From: simonpj <unknown>
Date: Wed, 11 Dec 2002 17:09:08 +0000 (+0000)
Subject: [project @ 2002-12-11 17:09:08 by simonpj]
X-Git-Tag: Approx_11550_changesets_converted~1359
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=f7e4ca26a0a91265573ea05ee46143787ace82f8;hp=63e510b508399d603045b3a628a3d765ca36ff8b;p=ghc-hetmet.git

[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
--- 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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