[GHC] #15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint`
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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: -------------------------------------+------------------------------------- {{{#!hs {-# Language DataKinds, TypeInType, TypeFamilies #-} import Data.Kind newtype I a = I a type C = Constraint type family UnitC :: Constraint where UnitC = () instance UnitC => Functor I where fmap = undefined }}} this works fine, but if I try to use the constraint synonym `C` (`UnitC :: C where`) fails with "Instance head is not headed by a class" which has '''no''' Google hits outside of the [https://hackage.haskell.org/package/ghc-8.4.1/docs/src/TcValidity.html#check... compiler] {{{ $ ghci -ignore-dot-ghci hs/249.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/249.hs, interpreted ) hs/249.hs:13:10: error: • Instance head is not headed by a class • In the instance declaration for ‘Functor I’ | 13 | instance UnitC => Functor I where | ^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: 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 simonpj): Can you give the code that fails? In general, in an instance {{{ instance ... => C t1 t2 where ... }}} `C` must be a class; at least that's what the error message is complaining about. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: 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 monoidal): The failing code is obtained by replacing `Constraint` by `C`: {{{ {-# Language DataKinds, TypeInType, TypeFamilies #-} import Data.Kind newtype I a = I a type C = Constraint type family UnitC :: C where UnitC = () instance UnitC => Functor I where fmap = undefined }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: 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): Yes sorry, I should have included the failing code. Here is a version without type families {{{#!hs {-# Language DataKinds, TypeInType, ConstraintKinds #-} import Data.Kind data A :: Type type C = Constraint type UnitC = (() :: C) instance UnitC => Eq A where (==) = undefined }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with
`type C = Constraint`
-------------------------------------+-------------------------------------
Reporter: Iceland_jack | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.6.1
Component: Compiler | Version: 8.4.3
Resolution: | Keywords:
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 Simon Peyton Jones
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: merge Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | testsuite/tests/typecheck/should_compile/T15412 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => merge * testcase: => testsuite/tests/typecheck/should_compile/T15412 Comment: Wow, that was bad! Thank you for reporting; now fixed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | testsuite/tests/typecheck/should_compile/T15412 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: merge => closed * resolution: => fixed Comment: Merged in 6a7cb80648253ebcc84be5f11e2cc78eae085aa8. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | testsuite/tests/typecheck/should_compile/T15412 Blocked By: | Blocking: Related Tickets: #15583 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Iceland_jack): * related: => #15583 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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