#14763: GHC 8.4.1-alpha regression with FunctionalDependencies -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.4.1-alpha3 checker) | Resolution: | Keywords: FunDeps 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: | -------------------------------------+------------------------------------- Description changed by RyanGlScott: Old description:

This regression prevents `esqeueleto-2.5.3` from building with GHC 8.4.1. Here is a minimized example of the problem:

{{{#!hs {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} module Bug where

data Value a = Value a

data SomeValue expr where SomeValue :: Esqueleto query expr backend => expr (Value a) -> SomeValue expr

class Esqueleto (query :: * -> *) (expr :: * -> *) backend | query -> expr backend, expr -> query backend

data SqlQuery a

data SqlBackend

data SqlExpr a where ECompositeKey :: SqlExpr (Value a)

instance Esqueleto SqlQuery SqlExpr SqlBackend

match' :: SomeValue SqlExpr -> a match' (SomeValue ECompositeKey) = undefined }}}

On GHC 8.2.2, this typechecks without issue. On GHC 8.4.1-alpha (version 8.4.0.20180204), this fails with:

{{{ $ /opt/ghc/8.4.1/bin/ghci Bug.hs GHCi, version 8.4.0.20180204: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted )

Bug.hs:25:19: error: • Could not deduce: query ~ SqlQuery arising from a functional dependency between: constraint ‘Esqueleto query SqlExpr backend’ arising from a pattern with constructor: SomeValue :: forall (query :: * -> *) (expr :: * -> *) backend a. Esqueleto query expr backend => expr (Value a) -> SomeValue expr, in an equation for ‘match'’ instance ‘Esqueleto SqlQuery SqlExpr SqlBackend’ at Bug.hs:22:10-46 from the context: Value a1 ~ Value a2 bound by a pattern with constructor: ECompositeKey :: forall a. SqlExpr (Value a), in an equation for ‘match'’ at Bug.hs:25:19-31 ‘query’ is a rigid type variable bound by a pattern with constructor: SomeValue :: forall (query :: * -> *) (expr :: * -> *) backend a. Esqueleto query expr backend => expr (Value a) -> SomeValue expr, in an equation for ‘match'’ at Bug.hs:25:9-31 Inaccessible code in a pattern with constructor: ECompositeKey :: forall a. SqlExpr (Value a), in an equation for ‘match'’ • In the pattern: ECompositeKey In the pattern: SomeValue ECompositeKey In an equation for ‘match'’: match' (SomeValue ECompositeKey) = undefined | 25 | match' (SomeValue ECompositeKey) = undefined | ^^^^^^^^^^^^^ }}}

New description: This regression prevents `esqueleto-2.5.3` from building with GHC 8.4.1. Here is a minimized example of the problem: {{{#!hs {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} module Bug where data Value a = Value a data SomeValue expr where SomeValue :: Esqueleto query expr backend => expr (Value a) -> SomeValue expr class Esqueleto (query :: * -> *) (expr :: * -> *) backend | query -> expr backend, expr -> query backend data SqlQuery a data SqlBackend data SqlExpr a where ECompositeKey :: SqlExpr (Value a) instance Esqueleto SqlQuery SqlExpr SqlBackend match' :: SomeValue SqlExpr -> a match' (SomeValue ECompositeKey) = undefined }}} On GHC 8.2.2, this typechecks without issue. On GHC 8.4.1-alpha (version 8.4.0.20180204), this fails with: {{{ $ /opt/ghc/8.4.1/bin/ghci Bug.hs GHCi, version 8.4.0.20180204: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:25:19: error: • Could not deduce: query ~ SqlQuery arising from a functional dependency between: constraint ‘Esqueleto query SqlExpr backend’ arising from a pattern with constructor: SomeValue :: forall (query :: * -> *) (expr :: * -> *) backend a. Esqueleto query expr backend => expr (Value a) -> SomeValue expr, in an equation for ‘match'’ instance ‘Esqueleto SqlQuery SqlExpr SqlBackend’ at Bug.hs:22:10-46 from the context: Value a1 ~ Value a2 bound by a pattern with constructor: ECompositeKey :: forall a. SqlExpr (Value a), in an equation for ‘match'’ at Bug.hs:25:19-31 ‘query’ is a rigid type variable bound by a pattern with constructor: SomeValue :: forall (query :: * -> *) (expr :: * -> *) backend a. Esqueleto query expr backend => expr (Value a) -> SomeValue expr, in an equation for ‘match'’ at Bug.hs:25:9-31 Inaccessible code in a pattern with constructor: ECompositeKey :: forall a. SqlExpr (Value a), in an equation for ‘match'’ • In the pattern: ECompositeKey In the pattern: SomeValue ECompositeKey In an equation for ‘match'’: match' (SomeValue ECompositeKey) = undefined | 25 | match' (SomeValue ECompositeKey) = undefined | ^^^^^^^^^^^^^ }}} -- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14763#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14763: GHC 8.4.1-alpha regression with FunctionalDependencies -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.4.1-alpha3 checker) | Resolution: | Keywords: FunDeps 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: | -------------------------------------+------------------------------------- Comment (by bgamari): Thank you RyanGlScott! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14763#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14763: GHC 8.4.1-alpha regression with FunctionalDependencies -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: merge Priority: highest | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.4.1-alpha3 checker) | Resolution: | Keywords: FunDeps Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | testsuite/tests/typecheck/should_compile/T14763 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => merge * testcase: => testsuite/tests/typecheck/should_compile/T14763 Comment: Thanks for reporting this with a nice small example. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14763#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler