From ab5c770bed51f08d56a0d61086988053b21aa461 Mon Sep 17 00:00:00 2001 From: Ian Lynagh Date: Sun, 30 Mar 2008 18:28:13 +0000 Subject: [PATCH] Derive a valid Ix instance for data Foo = Foo Int Int The old one didn't satisfy the axioms. See trac #2158 for details. --- compiler/typecheck/TcGenDeriv.lhs | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) 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 -- 1.7.10.4