#15474: Error message mentions Any -------------------------------------+------------------------------------- Reporter: monoidal | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by monoidal: Old description:

I'm not sure if this is a bug. File:

{{{#!hs {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeInType #-} module T15474 where

import Data.Kind (Type)

data Proxy a

type Forall = forall t. Proxy t

f1 :: forall (t :: Type). Proxy t f1 = f1

f2 :: Forall f2 = f1 }}}

gives an error message mentioning Any:

{{{ • Couldn't match type ‘GHC.Types.Any’ with ‘*’ Expected type: Proxy t Actual type: Proxy t0 }}}

The appearance of Any is suspicious - I thought it's an implementation detail?

New description: I'm not sure if this is a bug. File: {{{#!hs {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeInType #-} module T15474 where import Data.Kind (Type) data Proxy a type Forall = forall t. Proxy t f1 :: forall (t :: Type). Proxy t f1 = f1 f2 :: Forall f2 = f1 }}} gives an error message mentioning Any: {{{ • Couldn't match type ‘GHC.Types.Any’ with ‘*’ Expected type: Proxy t Actual type: Proxy t0 }}} The appearance of Any is suspicious to me - I thought it's an implementation detail? -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15474#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15474: Error message mentions Any -------------------------------------+------------------------------------- Reporter: monoidal | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):

So why doesn't something like that happen for Forall?

Ah, I think it's because * The kind of `Forall`'s RHS is just `Type` * And we do kind-generalise that kind; it has no free kind variables, so no kind-generalisation actually happens. And in any case the decl {{{ type Forall k = forall (t :: k). Proxy @k t }}} gives `Forall` the ambiguous kind `Forall :: forall k. Type`, which will do no one any good. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15474#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15474: Error message mentions Any -------------------------------------+------------------------------------- Reporter: monoidal | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #14198 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes, rejection is probably best for now -- we can always do something else later. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15474#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler