[GHC] #15593: QuantifiedConstraints: trouble with type family
#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints, | ConstraintKinds | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances #-} import Data.Kind data TreeF a b = T0 | T1 a | T2 b b -- from Data.Reify class MuRef (a :: Type) where type DeRef a :: Type -> Type class (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree }}} fails with {{{ $ ~/code/qc-ghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci 351.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( 351.hs, interpreted ) 351.hs:12:10: error: • Couldn't match type ‘DeRef (tree xx)’ with ‘TreeF xx’ arising from the superclasses of an instance declaration • In the instance declaration for ‘MuRef1 tree’ | 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} ---- What I want to write: {{{#!hs {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses, UndecidableInstances, AllowAmbiguousTypes #-} import Data.Kind -- from Data.Reify class MuRef (a :: Type) where type DeRef a :: Type -> Type type T = Type type TT = T -> T type TTT = T -> TT class (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1 (f :: TT) (deRef1 :: TT -> TTT) instance (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) => MuRef1 (f :: TT) (deRef1 :: TT -> TTT) }}} where I am trying to capture [https://hackage.haskell.org/package/folds-0.7.4/docs/src/Data-Fold- Internal.html MuRef1 & DeRef1] {{{#!hs class MuRef1 (f :: TT) where type DeRef1 f :: TTT muRef1 :: proxy (f a) -> Dict (MuRef (f a), DeRef (f a) ~ DeRef1 f a) }}} ---- Workarounds: I think splitting the class alias & quantification does the job (I haven't tested it but it compiles), I want to know if the first two programs are meant to compile or not {{{#!hs {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses, UndecidableInstances, AllowAmbiguousTypes, FlexibleContexts #-} -- .. class (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx instance (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx class (forall xx. cls xx) => Forall cls instance (forall xx. cls xx) => Forall cls class Forall (MuRef1_ f deRef1) => MuRef1 f deRef1 instance Forall (MuRef1_ f deRef1) => MuRef1 f deRef1 }}} or as a regular type/constraint synonym (at the loss of partial application) {{{#!hs type MuRef1 f deRef1 = (forall xx. (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx)) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints, | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Unfortunately, this is going to work, since you can't quantify over constraints that are headed by `(~)`. See #15359. GHC HEAD at least gives you a slightly more informative error message about this: {{{ $ /opt/ghc/head/bin/ghci Bug.hs GHCi, version 8.7.20180827: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:11:1: error: • Class ‘~’ does not support user-specified instances • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the context: forall xx. DeRef (tree xx) ~ TreeF xx While checking the super-classes of class ‘MuRef1’ In the class declaration for ‘MuRef1’ | 11 | class (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:12:10: error: • Class ‘~’ does not support user-specified instances • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the instance declaration for ‘MuRef1 tree’ | 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints, | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Oh dear can of worms, what about the workaround then: does that compile on HEAD? I will have to read that ticket {{{#!hs {-# Language KindSignatures, TypeFamilies, QuantifiedConstraints, FlexibleInstances, DataKinds, TypeInType, MultiParamTypeClasses, UndecidableInstances, AllowAmbiguousTypes, FlexibleContexts #-} import Data.Kind class MuRef (a :: Type) where type DeRef a :: Type -> Type class (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx instance (MuRef (f xx), DeRef (f xx) ~ deRef1 f xx) => MuRef1_ f deRef1 xx class (forall xx. cls xx) => Forall cls instance (forall xx. cls xx) => Forall cls class Forall (MuRef1_ f deRef1) => MuRef1 f deRef1 instance Forall (MuRef1_ f deRef1) => MuRef1 f deRef1 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedConstraints, | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): The program you posted in comment:2 compiles in both GHC 8.6.1 and HEAD. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15593: QuantifiedConstraints: trouble with type family -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: duplicate | Keywords: | QuantifiedConstraints, | ConstraintKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14860 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #14860 Comment: Update: GHC HEAD now //does// allow quantified constraints to be headed by `(~)`. Unfortunately, the original example still won't compile due to the fact that the quantified constraint's head mentions a type family: {{{ Bug.hs:11:1: error: • Illegal type synonym family application ‘DeRef (tree xx)’ in instance: DeRef (tree xx) ~ TreeF xx • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the context: forall xx. DeRef (tree xx) ~ TreeF xx While checking the super-classes of class ‘MuRef1’ In the class declaration for ‘MuRef1’ | 11 | class (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:12:10: error: • Illegal type synonym family application ‘DeRef (tree xx)’ in instance: DeRef (tree xx) ~ TreeF xx • In the quantified constraint ‘forall xx. DeRef (tree xx) ~ TreeF xx’ In the instance declaration for ‘MuRef1 tree’ | 12 | instance (forall xx. DeRef (tree xx) ~ TreeF xx) => MuRef1 tree | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} This is the subject of # , so I'm going to close this ticket in favor of that one. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15593#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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