[GHC] #8630: Kind inference fails to account for associated types
#8630: Kind inference fails to account for associated types -------------------------------------------+------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- Consider this: {{{ {-# LANGUAGE PolyKinds, TypeFamilies, FlexibleInstances #-} class C a where type F a instance C a where type F a = a -> a }}} HEAD gives me {{{ Expected a type, but ‛a’ has kind ‛k’ In the type ‛a -> a’ In the type instance declaration for ‛F’ In the instance declaration for ‛C a’ }}} The problem is that the use of `(->)` in the RHS of the definition for the `F a` instance constrains `a` to be of kind `*`, but GHC does not propagate this information back to the instance head. This decision is in conflict with the behavior for ''class'' declarations (as opposed to ''instance'' declarations), where a type variable's use in the definition informs its kind in the head. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8630 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8630: Kind inference fails to account for associated types --------------------------------------------+------------------------------ Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by simonpj): I suppose you are right here. But I think it'll take quite a bit of code to do the Right Thing here. The Right Thing is, I suppose, to walk over the associated-type declarations of the instance, and gather any kind constraints they generate. But consider something like this: {{{ instance C (T a) where type F (T a) b = b -> a }}} Here we must bring 'b' into scope before doing kind-checking on the RHS of the type instance. And then there are data families. This all duplicates the code we subsequently use for typechecking those type/data instances. Fiddly. Moreover, you could argue the same for ''value'' declarations too: {{{ instance C (T a) where op x y = [x, y::a -> a] }}} There's a type signature deep in the term that constrains the kind of 'a'. It would arguably be inconsistent to look at ''type'' instances but not ''value'' instances. And we really really are not going to look into the value terms! So I'm not sure it's worth fixing this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8630#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8630: Kind inference fails to account for associated types --------------------------------------------+------------------------------ Reporter: goldfire | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: wontfix | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by goldfire): * status: new => closed * resolution: => wontfix Comment: Fair enough. I took a look at what it would take to fix this, too, and came to much the same conclusion. I was rather hoping you might see some refactoring that would fix this bug ''and'' simplify gobs of other code in the process, as you sometimes do. When I have a chance, I might see if this behavior can be documented somewhere, as it did truly confuse me when I first hit it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8630#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8630: Kind inference fails to account for associated types
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Reporter: goldfire | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.7
Resolution: wontfix | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Test Case: | Difficulty: Unknown
Blocking: | Blocked By:
| Related Tickets:
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Comment (by Richard Eisenberg
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