#14146: Can GHC propose kind restrictions? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: Old description:

This is UX. As my code gets more polykinded I find myself in situations such as these all the more often

{{{#!hs instance (Comonad f, Representable f, Comonad g, Representable g) => Comonad (Compose f g) where extract = undefined duplicate = undefined

instance ComonadApply (Compose f g) where (<@>) = undefined }}}

{{{ /tmp/a.hs:20:10-35: error: • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration • In the instance declaration for ‘ComonadApply (Compose f g)’ | 20 | instance ComonadApply (Compose f g) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, modules loaded: none. }}}

It would help me if GHC looked up a `Comonad (Compose _ _)` instance, compared the kinds

{{{#!hs Comonad (Compose (f :: Type -> Type) (g :: Type -> Type) :: Type -> Type) ComonadApply (Compose (f :: k -> Type) (g :: Type -> k) :: Type -> Type) }}}

a simple suggestion like this would be helpful

{{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a kind signature (ComonadApply (Compose (f :: Type -> Type) g)). }}}

Even more amazing would be

{{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a context (Comonad f, Representable f, Comonad g, Representable g) => ComonadApply (Compose f g) }}}

Which the compiler could in theory guess, since following GHC's suggestion iteratively you end up with

{{{#!hs ComonadApply (Compose (f :: Type -> Type) g)

-- ==> add (Comonad f) to the context of the instance declaration Comonad f => ComonadApply (Compose f g)

-- ==> add (Comonad g) to the context of the instance declaration (Comonad f, Comonad g) => ComonadApply (Compose f g)

-- ==> add (Representable f) to the context of the instance declaration (Comonad f, Comonad g, Representable g) => ComonadApply (Compose f g)

-- ==> add (Representable f) to the context of the instance declaration (Comonad f, Comonad g, Representable g, Representable f) => ComonadApply (Compose f g) }}}

New description: This is UX. As my code gets more polykinded I find myself in situations such as these all the more often {{{#!hs instance (Comonad f, Representable f, Comonad g, Representable g) => Comonad (Compose f g) where extract = undefined duplicate = undefined instance ComonadApply (Compose f g) where (<@>) = undefined }}} {{{ /tmp/a.hs:20:10-35: error: • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration • In the instance declaration for ‘ComonadApply (Compose f g)’ | 20 | instance ComonadApply (Compose f g) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, modules loaded: none. }}} It would help me if GHC looked up a `Comonad (Compose _ _)` instance, compared the kinds {{{#!hs Comonad (Compose (f :: Type -> Type) (g :: Type -> Type) :: Type -> Type) ComonadApply (Compose (f :: k -> Type) (g :: Type -> k) :: Type -> Type) }}} a simple suggestion like this would be helpful {{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a kind signature (ComonadApply (Compose (f :: Type -> Type) g)). }}} Even more amazing would be {{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a context (Comonad f, Representable f, Comonad g, Representable g) => ComonadApply (Compose f g) }}} Which the compiler could in theory guess, since following GHC's suggestion iteratively you end up with {{{#!hs ComonadApply (Compose (f :: Type -> Type) g) -- ==> add (Comonad f) to the context of the instance declaration Comonad f => ComonadApply (Compose f g) -- ==> add (Comonad g) to the context of the instance declaration (Comonad f, Comonad g) => ComonadApply (Compose f g) -- ==> add (Representable g) to the context of the instance declaration (Comonad f, Comonad g, Representable g) => ComonadApply (Compose f g) -- ==> add (Representable f) to the context of the instance declaration (Comonad f, Comonad g, Representable g, Representable f) => ComonadApply (Compose f g) }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14146#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler