September 2016 - ghc-tickets

[GHC] #10241: BlockedIndefinitelyOnMVar thrown to the thread which is not blocked indefinitely
by GHC 21 May '18

21 May '18

#10241: BlockedIndefinitelyOnMVar thrown to the thread which is not blocked indefinitely -------------------------------------+------------------------------------- Reporter: asukamirai | Owner: simonmar Type: bug | Status: new Priority: normal | Milestone: Component: Runtime | Version: 7.8.3 System | Operating System: Unknown/Multiple Keywords: | Type of failure: Incorrect result Architecture: x86_64 | at runtime (amd64) | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- BlockedIndefinatelyOnMVar exception is thrown to the main thread in below source code although the thread is not blocked indefinitely. {{{#!hs module Main where import qualified Control.Concurrent.MVar as MV import qualified Control.Concurrent as CC import qualified Control.Exception as E main :: IO () main = do -- call this thread "threadA" mvar1 <- MV.newEmptyMVar :: IO (MV.MVar ()) mvar2 <- MV.newEmptyMVar :: IO (MV.MVar ()) _ <- CC.forkIO $ do -- call this thread "threadB" MV.takeMVar mvar1 `E.catch` errorHandler1 putStrLn "after error catch" CC.threadDelay 1000000 MV.putMVar mvar2 () putStrLn "after putMVar" MV.readMVar mvar2 `E.catch` errorHandler2 putStrLn "after readMVar" CC.threadDelay 5000000 where errorHandler1 :: E.BlockedIndefinitelyOnMVar -> IO () errorHandler1 e = putStrLn $ "errorHandler1 : " ++ show e errorHandler2 :: E.BlockedIndefinitelyOnMVar -> IO () errorHandler2 e = putStrLn $ "errorHandler2 : " ++ show e }}} Save above as "mvar.hs" and run by ghc as below. {{{ > runhaskell mvar.hs errorHandler1 : thread blocked indefinitely in an MVar operation errorHandler2 : thread blocked indefinitely in an MVar operation after error catch after readMVar after putMVar }}} BlockedIndefinitelyOnMVar thrown for mvar1 is correct. It will be caught by errorHandler1 and "threadB" can continue to put the value to mvar2. It means that "threadA" can wait for the value of mvar2 and it is not blocked indefinately. However, BlockedIndefinitelyOnMVar is thrown for mvar2 on "threadA" before "threadB" puts value to the mvar2. I think it is incorrect. ---- I tested another case that adding "CC.threadDelay 10000000" before "readMVar" as below. {{{#!hs module Main where import qualified Control.Concurrent.MVar as MV import qualified Control.Concurrent as CC import qualified Control.Exception as E main :: IO () main = do -- call this thread "threadA" mvar1 <- MV.newEmptyMVar :: IO (MV.MVar ()) mvar2 <- MV.newEmptyMVar :: IO (MV.MVar ()) _ <- CC.forkIO $ do -- call this thread "threadB" MV.takeMVar mvar1 `E.catch` errorHandler1 putStrLn "after error catch" CC.threadDelay 1000000 MV.putMVar mvar2 () putStrLn "after putMVar" CC.threadDelay 10000000 -- <-- this line is added MV.readMVar mvar2 `E.catch` errorHandler2 putStrLn "after readMVar" CC.threadDelay 5000000 where errorHandler1 :: E.BlockedIndefinitelyOnMVar -> IO () errorHandler1 e = putStrLn $ "errorHandler1 : " ++ show e errorHandler2 :: E.BlockedIndefinitelyOnMVar -> IO () errorHandler2 e = putStrLn $ "errorHandler2 : " ++ show e }}} And it will run correctly (BlockedIndefinitelyOnMVar is not thrown for mvar2). {{{ > runhaskell mvar.hs errorHandler1 : thread blocked indefinitely in an MVar operation after error catch after putMVar after readMVar }}} ---- I found this behavior is same on STM / BlockedIndefinitelyOnSTM. {{{#!hs module Main where import qualified Control.Concurrent.STM as STM import qualified Control.Concurrent as CC import qualified Control.Exception as E main :: IO () main = do tmv1 <- STM.newEmptyTMVarIO :: IO (STM.TMVar ()) tmv2 <- STM.newEmptyTMVarIO :: IO (STM.TMVar ()) _ <- CC.forkIO $ do STM.atomically (STM.takeTMVar tmv1) `E.catch` errorHandler1 putStrLn "after error catch" CC.threadDelay 1000000 STM.atomically $ STM.putTMVar tmv2 () putStrLn "after putTMVar" STM.atomically (STM.readTMVar tmv2) `E.catch` errorHandler2 putStrLn "after readTMVar" CC.threadDelay 5000000 where errorHandler1 :: E.BlockedIndefinitelyOnSTM -> IO () errorHandler1 e = putStrLn $ "errorHandler1 : " ++ show e errorHandler2 :: E.BlockedIndefinitelyOnSTM -> IO () errorHandler2 e = putStrLn $ "errorHandler2 : " ++ show e }}} {{{ > runhaskell stm.hs errorHandler1 : thread blocked indefinitely in an STM transaction errorHandler2 : thread blocked indefinitely in an STM transaction after error catch after readTMVar after putTMVar }}} ---- I tested this in below versions/OSs and got same result (exception thrown for mvar2/tmv2). ghc7.8.3 on Windows7 ghc7.8.3 on lubuntu14.04 on VirtualBox on Windows7 ghc7.8.4 on lubuntu14.04 on VirtualBox on Windows7 ghc7.10.1 on lubuntu14.04 on VirtualBox on Windows7 Similar report https://ghc.haskell.org/trac/ghc/ticket/8804 found but not the same. (In this case, the reference to the MVar is not weak) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10241> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9640: Invalid object in processHeapClosureForDead(): 2130762223
by GHC 19 May '18

