[GHC] #13788: TypeInType fails to compile old code
#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | 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: -------------------------------------+------------------------------------- {{{ {-# LANGUAGE TypeInType #-} module PP where newtype Field l v = Field { value :: v } label :: Field l v -> l label = undefined class HEq x hEq :: HEq x => x -> Int hEq = undefined class HListGet r where hListGet :: r -> Int instance HEq l => HListGet (Field l v) where hListGet f = hEq (label f) }}} fails with {{{ mini.hs:18:29: error: • Couldn't match type ‘k’ with ‘*’ ‘k’ is a rigid type variable bound by the instance declaration at mini.hs:17:5-33 Expected type: Field * l v Actual type: Field k l v • In the first argument of ‘label’, namely ‘f’ In the first argument of ‘hEq’, namely ‘(label f)’ In the expression: hEq (label f) • Relevant bindings include f :: Field k l v (bound at mini.hs:18:14) hListGet :: Field k l v -> Int (bound at mini.hs:18:5) | 18 | hListGet f = hEq (label f) | }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: 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 goldfire): This looks like correct behavior to me. With `-XTypeInType`, GHC will generalize kinds, including the kind of `HEq` (now `forall k. k -> Constraint`) and of `Field` (now `forall k. k -> Type -> Type`). However, `label` can't be generalized; it has type `forall (l :: Type) (v :: Type). Field @Type l v -> l`. (I'm using `@` to denote a normally-elided parameter.) Because the instance at the end doesn't constraint the kind of `l`, the type of `f` is `Field @k l v`, where `k` is implicitly bound in the instance declaration. Accordingly, it can't be passed to `label`. The solution: add a kind signature: `instance HEq l => HListGet (Field (l :: Type) v) where ...`. (You will have to import `Type` from `Data.Kind`.) Please close the ticket if you agree with my analysis. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: 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 br1): Your solution works. The problem is that adding TypeInType requires adding kind signatures all over. Then TypeInType is not an extension but a breaking change. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: 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 goldfire): This isn't an issue with `TypeInType`, per se, but with `PolyKinds`, which originally added kind generalization. (You'll find that compiling with just `PolyKinds` causes the same effect as you're observing with `TypeInType`.) Many of the language "extensions" have the potential to break code. For example, `OverloadedStrings` can cause declarations like `x = length "hi"` to fail, because GHC can no longer tell what type `"hi"` should be. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#13788: TypeInType fails to compile old code -------------------------------------+------------------------------------- Reporter: br1 | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: invalid | Keywords: 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: | -------------------------------------+------------------------------------- Changes (by br1): * status: new => closed * resolution: => invalid Comment: I see now. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13788#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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