+
+
+
+%************************************************************************
+%* *
+ Any
+%* *
+%************************************************************************
+
+Note [Any types]
+~~~~~~~~~~~~~~~~
+The type constructor Any::* has these properties
+
+ * It is defined in module GHC.Prim, and exported so that it is
+ available to users. For this reason it's treated like any other
+ primitive type:
+ - has a fixed unique, anyTyConKey,
+ - lives in the global name cache
+ - built with TyCon.PrimTyCon
+
+ * It is lifted, and hence represented by a pointer
+
+ * It is inhabited by at least one value, namely bottom
+
+ * You can unsafely coerce any lifted type to Ayny, and back.
+
+ * It does not claim to be a *data* type, and that's important for
+ the code generator, because the code gen may *enter* a data value
+ but never enters a function value.
+
+ * It is used to instantiate otherwise un-constrained type variables of kind *
+ For example length Any []
+ See Note [Strangely-kinded void TyCons]
+
+In addition, we have a potentially-infinite family of types, one for
+each kind /other than/ *, needed to instantiate otherwise
+un-constrained type variables of kinds other than *. This is a bit
+like tuples; there is a potentially-infinite family. They have slightly
+different characteristics to Any::*:
+
+ * They are built with TyCon.AnyTyCon
+ * They have non-user-writable names like "Any(*->*)"
+ * They are not exported by GHC.Prim
+ * They are uninhabited (of course; not kind *)
+ * They have a unique derived from their OccName (see Note [Uniques of Any])
+ * Their Names do not live in the global name cache
+
+Note [Uniques of Any]
+~~~~~~~~~~~~~~~~~~~~~
+Although Any(*->*), say, doesn't have a binding site, it still needs
+to have a Unique. Unlike tuples (which are also an infinite family)
+there is no convenient way to index them, so we use the Unique from
+their OccName instead. That should be unique,
+ - both wrt each other, because their strings differ
+
+ - and wrt any other Name, because Names get uniques with
+ various 'char' tags, but the OccName of Any will
+ get a Unique built with mkTcOccUnique, which has a particular 'char'
+ tag; see Unique.mkTcOccUnique!
+
+Note [Strangely-kinded void TyCons]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+See Trac #959 for more examples
+
+When the type checker finds a type variable with no binding, which
+means it can be instantiated with an arbitrary type, it usually
+instantiates it to Void. Eg.
+
+ length []
+===>
+ length Any (Nil Any)
+
+But in really obscure programs, the type variable might have a kind
+other than *, so we need to invent a suitably-kinded type.
+
+This commit uses
+ Any for kind *
+ Any(*->*) for kind *->*
+ etc
+
+\begin{code}
+anyTyConName :: Name
+anyTyConName = mkPrimTc (fsLit "Any") anyTyConKey anyTyCon
+
+anyTyCon :: TyCon
+anyTyCon = mkLiftedPrimTyCon anyTyConName liftedTypeKind 0 PtrRep
+
+anyTypeOfKind :: Kind -> Type
+anyTypeOfKind kind = mkTyConApp (anyTyConOfKind kind) []
+
+anyTyConOfKind :: Kind -> TyCon
+-- Map all superkinds of liftedTypeKind to liftedTypeKind
+anyTyConOfKind kind
+ | isLiftedTypeKind kind = anyTyCon
+ | otherwise = tycon
+ where
+ -- Derive the name from the kind, thus:
+ -- Any(*->*), Any(*->*->*)
+ -- These are names that can't be written by the user,
+ -- and are not allocated in the global name cache
+ str = "Any" ++ showSDoc (pprParendKind kind)
+
+ occ = mkTcOcc str
+ uniq = getUnique occ -- See Note [Uniques of Any]
+ name = mkWiredInName gHC_PRIM occ uniq (ATyCon tycon) UserSyntax
+ tycon = mkAnyTyCon name kind
+\end{code}
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