Implement -XConstraintKind

Basically as documented in http://hackage.haskell.org/trac/ghc/wiki/KindFact,
this patch adds a new kind Constraint such that:

  Show :: * -> Constraint
  (?x::Int) :: Constraint
  (Int ~ a) :: Constraint

And you can write *any* type with kind Constraint to the left of (=>):
even if that type is a type synonym, type variable, indexed type or so on.

The following (somewhat related) changes are also made:
 1. We now box equality evidence. This is required because we want
    to give (Int ~ a) the *lifted* kind Constraint
 2. For similar reasons, implicit parameters can now only be of
    a lifted kind. (?x::Int#) => ty is now ruled out
 3. Implicit parameter constraints are now allowed in superclasses
    and instance contexts (this just falls out as OK with the new
    constraint solver)

Internally the following major changes were made:
 1. There is now no PredTy in the Type data type. Instead
    GHC checks the kind of a type to figure out if it is a predicate
 2. There is now no AClass TyThing: we represent classes as TyThings
    just as a ATyCon (classes had TyCons anyway)
 3. What used to be (~) is now pretty-printed as (~#). The box
    constructor EqBox :: (a ~# b) -> (a ~ b)
 4. The type LCoercion is used internally in the constraint solver
    and type checker to represent coercions with free variables
    of type (a ~ b) rather than (a ~# b)

1 files changed, 41 insertions, 0 deletions

diff --git a/compiler/types/IParam.lhs b/compiler/types/IParam.lhs
new file mode 100644
index 0000000000..67d46c3a82
--- /dev/null
+++ b/compiler/types/IParam.lhs
@@ -0,0 +1,41 @@
+%
+% (c) The University of Glasgow 2006
+% (c) The GRASP/AQUA Project, Glasgow University, 1998
+%
+
+\begin{code}
+module IParam (
+        ipFastString, ipTyConName, ipTyCon, ipCoAxiom
+    ) where
+
+#include "HsVersions.h"
+
+import Name
+import TyCon    (CoAxiom, TyCon, newTyConCo_maybe)
+import Type
+
+import BasicTypes (IPName(..), ipNameName)
+import FastString
+import Outputable
+\end{code}
+
+\begin{code}
+ipFastString :: IPName Name -> FastString
+ipFastString = occNameFS . nameOccName . ipTyConName
+
+ipTyConName :: IPName Name -> Name
+ipTyConName = ipNameName
+
+ipTyCon :: IPName Name -> TyCon
+ipTyCon ip = case wiredInNameTyThing_maybe (ipTyConName ip) of
+    Just (ATyCon tc) -> tc
+    _                -> pprPanic "ipTyCon" (ppr ip)
+
+ipCoAxiom :: IPName Name -> CoAxiom
+ipCoAxiom ip = case newTyConCo_maybe (ipTyCon ip) of
+    Just ax -> ax
+    _       -> pprPanic "ipCoAxiom" (ppr ip)
+
+-- The IParam DataCon never gets any code generated for it, so it's
+-- a bit dangerous to actually make use of it, hence no ipDataCon function
+\end{code}