#12679: Permit abstract data types in signatures that don't have kind * -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ezyang): Also, you may be tempted to implement this using a constraint family: {{{ signature Key where import GHC.Exts (Constraint) type family Key k :: Constraint instance Key Bool }}} But this doesn't work, because GHC expects an instance head to be a type class. (This will probably be a problem even if we allow data types that are not kind Constraint.) The "silver lining" is that you can just defer the "insoluble" constraint to the use-site. But this is terribel, and there are other problems too. Here is some code that type checks with HEAD, but it is not pretty. {{{ {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE RoleAnnotations #-} unit p where signature Key where import GHC.Exts (Constraint) type family Key :: * -> Constraint signature Map where import Key type role Map nominal representational data Map k a empty :: Map k a insert :: Key k => k -> a -> Map k a -> Map k a lookup :: Key k => k -> Map k a -> Maybe a module M where import Map import Key -- Need to stick the instance constraint here -- (and need to do it explicitly! GHC won't infer it.) x :: Key Bool => Map Bool String x = insert True "foo" empty unit q where module Key where import GHC.Exts (Constraint) type family Key :: * -> Constraint type instance Key = Ord module Map(Map, M.empty, insert, lookup) where import Prelude hiding (lookup) import Data.Map (Map) import qualified Data.Map as M import Key -- Ord is NOT definitionally equal to Key, which means -- we have to massage the types insert :: Key k => k -> a -> Map k a -> Map k a insert = M.insert lookup :: Key k => k -> Map k a -> Maybe a lookup = M.lookup unit r where dependency p[Key=q:Key, Map=q:Map] -- OK! }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12679#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#12679: Permit abstract data types in signatures that don't have kind * -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: patch Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2595 Wiki Page: | -------------------------------------+------------------------------------- Changes (by ezyang): * status: new => patch * differential: => Phab:D2595 Comment: Richard is right. I put in a patch. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12679#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#12679: Permit abstract data types in signatures that don't have kind * -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: closed Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: fixed | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2595 Wiki Page: | -------------------------------------+------------------------------------- Changes (by ezyang): * status: patch => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12679#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler