Turn ElField into a newtype (VinylRecords#150)

* Saves a 1 word per field
* Improves accessor performance significantly both for Rec as well as ARec

Data/Vinyl/Class/Method.hs

@@ -142,7 +142,7 @@ data FieldTyper = FieldId | FieldSnd
 -- | The interpretation function of the 'FieldTyper' symbols.
 type family ApplyFieldTyper (f :: FieldTyper) (a :: k) :: * where
   ApplyFieldTyper 'FieldId a = a
-  ApplyFieldTyper 'FieldSnd '(s, b) = b
+  ApplyFieldTyper 'FieldSnd a = Snd a
 
 -- | A mapping of record contexts into the 'FieldTyper' function
 -- space. We explicitly match on 'ElField' to pick out the payload

Data/Vinyl/Derived.hs

@@ -46,7 +46,7 @@ getField :: ElField '(s,t) -> t
 getField (Field x) = x
 
 -- | Get the label name of an 'ElField'.
-getLabel :: forall s t. ElField '(s,t) -> String
+getLabel :: forall s t. KnownSymbol s => ElField '(s,t) -> String
 getLabel (Field _) = symbolVal (Proxy::Proxy s)
 
 -- | 'ElField' is isomorphic to a functor something like @Compose

Data/Vinyl/Functor.hs

@@ -42,6 +42,7 @@ import Foreign.Storable
 import GHC.Generics
 import GHC.TypeLits
 import GHC.Types (Type)
+import Data.Vinyl.TypeLevel (Snd)
 
 {- $introduction
     This module provides functors and functor compositions
@@ -107,8 +108,10 @@ newtype Const (a :: *) (b :: k)
 -- | A value with a phantom 'Symbol' label. It is not a
 -- Haskell 'Functor', but it is used in many of the same places a
 -- 'Functor' is used in vinyl.
-data ElField (field :: (Symbol, Type)) where
-  Field :: KnownSymbol s => !t -> ElField '(s,t)
+--
+-- Morally: newtype ElField (s, t) = Field t
+-- But GHC doesn't allow that
+newtype ElField (t :: (Symbol, Type)) = Field (Snd t)
 
 deriving instance Eq t => Eq (ElField '(s,t))
 deriving instance Ord t => Ord (ElField '(s,t))