Right. There are two things here:
1) -frewrite-rules
enables rules to fire.
2) -fglasgow-exts
enables parsing of RULES pragmas, and their interpretation.
You need both if you wish to both write your own rules, and have them fire.
nope!-) -fglasgow-exts is sufficient for the RULE to be parsed and applied in Lennart's code, -frewrite-rules doesn't seem to serve any noticable purpose. but i see now this is under re-evaluation, aptly titled: "Confusing flags for rewrite rules" http://hackage.haskell.org/trac/ghc/ticket/2213 (see simonpj's comment at the end) claus $ head -1 Rules.hs {-# OPTIONS_GHC -O2 -Wall -fglasgow-exts #-} $ /cygdrive/c/fptools/ghc/ghc-6.9.20080514/bin/ghc -fno-method-sharing -ddump-simpl-stats -c Rules. hs ==================== FloatOut stats: ==================== 0 Lets floated to top level; 0 Lets floated elsewhere; from 3 Lambda groups ==================== FloatOut stats: ==================== 0 Lets floated to top level; 0 Lets floated elsewhere; from 3 Lambda groups ==================== Grand total simplifier statistics ==================== Total ticks: 60 18 PreInlineUnconditionally 11 PostInlineUnconditionally 5 UnfoldingDone 1 RuleFired 1 toInt/fromInt 2 EtaReduction 22 BetaReduction 1 KnownBranch 11 SimplifierDone