#8359: ConstraintKinds require UndecidableInstances when it doesn't need it ----------------------------------------------+---------------------------- Reporter: thomaseding | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.4.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Description changed by simonpj: Old description:

Using ConstraintKinds to alias a bunch of class constraints fails to compile without UndecidableInstances. The same code that manually spells out class constraints without ConstraintKinds compiles just fine though.

Test case below

-------------------------------------

{-# LANGUAGE ConstraintKinds #-}

{-# LANGUAGE MultiParamTypeClasses #-}

class DifferentTypes a b

type DifferentTypes3 a b c = (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c)

class Foo a

class Bar a

-- Buggy instance requires UndecidableInstances to compile

instance (DifferentTypes3 a b c, Bar a, Bar b, Bar c) => Foo (a, b, c)

-- Equivalent instance compiles when manually expanding constraint type

instance (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c, Bar a, Bar b, Bar c) => Foo (a, b, c)

New description: Using `ConstraintKinds` to alias a bunch of class constraints fails to compile without `UndecidableInstances`. The same code that manually spells out class constraints without `ConstraintKinds` compiles just fine though. Test case below {{{ ------------------------------------- {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE MultiParamTypeClasses #-} class DifferentTypes a b type DifferentTypes3 a b c = (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c) class Foo a class Bar a -- Buggy instance requires UndecidableInstances to compile instance (DifferentTypes3 a b c, Bar a, Bar b, Bar c) => Foo (a, b, c) -- Equivalent instance compiles when manually expanding constraint type instance (DifferentTypes a b, DifferentTypes b c, DifferentTypes a c, Bar a, Bar b, Bar c) => Foo (a, b, c) }}} -- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8359#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8359: ConstraintKinds require UndecidableInstances when it doesn't need it ----------------------------------------------+---------------------------- Reporter: thomaseding | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.4.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by unknown <simonpj@…>): In [changeset:4407614cb19a65ddb2f9ede4778a0b47396f8ec9/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="4407614cb19a65ddb2f9ede4778a0b47396f8ec9" Look inside tuple predicates when checking instance declaration contexts This fixes Trac #8359 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8359#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#8359: ConstraintKinds require UndecidableInstances when it doesn't need it ----------------------------------------------+---------------------------- Reporter: thomaseding | Owner: simonpj Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.4.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: polykinds/T8359 | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Changes (by simonpj): * status: new => closed * testcase: => polykinds/T8359 * resolution: => fixed Comment: Thanks for pointing this out. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8359#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler