{-# LANGUAGE CPP #-} {- | Description : namespaced labels The HList library (C) 2004-2006, Oleg Kiselyov, Ralf Laemmel, Keean Schupke A model of labels as needed for extensible records. As before, all the information about labels is recorded in their type, so the labels of records may be purely phantom. In general, Labels are exclusively type-level entities and have no run-time representation. Record labels are triplets of type-level naturals, namespace, and description. The namespace part helps avoid confusions between labels from different Haskell modules. The description is an arbitrary nullary type constructor. For the sake of printing, the description is required to be the instance of Show. One must make sure that the show functions does not examine the value, as descr is purely phantom. Here's an example of the good Label description: > data MyLabelDescr; instance Show MyLabelDescr where show _ = "descr" which are automated by makeLabels3 from "Data.HList.MakeLabel". This model even allows the labels in a record to belong to different namespaces. To this end, the model employs the predicate for type equality. -} module Data.HList.Label3 where import Data.HList.FakePrelude import Data.HList.HListPrelude import GHC.TypeLits import Data.Typeable {- $setup >>> let label3 = Label :: Label (Lbl HZero () ()) >>> let label6 = Label :: Label "6" -} data Lbl (x :: HNat) (ns :: *) (desc :: *) -- labels are exclusively type-level entities #if !OLD_TYPEABLE deriving Typeable #else instance (ShowLabel x) => Typeable2 (Lbl x) where typeOf2 _ = mkTyConApp (mkTyCon3 "HList" "Data.HList.Label3" "Lbl") [mkTyConApp (mkTyCon3 "GHC" "GHC.TypeLits" (showLabel (Label :: Label x))) []] #endif type instance ZipTagged (Lbl ix ns n ': ts) (v ': vs) = Tagged (Lbl ix ns n) v ': ZipTagged ts vs instance (Label t ~ Label (Lbl ix ns n)) => SameLabels (Label t) (Lbl ix ns n) -- * Public constructors for labels -- | Construct the first label firstLabel :: ns -> desc -> Label (Lbl HZero ns desc) firstLabel _ _ = Label -- | Construct the next label nextLabel :: Label (Lbl x ns desc) -> desc' -> Label (Lbl (HSucc x) ns desc') nextLabel _ _ = Label -- | Equality on labels (descriptions are ignored) -- Use generic instance {- instance ( HEq x x' b , HEq ns ns' b' , bres ~ HAnd b b' ) => HEq (Lbl x ns desc) (Lbl x' ns' desc') bres -} -- | Show label instance Show desc => ShowLabel (Lbl x ns desc) where showLabel = show . getd where getd :: Label (Lbl x ns desc) -> desc -- for the sake of Hugs getd = error "Data.HList.Label3 desc" instance Show desc => Show (Label (Lbl x ns desc)) where show = show . getd where getd :: Label (Lbl x ns desc) -> desc -- for the sake of Hugs getd = error "Data.HList.Label3 desc" {- | If possible, Label is left off: >>> let q = label3 .*. label3 .*. emptyProxy >>> :t q q :: Proxy '[Lbl 'HZero () (), Lbl 'HZero () ()] -} instance HExtend (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs)) where type HExtendR (Label (Lbl n ns desc)) (Proxy (Lbl n' ns' desc' ': xs)) = Proxy (Lbl n ns desc ': Lbl n' ns' desc' ': xs) (.*.) _ _ = Proxy {- | Mixing two label kinds means we have to include 'Label': >>> let r = label3 .*. label6 .*. emptyProxy >>> :t r r :: Proxy '[Label (Lbl 'HZero () ()), Label "6"] -} instance HExtend (Label (Lbl n ns desc)) (Proxy (x ': xs :: [Symbol])) where type HExtendR (Label (Lbl n ns desc)) (Proxy (x ': xs)) = Proxy (Label (Lbl n ns desc) ': MapLabel (x ': xs)) (.*.) _ _ = Proxy {- | Mixing two label kinds means we have to include 'Label': >>> let s = label6 .*. label3 .*. emptyProxy >>> :t s s :: Proxy '[Label "6", Label (Lbl 'HZero () ())] -} instance HExtend (Label (y :: Symbol)) (Proxy ((x :: *) ': xs)) where type HExtendR (Label (y :: Symbol)) (Proxy (x ': xs)) = Proxy (Label y ': (MapLabel (x ': xs))) (.*.) _ _ = Proxy instance HExtend (Label (y :: Symbol)) (Proxy ((x :: Nat) ': xs)) where type HExtendR (Label (y :: Symbol)) (Proxy (x ': xs)) = Proxy (Label y ': (MapLabel (x ': xs))) (.*.) _ _ = Proxy instance HExtend (Label (y :: Nat)) (Proxy ((x :: *) ': xs)) where type HExtendR (Label (y :: Nat)) (Proxy (x ': xs)) = Proxy (Label y ': (MapLabel (x ': xs))) (.*.) _ _ = Proxy instance HExtend (Label (y :: Nat)) (Proxy ((x :: Symbol) ': xs)) where type HExtendR (Label (y :: Nat)) (Proxy (x ': xs)) = Proxy (Label y ': (MapLabel (x ': xs))) (.*.) _ _ = Proxy -- | similar to Data.HList.Record.Labels1, but avoids producing Label (Label x) type family MapLabel (xs :: [k]) :: [*] type instance MapLabel '[] = '[] #if NO_CLOSED_TF -- if we can't have any overlap in the TF (ghc-7.6), cover the Label3/Label6 case type instance MapLabel ((x :: Symbol) ': xs) = Label x ': MapLabel xs type instance MapLabel (Lbl n ns desc ': xs) = Label (Lbl n ns desc) ': MapLabel xs type instance MapLabel (Label x ': xs) = Label x ': MapLabel xs #else type instance MapLabel (x ': xs) = AddLabel x ': MapLabel xs type family AddLabel (x :: k) :: * where AddLabel (Label x) = Label x AddLabel x = Label x #endif