merge GHC HEAD

[ghc-hetmet.git] / compiler / prelude / TysPrim.lhs

diff --git a/compiler/prelude/TysPrim.lhs b/compiler/prelude/TysPrim.lhs
index 4b3492b..4c70bcb 100644 (file)
--- a/compiler/prelude/TysPrim.lhs
+++ b/compiler/prelude/TysPrim.lhs
@@ -10,7 +10,7 @@
 --   They are all, therefore, wired-in TyCons.  C.f module TysWiredIn
 module TysPrim(
        alphaTyVars, betaTyVars, alphaTyVar, betaTyVar, gammaTyVar, deltaTyVar,
-       alphaTy, betaTy, gammaTy, deltaTy,
+       alphaTy, betaTy, gammaTy, deltaTy, ecTyVars,
        openAlphaTy, openBetaTy, openAlphaTyVar, openBetaTyVar, openAlphaTyVars,
         argAlphaTy, argAlphaTyVar, argBetaTy, argBetaTyVar,
 
@@ -74,6 +74,8 @@ import Name           ( Name, BuiltInSyntax(..), mkInternalName, mkWiredInName )
 import OccName          ( mkTcOcc,mkTyVarOccFS, mkTcOccFS )
 import TyCon
 import TypeRep
+import Type
+import Coercion
 import SrcLoc
 import Unique          ( mkAlphaTyVarUnique )
 import PrelNames
@@ -175,6 +177,9 @@ tyVarList kind = [ mkTyVar (mkInternalName (mkAlphaTyVarUnique u)
                            where c = chr (u-2 + ord 'a')
                 ]
 
+ecTyVars :: [TyVar]
+ecTyVars = tyVarList ecKind
+
 alphaTyVars :: [TyVar]
 alphaTyVars = tyVarList liftedTypeKind
 
@@ -379,6 +384,22 @@ doublePrimTyCon    = pcPrimTyCon0 doublePrimTyConName DoubleRep
 %*                                                                     *
 %************************************************************************
 
+Note [The (~) TyCon)
+~~~~~~~~~~~~~~~~~~~~
+There is a perfectly ordinary type constructor (~) that represents the type
+of coercions (which, remember, are values).  For example
+   Refl Int :: Int ~ Int
+
+Atcually it is not quite "perfectly ordinary" because it is kind-polymorphic:
+   Refl Maybe :: Maybe ~ Maybe
+
+So the true kind of (~) :: forall k. k -> k -> #.  But we don't have
+polymorphic kinds (yet). However, (~) really only appears saturated in
+which case there is no problem in finding the kind of (ty1 ~ ty2). So
+we check that in CoreLint (and, in an assertion, in Kind.typeKind).
+
+Note [The State# TyCon]
+~~~~~~~~~~~~~~~~~~~~~~~
 State# is the primitive, unlifted type of states.  It has one type parameter,
 thus
        State# RealWorld
@@ -392,10 +413,11 @@ keep different state threads separate.  It is represented by nothing at all.
 mkStatePrimTy :: Type -> Type
 mkStatePrimTy ty = mkTyConApp statePrimTyCon [ty]
 
-statePrimTyCon :: TyCon
+statePrimTyCon :: TyCon   -- See Note [The State# TyCon]
 statePrimTyCon  = pcPrimTyCon statePrimTyConName 1 VoidRep
 
 eqPredPrimTyCon :: TyCon  -- The representation type for equality predicates
+                         -- See Note [The (~) TyCon]
 eqPredPrimTyCon  = pcPrimTyCon eqPredPrimTyConName 2 VoidRep
 \end{code}
 
@@ -415,7 +437,6 @@ realWorldStatePrimTy = mkStatePrimTy realWorldTy    -- State# RealWorld
 Note: the ``state-pairing'' types are not truly primitive, so they are
 defined in \tr{TysWiredIn.lhs}, not here.
 
-
 %************************************************************************
 %*                                                                     *
 \subsection[TysPrim-arrays]{The primitive array types}