From: Ian Lynagh <igloo@earth.li>
Date: Sun, 30 Mar 2008 18:28:13 +0000 (+0000)
Subject: Derive a valid Ix instance for data Foo = Foo Int Int
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=ab5c770bed51f08d56a0d61086988053b21aa461

Derive a valid Ix instance for data Foo = Foo Int Int
The old one didn't satisfy the axioms. See trac #2158 for details.
---

diff --git a/compiler/typecheck/TcGenDeriv.lhs b/compiler/typecheck/TcGenDeriv.lhs
index 6d9fc55..f248674 100644
--- a/compiler/typecheck/TcGenDeriv.lhs
+++ b/compiler/typecheck/TcGenDeriv.lhs
@@ -730,7 +730,11 @@ gen_Ix_binds tycon
       = mk_easy_FunBind tycon_loc unsafeIndex_RDR 
 		[nlTuplePat [con_pat as_needed, con_pat bs_needed] Boxed, 
 		 con_pat cs_needed] 
-		(mk_index (zip3 as_needed bs_needed cs_needed))
+        -- We need to reverse the order we consider the components in
+        -- so that
+        --     range (l,u) !! index (l,u) i == i   -- when i is in range
+        -- (from http://haskell.org/onlinereport/ix.html) holds.
+		(mk_index (reverse $ zip3 as_needed bs_needed cs_needed))
       where
 	-- index (l1,u1) i1 + rangeSize (l1,u1) * (index (l2,u2) i2 + ...)
 	mk_index [] 	   = nlHsIntLit 0