#14869: Documentation for isLiftedTypeKind is incorrect -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by RyanGlScott: Old description:

I [https://github.com/ghc-proposals/ghc- proposals/pull/32#issuecomment-369252383 noticed recently] that Template Haskell reifies `Constraint` as `Type`:

{{{#!hs $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive GHCi, version 8.5.20180221: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XTypeFamilies -XTemplateHaskell λ> :m + Data.Kind Language.Haskell.TH λ> type family Foo :: Constraint λ> putStrLn $(reify ''Foo >>= stringE . pprint) type family Ghci1.Foo :: * }}}

The root of the issue can be traced back to the `isLiftedTypeKind` function, which `TcSplice` uses to distinguish `Type` from `Constraint`. At least, that's what its [http://git.haskell.org/ghc.git/blob/df2c3b3364834d2fd038192c89348fc50a2e0475... documentation] claims:

{{{ -- | This version considers Constraint to be distinct from *. Returns True -- if the argument is equivalent to Type and False otherwise. isLiftedTypeKind :: Kind -> Bool isLiftedTypeKind = is_TYPE is_lifted where is_lifted (TyConApp lifted_rep []) = lifted_rep `hasKey` liftedRepDataConKey is_lifted _ = False }}}

However, in practice this claim about treating `Constraint` and `Type` as distinct is false:

{{{ $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive -package ghc GHCi, version 8.5.20180221: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :m + TyCoRep TysWiredIn λ> isLiftedTypeKind liftedTypeKind True λ> isLiftedTypeKind constraintKind True }}}

Either we should change the implementation of `isLiftedTypeKind` to match the documentation's claim, or change the documentation.

New description: I [https://github.com/ghc-proposals/ghc- proposals/pull/32#issuecomment-369252383 noticed recently] that Template Haskell reifies `Constraint` as `Type`: {{{ $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive GHCi, version 8.5.20180221: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XTypeFamilies -XTemplateHaskell λ> :m + Data.Kind Language.Haskell.TH λ> type family Foo :: Constraint λ> putStrLn $(reify ''Foo >>= stringE . pprint) type family Ghci1.Foo :: * }}} The root of the issue can be traced back to the `isLiftedTypeKind` function, which `TcSplice` uses to distinguish `Type` from `Constraint`. At least, that's what its [http://git.haskell.org/ghc.git/blob/df2c3b3364834d2fd038192c89348fc50a2e0475... documentation] claims: {{{ -- | This version considers Constraint to be distinct from *. Returns True -- if the argument is equivalent to Type and False otherwise. isLiftedTypeKind :: Kind -> Bool isLiftedTypeKind = is_TYPE is_lifted where is_lifted (TyConApp lifted_rep []) = lifted_rep `hasKey` liftedRepDataConKey is_lifted _ = False }}} However, in practice this claim about treating `Constraint` and `Type` as distinct is false: {{{ $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive -package ghc GHCi, version 8.5.20180221: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :m + TyCoRep TysWiredIn λ> isLiftedTypeKind liftedTypeKind True λ> isLiftedTypeKind constraintKind True }}} Either we should change the implementation of `isLiftedTypeKind` to match the documentation's claim, or change the documentation. -- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14869#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14869: Documentation for isLiftedTypeKind is incorrect -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):

I propose that we:

Yes that sounds reasonable. Thanks -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14869#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14869: Documentation for isLiftedTypeKind is incorrect -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Documentation | Unknown/Multiple bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4474 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott <ryan.gl.scott@…>): In [changeset:"49ac3f0f2a13f66fea31a258fa98b0de39bfbf10/ghc" 49ac3f0f/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="49ac3f0f2a13f66fea31a258fa98b0de39bfbf10" Fix #14869 by being more mindful of Type vs. Constraint Summary: Before, we were using `isLiftedTypeKind` in `reifyType` before checking if a type was `Constraint`. But as it turns out, `isLiftedTypeKind` treats `Constraint` the same as `Type`, so every occurrence of `Constraint` would be reified as `Type`! To make things worse, the documentation for `isLiftedTypeKind` stated that it treats `Constraint` //differently// from `Type`, which simply isn't true. This revises the documentation for `isLiftedTypeKind` to reflect reality, and defers the `isLiftedTypeKind` check in `reifyType` so that it does not accidentally swallow `Constraint`. Test Plan: make test TEST=T14869 Reviewers: goldfire, bgamari Reviewed By: goldfire Subscribers: rwbarton, thomie, carter GHC Trac Issues: #14869 Differential Revision: https://phabricator.haskell.org/D4474 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14869#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler