Implement an unlifted Proxy type, Proxy#

A value of type 'Proxy# a' can only be created through the new,
primitive witness 'proxy# :: Proxy# a' - a Proxy# has no runtime
representation and is thus free.

This lets us clean up the internals of TypeRep, as well as Adam's future
work concerning records (by using a zero-width primitive type.)
Authored-by: Edward Kmett <ekmett@gmail.com>
Authored-by: Austin Seipp <austin@well-typed.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>

compiler/basicTypes/MkId.lhs

@@ -138,7 +138,8 @@ ghcPrimIds
     nullAddrId,
     seqId,
     magicSingIId,
-    coerceId
+    coerceId,
+    proxyHashId
     ]
 \end{code}
 
@@ -1037,7 +1038,7 @@ they can unify with both unlifted and lifted types.  Hence we provide
 another gun with which to shoot yourself in the foot.
 
 \begin{code}
-lazyIdName, unsafeCoerceName, nullAddrName, seqName, realWorldName, coercionTokenName, magicSingIName, coerceName :: Name
+lazyIdName, unsafeCoerceName, nullAddrName, seqName, realWorldName, coercionTokenName, magicSingIName, coerceName, proxyName :: Name
 unsafeCoerceName  = mkWiredInIdName gHC_PRIM (fsLit "unsafeCoerce#") unsafeCoerceIdKey  unsafeCoerceId
 nullAddrName      = mkWiredInIdName gHC_PRIM (fsLit "nullAddr#")     nullAddrIdKey      nullAddrId
 seqName           = mkWiredInIdName gHC_PRIM (fsLit "seq")           seqIdKey           seqId
@@ -1046,9 +1047,23 @@ lazyIdName        = mkWiredInIdName gHC_MAGIC (fsLit "lazy")         lazyIdKey
 coercionTokenName = mkWiredInIdName gHC_PRIM (fsLit "coercionToken#") coercionTokenIdKey coercionTokenId
 magicSingIName    = mkWiredInIdName gHC_PRIM (fsLit "magicSingI")    magicSingIKey magicSingIId
 coerceName        = mkWiredInIdName gHC_PRIM (fsLit "coerce")        coerceKey          coerceId
+proxyName         = mkWiredInIdName gHC_PRIM (fsLit "proxy#")        proxyHashKey       proxyHashId
 \end{code}
 
 \begin{code}
+
+------------------------------------------------
+-- proxy# :: forall a. Proxy# a
+proxyHashId :: Id
+proxyHashId
+  = pcMiscPrelId proxyName ty noCafIdInfo
+  where
+    ty      = mkForAllTys [kv, tv] (mkProxyPrimTy k t)
+    kv      = kKiVar
+    k       = mkTyVarTy kv
+    tv:_    = tyVarList k
+    t       = mkTyVarTy tv
+
 ------------------------------------------------
 -- unsafeCoerce# :: forall a b. a -> b
 unsafeCoerceId :: Id

compiler/ghci/RtClosureInspect.hs

@@ -508,6 +508,7 @@ repPrim t = rep where
     | t == stablePtrPrimTyCon        = text "<stablePtr>"
     | t == stableNamePrimTyCon       = text "<stableName>"
     | t == statePrimTyCon            = text "<statethread>"
+    | t == proxyPrimTyCon            = text "<proxy>"
     | t == realWorldTyCon            = text "<realworld>"
     | t == threadIdPrimTyCon         = text "<ThreadId>"
     | t == weakPrimTyCon             = text "<Weak>"

compiler/prelude/PrelNames.lhs

@@ -1480,6 +1480,9 @@ ntTyConKey = mkPreludeTyConUnique 174
 coercibleTyConKey :: Unique
 coercibleTyConKey = mkPreludeTyConUnique 175
 
+proxyPrimTyConKey :: Unique
+proxyPrimTyConKey = mkPreludeTyConUnique 176
+
 ---------------- Template Haskell -------------------
 --      USES TyConUniques 200-299
 -----------------------------------------------------
@@ -1793,6 +1796,9 @@ fromListClassOpKey = mkPreludeMiscIdUnique 199
 fromListNClassOpKey = mkPreludeMiscIdUnique 500
 toListClassOpKey = mkPreludeMiscIdUnique 501
 
+proxyHashKey :: Unique
+proxyHashKey = mkPreludeMiscIdUnique 502
+
 ---------------- Template Haskell -------------------
 --      USES IdUniques 200-499
 -----------------------------------------------------

compiler/prelude/TysPrim.lhs

@@ -48,6 +48,8 @@ module TysPrim(
 	statePrimTyCon,		mkStatePrimTy,
 	realWorldTyCon,		realWorldTy, realWorldStatePrimTy,
 
