I suspect I'm tripping on a gap in the PolyKinds support. I'm trying to package up a type-level generic view. It uses PolyKinds — and DataKinds, but I think it's the PolyKinds that matter. If I compile everything locally in the same build, it works fine. If I isolate the spine view declarations in their own package, I get type errors.
A quick search turned up this omnibus, which I'm guessing might fix my problem. http://hackage.haskell.org/trac/ghc/changeset/3bf54e78cfd4b94756e3f21c00ae187f80c3341d I was hoping someone might be able to identify if that's the case. I can wait for 7.6.1 if I must, but I was wondering if there's a workaround. (If I avoid PolyKinds, it works. But I have to simulate PolyKinds for a finite set of kinds, which is pretty obfuscating and not general. If you're curious, checkout the type-spine package. This whole email regards my trying to generalize and simplify that package with PolyKinds.) In a distillation of my use case, we have two modules. The first is the type-level spine view. > {-# LANGUAGE TypeFamilies, PolyKinds, DataKinds, TypeOperators, > UndecidableInstances, TemplateHaskell, EmptyDataDecls #-} > module Spine where > > type family Spine a :: * > > data Atom n -- represents a totally unapplied type name > data f :@ a -- |represents a type-level application > > -- this is an "unsaturated instance", which might be a no-no, but > -- at least isn't obviously causing the current problem > type instance Spine (f a) = f :@ a > > -- all other instances are for unapplied types names: > ----- e.g. type instance Spine N = Atom N > spineType :: Name -> Q [Dec] > spineType n = let t = conT n in > (:[]) `fmap` tySynInstD ''Spine [t] [t| Atom $t |] The second module is a distilled use case. > {-# LANGUAGE TypeFamilies, PolyKinds, DataKinds, TypeOperators, > TemplateHaskell #-} > > module Test where > > import Spine -- our first module above > > type family IsApp (a :: *) :: Bool > type instance IsApp (Atom n) = False > type instance IsApp (a :@ b) = True > > -- example types and use > data A = A > data F a = F a > > concat `fmap` mapM spineType [''A, ''F] > > isApp :: (True ~ IsApp (Spine a)) => a -> () > isApp _ = () > > test :: () > test = isApp (F A) If Spine.hs and Test.hs are in the same directory and I load Test in ghci, it type-checks fine. If I instead use cabal to install Spine as its own package, the subsequent type-checking of Test.test fails with: > Couldn't match type `IsApp ((:@) (* -> *) * F A)' with `'True' The :bro Spine command returns the same information regardless of how Test imports Spine. > *Test> :bro Spine > type family Spine k a :: * > data Atom k n > data (:@) k k f a > spineType :: ...snip... Why can "IsApp (... :@ ...)" reduce if Spine was locally compiled but not if it's pulled from a package? Is there some crucial info that the package interfaces can't yet express? Is there an open bug for this? Thanks for your time, Nick _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