19 May '18

#9640: Invalid object in processHeapClosureForDead(): 2130762223 -------------------------------------+------------------------------------- Reporter: Fuuzetsu | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I just tried using {{{-hb}}} profiling for Yi and hit {{{ yi: internal error: Invalid object in processHeapClosureForDead(): 2130762223 (GHC version 7.8.3 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Aborted }}} Unfortunately I don't have an easy way to replicate. In my case it's to check out HEAD version of Yi, build with profiling and run {{{ [nix-shell:~/programming/yi/yi]$ TERM=xterm yi_datadir=. dist/build/yi/yi -fpango --as=emacs -y /tmp /tmp/long +RTS -p -hb }}} where {{{/tmp/long}}} here is just a file with 1MB of the letter {{{A}}} repeated on a single line. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9640> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11515: PartialTypeSignatures suggests a redundant constraint with constraint families
by GHC 18 May '18

18 May '18

#11515: PartialTypeSignatures suggests a redundant constraint with constraint families -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 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: -------------------------------------+------------------------------------- When I say {{{ {-# LANGUAGE ConstraintKinds, TypeFamilies #-} module Bug where type family ShowSyn a where ShowSyn a = Show a foo :: (ShowSyn a, _) => a -> String foo x = show x }}} I get {{{ Bug.hs:7:20: error: Found constraint wildcard ‘_’ standing for ‘Show a’ To use the inferred type, enable PartialTypeSignatures In the type signature: foo :: (ShowSyn a, _) => a -> String }}} But if I fill in my wildcard with `Show a`, I'll rightly get a redundant constraint. Just a vanilla type synonym works there, but the type family causes trouble. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11515> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12563: Bad error message around lack of impredicativity
by GHC 18 May '18

18 May '18

#12563: Bad error message around lack of impredicativity -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- (Broken out from #12557.) Consider {{{ foo :: ((forall a. f a) -> f r) -> f r foo g = undefined x = \g -> foo g }}} Compiling this code produces {{{ Scratch.hs:24:15: error: • Couldn't match expected type ‘(forall (a :: k). f a) -> f r’ with actual type ‘t’ ‘t’ is a rigid type variable bound by the inferred type of x :: t -> f r at Scratch.hs:24:1-15 • In the first argument of ‘foo’, namely ‘g’ In the expression: foo g In the expression: \ g -> foo g • Relevant bindings include g :: t (bound at Scratch.hs:24:6) x :: t -> f r (bound at Scratch.hs:24:1) }}} The code should be rejected. But, really, the problem is that the type of a lambda-bound variable can't be a polytype (unless the type is obvious at the binding site). We should improve this error message. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12563> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11513: Work out when GADT parameters should be specified
by GHC 18 May '18

18 May '18

#11513: Work out when GADT parameters should be specified -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple TypeApplications | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Right now, there's not a clear specification of when GADT data constructor parameters should be specified, and what order they appear in. For example: {{{ data G a where MkG :: b -> a -> G a }}} To my eye, we should get `MkG :: forall b a. b -> a -> G a`. But GHC now gives `MkG :: a {b}. b -> a -> G a`. At least two things are going on here: 1. GHC puts all universals before existentials. 2. There is an outright bug in `mkDataCon` that makes existentials act as inferred variables, but only for the representation type, not the wrapper type. You can witness (2) by noting that the `b` magically becomes specified if you put a strictness annotation (thereby necessitating the construction of a wrapper) anywhere. I'm not sure what to do about (1), from a design standpoint. Here are some thoughts. A. Having universals always come before existentials is convenient in pattern matching. When we have type application in patterns, you'll want to match only on existentials, never universals. So keeping the existentials together makes some sense. The universals will be omitted from the match entirely. B. FC absolutely requires that the universals come first. So if we allow the user to reorder the variables, that will necessitate creating a wrapper. Are there performance implications? That would be sad. C. We could tread a middle path, where if the user writes a `forall`, they get the order requested. Otherwise, they get universals first (whose order is taken from the ordering in the `data` declaration head) followed by existentials (whose order is taken from left-to-right first occurrence in the constructor type signature). But this is different than for functions, where we always use left-to-right ordering, even when lacking a `forall`. I tend to think that we should always just do what the user asks, but I'm worried about performance implications of this decision. It will make wrappers necessary for existential constructors that otherwise don't need them. Note that this whole debate is about only GADT constructors, not Haskell98 ones. Haskell98 constructors will always have universals before existentials, because that's quite obvious from the way they're declared. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11513> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9840: Permit empty closed type families
by GHC 18 May '18

18 May '18

#9840: Permit empty closed type families -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- At the moment, closed type families without any equations fail with a parse error. In addition, they cannot be created by TH (see #8028). Would it be possible to permit these instead? My use case is in my typechecker plugin for units of measure, where I want to add new type-level operators without any equational theory (because it will be supplied by the plugin) and without users having the ability to introduce their own type family instances. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9840> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10078: tcPluginStop of a type checker plugin is not called if an error occurs
by GHC 18 May '18

18 May '18

#10078: tcPluginStop of a type checker plugin is not called if an error occurs -------------------------------------+------------------------------------- Reporter: jbracker | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 (Type checker) | Operating System: Linux Keywords: | Type of failure: Incorrect result Architecture: x86_64 | at runtime (amd64) | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- When a module using a type checker plugin produces a compiler error the clean up function `tcPluginStop` of the plugin is not called. I am not sure if this is intended, but according to the description of the wiki page (Plugins/TypeChecker) this should always be called. === Test plugin `MyPlugin.hs`: {{{#!hs module MyPlugin ( plugin ) where import Plugins import TcRnTypes import TcPluginM plugin :: Plugin plugin = defaultPlugin { tcPlugin = \clos -> Just $ TcPlugin { tcPluginInit = tcPluginIO $ putStrLn ">>> Plugin Init" , tcPluginSolve = \_ _ _ _ -> do tcPluginIO $ putStrLn ">>> Plugin Solve" return $ TcPluginOk [] [] , tcPluginStop = \_ -> tcPluginIO $ putStrLn ">>> Plugin Stop" } } }}} === Minimal example (with type error) `Main.hs`: {{{#!hs {-# OPTIONS_GHC -fplugin MyPlugin #-} module Main where main :: (Monad m) => m () main = do return 1 }}} Compiling this will lead to the following output: {{{ $ ~/ghc/inplace/bin/ghc-stage2 --make -package ghc-7.11.20150209 -dynamic Main.hs [2 of 2] Compiling Main ( Main.hs, Main.o ) >>> Plugin Init >>> Plugin Solve >>> Plugin Solve >>> Plugin Solve Main.hs:6:10: Could not deduce (Num ()) arising from the literal ‘1’ from the context: Monad m bound by the type signature for: main :: Monad m => m () at Main.hs:4:9-25 In the first argument of ‘return’, namely ‘1’ In a stmt of a 'do' block: return 1 In the expression: do { return 1 } }}} Which means `tcPluginStop` was _not_ called. === Minimal example (without type error) `Main.hs`: {{{#!hs {-# OPTIONS_GHC -fplugin MyPlugin #-} module Main where main :: (Monad m) => m () main = do return () }}} Compiling this will lead to the following output: {{{ $ ~/ghc/inplace/bin/ghc-stage2 --make -package ghc-7.11.20150209 -dynamic Main.hs [2 of 2] Compiling Main ( Main.hs, Main.o ) [MyPlugin changed] >>> Plugin Init >>> Plugin Solve >>> Plugin Solve >>> Plugin Stop Linking Main ... }}} Which means `tcPluginStop` _was_ called. === Possible solution As far as I can see, the solution to this should be to change the plugin code at the bottom of `typechecker/TcRnDriver.hs` from {{{#!hs withTcPlugins :: HscEnv -> TcM a -> TcM a withTcPlugins hsc_env m = do plugins <- liftIO (loadTcPlugins hsc_env) case plugins of [] -> m -- Common fast case _ -> do (solvers,stops) <- unzip `fmap` mapM startPlugin plugins res <- updGblEnv (\e -> e { tcg_tc_plugins = solvers }) m mapM_ runTcPluginM stops return res where startPlugin (TcPlugin start solve stop) = do s <- runTcPluginM start return (solve s, stop s) }}} to {{{#!hs withTcPlugins :: HscEnv -> TcM a -> TcM a withTcPlugins hsc_env m = do plugins <- liftIO (loadTcPlugins hsc_env) case plugins of [] -> m -- Common fast case _ -> do (solvers,stops) <- unzip `fmap` mapM startPlugin plugins eitherRes <- tryM $ do updGblEnv (\e -> e { tcg_tc_plugins = solvers }) m mapM_ runTcPluginM stops case eitherRes of Left e -> failM Right res -> return res where startPlugin (TcPlugin start solve stop) = do s <- runTcPluginM start return (solve s, stop s) }}} . I have tried this. It compiles and my minimal example delivers the correct result. Are there any arguments against this change? If not, I would try to commit a patch for this problem sometime this weekend. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10078> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11462: Use of typechecker plugin erroneously triggers "unbound implicit parameter" error
by GHC 18 May '18

18 May '18

#11462: Use of typechecker plugin erroneously triggers "unbound implicit parameter" error -------------------------------------+------------------------------------- Reporter: kwf | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 7.10.3 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- To document this bug, we're going to need a typechecker plugin to test it with. I've built a dummy plugin for this purpose, so we can be sure it is not interference from a particular plugin. {{{dummy-plugin/dummy-plugin.cabal}}} {{{ name: dummy-plugin version: 0.1.0.0 category: Development build-type: Simple cabal-version: >=1.10 library hs-source-dirs: . exposed-modules: DummyPlugin build-depends: base, ghc default-language: Haskell2010 GHC-options: -Wall -O2 }}} {{{dummy-plugin/DummyPlugin.hs}}} {{{#!hs module DummyPlugin(plugin) where import TcRnMonad ( TcPlugin(..), TcPluginResult(..) ) import Plugins ( defaultPlugin, Plugin(..), CommandLineOption ) plugin :: Plugin plugin = defaultPlugin { tcPlugin = Just . thePlugin } thePlugin :: [CommandLineOption] -> TcPlugin thePlugin opts = TcPlugin { tcPluginInit = return () , tcPluginSolve = \_ _ _ _ -> return $ TcPluginOk [] [] , tcPluginStop = \_ -> return () } }}} {{{Bug.hs}}} {{{#!hs {-# OPTIONS_GHC -fplugin=DummyPlugin #-} module Bug where impossible :: a impossible = undefined }}} First, compile the dummy plugin. From its directory, run {{{cabal install}}} to install the plugin. Then, from the main directory, run {{{ghc Bug.hs}}}. Expected result: the file compiles. Actual result: {{{ Bug.hs:6:14: error: • Unbound implicit parameter ?callStack::GHC.Stack.Types.CallStack arising from a use of implicit parameter ‘?callStack’ • In the expression: undefined In an equation for ‘impossible’: impossible = undefined }}} Further, observe that commenting out the line which invokes the type- checker plugin (the pragma on line 1) causes the file to compile correctly. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11462> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11525: Using a dummy typechecker plugin causes an ambiguity check error
by GHC 18 May '18

