[project @ 1999-03-17 08:26:30 by simonpj]

Complete the hack that lets us give 'error' the type

	error :: forall a:?. String -> a

The kind '?' (also known as 'Type AnyBox') means that
error can be applied to unboxed types too.  unsafeCoerce is
similar.  But the 'real' type of error is:

	error :: forall bv. forall a:Type bv. String -> a

That is, it's really polymorphic in the boxity of a; that's
different to saying that the type bound to 'a' must have
kind '?'.

The fix was in TcType.inst_tyvar, where we look for the kind '?'
and generate a boxity variable instead.  Sigh.

diff --git a/ghc/compiler/typecheck/TcType.lhs b/ghc/compiler/typecheck/TcType.lhs
index 651c76e..7305d51 100644 (file)
--- a/ghc/compiler/typecheck/TcType.lhs
+++ b/ghc/compiler/typecheck/TcType.lhs
@@ -181,7 +181,6 @@ tcInstTyVars tyvars
 inst_tyvar tyvar       -- Could use the name from the tyvar?
   = tcGetUnique                `thenNF_Tc` \ uniq ->
     let
-       kind = tyVarKind tyvar
        name = setNameUnique (tyVarName tyvar) uniq
        -- Note that we don't change the print-name
        -- This won't confuse the type checker but there's a chance
@@ -189,8 +188,26 @@ inst_tyvar tyvar   -- Could use the name from the tyvar?
        -- in an error message.  -dppr-debug will show up the difference
        -- Better watch out for this.  If worst comes to worst, just
        -- use mkSysLocalName.
+
+       kind = tyVarKind tyvar
     in
-    tcNewMutTyVar name kind
+
+       -- Hack alert!  Certain system functions (like error) are quantified
+       -- over type variables with an 'open' kind (a :: ?).  When we instantiate
+       -- these tyvars we want to make a type variable whose kind is (Type bv)
+       -- where bv is a boxity variable.  This makes sure it's a type, but 
+       -- is open about its boxity.  We *don't* want to give the thing the
+       -- kind '?' (= Type AnyBox).  
+       --
+       -- This is all a hack to avoid giving error it's "proper" type:
+       --       error :: forall bv. forall a::Type bv. String -> a
+
+    (if kind == openTypeKind then
+       newOpenTypeKind 
+     else
+       returnNF_Tc kind)       `thenNF_Tc` \ kind' ->
+
+    tcNewMutTyVar name kind'
 \end{code}
 
 @tcInstTcType@ instantiates the outer-level for-alls of a TcType with