+	proxyPrimTyCon,		mkProxyPrimTy,
+
 	arrayPrimTyCon,	mkArrayPrimTy, 
 	byteArrayPrimTyCon,	byteArrayPrimTy,
 	arrayArrayPrimTyCon, mkArrayArrayPrimTy, 
@@ -126,6 +128,7 @@ primTyCons
     , stablePtrPrimTyCon
     , stableNamePrimTyCon
     , statePrimTyCon
+    , proxyPrimTyCon
     , threadIdPrimTyCon
     , wordPrimTyCon
     , word32PrimTyCon
@@ -151,7 +154,7 @@ mkPrimTc fs unique tycon
 		  (ATyCon tycon)	-- Relevant TyCon
 		  UserSyntax		-- None are built-in syntax
 
-charPrimTyConName, intPrimTyConName, int32PrimTyConName, int64PrimTyConName, wordPrimTyConName, word32PrimTyConName, word64PrimTyConName, addrPrimTyConName, floatPrimTyConName, doublePrimTyConName, statePrimTyConName, realWorldTyConName, arrayPrimTyConName, arrayArrayPrimTyConName, byteArrayPrimTyConName, mutableArrayPrimTyConName, mutableByteArrayPrimTyConName, mutableArrayArrayPrimTyConName, mutVarPrimTyConName, mVarPrimTyConName, tVarPrimTyConName, stablePtrPrimTyConName, stableNamePrimTyConName, bcoPrimTyConName, weakPrimTyConName, threadIdPrimTyConName, eqPrimTyConName, eqReprPrimTyConName :: Name
+charPrimTyConName, intPrimTyConName, int32PrimTyConName, int64PrimTyConName, wordPrimTyConName, word32PrimTyConName, word64PrimTyConName, addrPrimTyConName, floatPrimTyConName, doublePrimTyConName, statePrimTyConName, proxyPrimTyConName, realWorldTyConName, arrayPrimTyConName, arrayArrayPrimTyConName, byteArrayPrimTyConName, mutableArrayPrimTyConName, mutableByteArrayPrimTyConName, mutableArrayArrayPrimTyConName, mutVarPrimTyConName, mVarPrimTyConName, tVarPrimTyConName, stablePtrPrimTyConName, stableNamePrimTyConName, bcoPrimTyConName, weakPrimTyConName, threadIdPrimTyConName, eqPrimTyConName, eqReprPrimTyConName :: Name
 charPrimTyConName    	      = mkPrimTc (fsLit "Char#") charPrimTyConKey charPrimTyCon
 intPrimTyConName     	      = mkPrimTc (fsLit "Int#") intPrimTyConKey  intPrimTyCon
 int32PrimTyConName	      = mkPrimTc (fsLit "Int32#") int32PrimTyConKey int32PrimTyCon
@@ -163,6 +166,7 @@ addrPrimTyConName    	      = mkPrimTc (fsLit "Addr#") addrPrimTyConKey addrPrim
 floatPrimTyConName   	      = mkPrimTc (fsLit "Float#") floatPrimTyConKey floatPrimTyCon
 doublePrimTyConName  	      = mkPrimTc (fsLit "Double#") doublePrimTyConKey doublePrimTyCon
 statePrimTyConName            = mkPrimTc (fsLit "State#") statePrimTyConKey statePrimTyCon
+proxyPrimTyConName            = mkPrimTc (fsLit "Proxy#") proxyPrimTyConKey proxyPrimTyCon
 eqPrimTyConName               = mkPrimTc (fsLit "~#") eqPrimTyConKey eqPrimTyCon
 eqReprPrimTyConName           = mkPrimTc (fsLit "~R#") eqReprPrimTyConKey eqReprPrimTyCon
 realWorldTyConName            = mkPrimTc (fsLit "RealWorld") realWorldTyConKey realWorldTyCon
@@ -473,6 +477,15 @@ mkStatePrimTy ty = TyConApp statePrimTyCon [ty]
 statePrimTyCon :: TyCon   -- See Note [The State# TyCon]
 statePrimTyCon	 = pcPrimTyCon statePrimTyConName [Nominal] VoidRep
 
+mkProxyPrimTy :: Type -> Type -> Type
+mkProxyPrimTy k ty = TyConApp proxyPrimTyCon [k, ty]
+
+proxyPrimTyCon :: TyCon
+proxyPrimTyCon = mkPrimTyCon proxyPrimTyConName kind [Nominal,Nominal] VoidRep
+  where kind = ForAllTy kv $ mkArrowKind k unliftedTypeKind
+        kv   = kKiVar
+        k    = mkTyVarTy kv
+
 eqPrimTyCon :: TyCon  -- The representation type for equality predicates
 		      -- See Note [The ~# TyCon]
 eqPrimTyCon  = mkPrimTyCon eqPrimTyConName kind [Nominal, Nominal, Nominal] VoidRep