18 May '18

#11525: Using a dummy typechecker plugin causes an ambiguity check error -------------------------------------+------------------------------------- Reporter: jme | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following variation on [[GhcFile(testsuite/tests/typecheck/should_compile/T10009.hs)]] compiles cleanly: {{{#!hs {-# LANGUAGE TypeFamilies, ScopedTypeVariables #-} {-# OPTIONS_GHC -fno-warn-redundant-constraints #-} module T10009WithClass where type family F a type family UnF a class (UnF (F b) ~ b) => C b where f :: F b -> () g :: forall a. (C a) => a -> () g _ = f (undefined :: F a) }}} But compiling it with the dummy typechecker plugin [[GhcFile(testsuite/tests/typecheck/should_compile/T11462_Plugin.hs)]] yields {{{ $ ghc -dynamic -c T11462_Plugin.hs $ ghc -fplugin=T11462_Plugin -dynamic -c T10009WithClass.hs T10009WithClass.hs:10:5: error: - Could not deduce: F b0 ~ F b from the context: C b bound by the type signature for: f :: C b => F b -> () at T10009WithClass.hs:10:5-18 NB: 'F' is a type function, and may not be injective The type variable 'b0' is ambiguous Expected type: F b -> () Actual type: F b0 -> () - In the ambiguity check for 'f' To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the class method: f :: forall b. C b => F b -> () In the class declaration for 'C' }}} Superficially, the problem is that the presence of the plugin causes `runTcPluginsWanted` to zonk the pending superclass constraint, changing it to a `CNonCanonical`. This in turn prevents `solveWanteds` from making any further progress (which ultimately leads to the error): {{{ getUnsolvedInerts tv eqs = {[W] hole{aIO} :: s_aIL[fuv:2] GHC.Prim.~# fsk_aIC[fsk] (CTyEqCan)} fun eqs = {[W] hole{aIM} :: F b_aIu[tau:3] GHC.Prim.~# s_aIL[fuv:2] (CFunEqCan)} insols = {} others = {[W] $dC_aIv :: C b_aIu[tau:3] (CDictCan(psc))} <===== BEFORE plugin implics = {} Unflattening {Funeqs = [W] hole{aIM} :: F b_aIu[tau:3] GHC.Prim.~# s_aIL[fuv:2] (CFunEqCan) Tv eqs = [W] hole{aIO} :: s_aIL[fuv:2] GHC.Prim.~# fsk_aIC[fsk] (CTyEqCan)} Filling coercion hole aIM := <F b_aIu[tau:3]>_N unflattenFmv s_aIL[fuv:2] := F b_aIu[tau:3] writeMetaTyVar s_aIL[fuv:2] :: * := F b_aIu[tau:3] Unflattening 1 {} Unflattening 2 {[W] hole{aIO} :: s_aIL[fuv:2] GHC.Prim.~# fsk_aIC[fsk] (CTyEqCan)} Unflattening 3 {} Unflattening done {[W] hole{aIO} :: s_aIL[fuv:2] GHC.Prim.~# fsk_aIC[fsk] (CNonCanonical)} zonkSimples done: {[W] hole{aIO} :: F b_aIu[tau:3] GHC.Prim.~# F b_aIs[sk] (CNonCanonical)} zonkSimples done: {[W] $dC_aIv :: C b_aIu[tau:3] (CNonCanonical), <===== AFTER plugin [W] hole{aIO} :: F b_aIu[tau:3] GHC.Prim.~# F b_aIs[sk] (CNonCanonical)} solveSimples end } iterations = 1 residual = WC {wc_simple = [W] $dC_aIv :: C b_aIu[tau:3] (CNonCanonical) [W] hole{aIO} :: F b_aIu[tau:3] GHC.Prim.~# F b_aIs[sk] (CNonCanonical)} solveWanteds } }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11525> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11457: Run type-checker plugins before GHC's solver
by GHC 18 May '18

18 May '18

#11457: Run type-checker plugins before GHC's solver -------------------------------------+------------------------------------- Reporter: gridaphobe | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc1 (Type checker) | Keywords: plugin | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here's an odd use-case for type-checker plugins: rejecting ''valid'' programs that the user doesn't like, for example, programs that use a class instance with confusing behavior (like the functor/traversable instances on tuples). Unfortunately, we can't write such a plugin at the moment because GHC runs its own solver first, and will (I think) discharge any dictionary constraints before the plugin can run. If instead we run the plugins first, a plugin could mark undesired instances as insoluble. See the discussion at http://mail.haskell.org/pipermail/libraries/2016-January/026602.html for the original motivation. Thoughts? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11457> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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