluginTcM)

newtype State = State { callref :: IORef Int }

callCount :: TcPlugin
callCount =
    TcPlugin
        { tcPluginInit = return . State =<< (unsafeTcPluginTcM $ newMutVar 1)

        , tcPluginSolve = \State{callref = c} _ _ _ -> do
            n <- unsafeTcPluginTcM $ readMutVar c
            tcPluginIO . putStrLn $ [s|>>> GHC-TcPlugin #%d|] n
            unsafeTcPluginTcM $ writeMutVar c (n + 1)
            return $ TcPluginOk [] []

        , tcPluginStop = const $ return ()
        }

Plugins are flagged for recompilation in their pluginRecompile field. Let’s now wire up and test the pure CallCount.Pure.Plugin and the impure CallCount.Impure.Plugin. The recommended way to wire up a plugin is with a pragma, only in source files that need the plugin.

{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
module Main where

main :: IO a
main = undefined

The call count prints on first build but not when there’s no work to do.

> cabal build test-wireup-pure-by-pragma
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1

> cabal build test-wireup-pure-by-pragma
Up to date

A plugin can also be wired up with an option, say in a cabal file. This is probably fine if all your modules need a plugin.

test-suite test-wireup-pure-by-option
  import: opts
  type: exitcode-stdio-1.0
  main-is: Main.hs
  hs-source-dirs: test-suites/wireup-pure-by-option
  ghc-options: -Wall -fplugin CallCount.Pure.Plugin
  build-depends: base, call-count-plugin

> cabal build test-wireup-pure-by-option
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1

If you mix and match both ways of doing the wiring you’ll end up with two instances of the plugin in the compilation.

> cabal build test-wireup-pure-by-both
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1
>>> GHC-TcPlugin #1

If your plugin is impure, it’s going to force a recompilation.

> cabal build test-wireup-impure-by-pragma
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1
[1 of 1] Compiling Main [Impure plugin forced recompilation]
>>> GHC-TcPlugin #1

> cabal build test-wireup-impure-by-option
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1
[1 of 1] Compiling Main [Impure plugin forced recompilation]
>>> GHC-TcPlugin #1

> cabal build test-wireup-impure-by-both
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1
>>> GHC-TcPlugin #1
[1 of 1] Compiling Main [Impure plugin forced recompilation]
>>> GHC-TcPlugin #1
>>> GHC-TcPlugin #1

Modularity

GHC compiles modules. We see the counter plugin is called on twice when functions foo and bar are in module Main.

{-# OPTIONS_GHC -fplugin CallCount.Pure.Plugin #-}
module Main where

foo :: IO a
foo = undefined

bar :: IO a
bar = undefined

main :: IO ()
main = return ()

> cabal build test-counter-main
[1 of 1] Compiling Main
>>> GHC-TcPlugin #1
>>> GHC-TcPlugin #2

Moving foo and bar to module FooBar and the counter plugin reports two calls again.

> cabal build test-counter-foobar-main
[1 of 2] Compiling FooBar
>>> GHC-TcPlugin #1
>>> GHC-TcPlugin #2
[2 of 2] Compiling Main

Move these functions into separate modules and we count one call for each module.

> cabal build test-counter-foo-bar-main
[1 of 3] Compiling Bar
>>> GHC-TcPlugin #1
[2 of 3] Compiling Foo
>>> GHC-TcPlugin #1
[3 of 3] Compiling Main

Undefined is not a Function

If your plugin behaves badly it is going to hurt. GHC takes the blame for the panic when any one of the functions required of a type checker plugin is implemented undefined².

plugin :: Plugin
plugin = mkPureTcPlugin undefSolve

undefSolve :: TcPlugin
undefSolve = noOp { tcPluginSolve = \_ _ _ _ -> undefined }

noOp :: TcPlugin
noOp =
    TcPlugin
        { tcPluginInit = return ()
        , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] []
        , tcPluginStop = const $ return ()
        }

mkPureTcPlugin :: TcPlugin -> Plugin
mkPureTcPlugin p =
    defaultPlugin
        { tcPlugin = const $ Just p
        , pluginRecompile = purePlugin
        }

> cabal build test-undefined-solve
[1 of 1] Compiling Undefined.Solve.Plugin
[1 of 1] Compiling Main
ghc: panic! (the 'impossible' happened)
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I would have liked to use record update syntax for undefSolve as shown above but this is not yet possible³ with GHC when the data type has an existential qualifier and that is how TcPlugin is defined⁴.

data TcPlugin = forall s. TcPlugin
    { tcPluginInit  :: TcPluginM s
    -- ^ Initialize plugin, when entering type-checker.
    , tcPluginSolve :: s -> TcPluginSolver
    -- ^ Solve some constraints.
    -- TODO: WRITE MORE DETAILS ON HOW THIS WORKS.
    , tcPluginStop  :: s -> TcPluginM ()
    -- ^ Clean up after the plugin, when exiting the type-checker.
    }

Care Free

Type checker plugins are of course called on by GHC to resolve constraints. Some need solving and others don’t. GHC knows that it can get an a from undefined but maybe a plugin can do better so we get called.

{-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
module Main where

main :: IO a
main = undefined

Going from () to () needs no further resolution. GHC can handle this by itself. The test-undefined-*-carefree test suites have these mains. The ones without carefree in their name don’t. They have the a from undefined mains.

{-# OPTIONS_GHC -fplugin Undefined.Solve.Plugin #-}
module Main where

main :: IO ()
main = return ()

We see that solve may be called but init and stop functions are always called.

+-------------------------------+------------+
| Test Suite                    | GHC Panics |
+===============================+============+
| test-undefined-init           |     x      |
+-------------------------------+------------+
| test-undefined-init-carefree  |     x      |
+-------------------------------+------------+
| test-undefined-solve          |     x      |
+-------------------------------+------------+
| test-undefined-solve-carefree |            |
+-------------------------------+------------+
| test-undefined-stop           |     x      |
+-------------------------------+------------+
| test-undefined-stop-carefree  |     x      |
+-------------------------------+------------+

Takeaways

• We should wire up type checker plugins with pragmas only in modules that need it.
• Don’t forget to flag pure plugins as such.
• If GHC doesn’t need help resolving constraints then it won’t call out to your plugin.
• Modules are the units of compilation.