[project @ 2002-12-11 17:09:08 by simonpj]

Back out the function tycon kind change (big comment to explain why)

diff --git a/ghc/compiler/types/TypeRep.lhs b/ghc/compiler/types/TypeRep.lhs
index 1ea81e7..c8e9f46 100644 (file)
--- a/ghc/compiler/types/TypeRep.lhs
+++ b/ghc/compiler/types/TypeRep.lhs
@@ -364,8 +364,14 @@ instance Binary Kind where
 We define a few wired-in type constructors here to avoid module knots
 
 \begin{code}
-funTyCon = mkFunTyCon funTyConName (mkArrowKinds [openTypeKind, openTypeKind] liftedTypeKind)
-       -- Functions can take and return either lifted or unlifted types
+funTyCon = mkFunTyCon funTyConName (mkArrowKinds [liftedTypeKind, liftedTypeKind] liftedTypeKind)
+       -- You might think that (->) should have type (? -> ? -> *), and you'd be right
+       -- But if we do that we get kind errors when saying
+       --      instance Control.Arrow (->)
+       -- becuase the expected kind is (*->*->*).  The trouble is that the
+       -- expected/actual stuff in the unifier does not go contra-variant, whereas
+       -- the kind sub-typing does.  Sigh.  It really only matters if you use (->) in
+       -- a prefix way, thus:  (->) Int# Int#.  And this is unusual.
 \end{code}
 
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