Hi All. I am a student and a noob to Haskell. I am having some trouble with an example from the paper "Playing by the rules: Rewriting as a practical optimisation technique in GHC" by Simon Peyton Jones, Andrew Tolmach and Tony Hoare, specifically, the Short- cut Deforestation example in section 3.1. I was trying to compile the following using GHC version 6.4 on Mac OS X 10.4. The definition for build and the rule are from the paper (the rule also appears in the GHC online doc in section 7.10.1). ----------------------------------------------------- -- BOF -- File: Main.hs module Main where build :: (forall b. (a->b->b) -> b -> b) -> [a] build' g = g (:) [] {-# RULES "foldr/build" forall k z (g::forall b. (a->b->b) -> b -> b) . foldr k z (build g) = g k z #-} main :: IO () main = do putStr "" -- EOF ----------------------------------------------------- When I enable the extensions for GHC I get the following error: chris$ ghc -fglasgow-exts --make Main.hs Chasing modules from: Main.hs Compiling Main ( Main.hs, Main.o ) Main.hs:15:1: lexical error When I don't have them enabled it gives this error: chris$ ghc --make Main.hs Chasing modules from: Main.hs Compiling Main ( Main.hs, Main.o ) Main.hs:8:18: parse error on input `.' I have also tried moving the RULES option to the top of of the file above "module Main", but I still get the same errors. Also, should the definition of build be: build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] build g = g (:) [] If I try to load the Main.hs file in HUGS with the -98 option and the above version of build (with the forall a.), it works without a problem. However, it still gives the same errors in GHC. Additionally, I tried this on a different version of GHC, 6.2.2 on a x86 box running Gentoo Linux, and it yielded the same results. I am completely lost and would greatly appreciate any help. Thanks so much. - Chris.