Hi, I've successfully built ghc-6.10 with ghc-6.6.1; there was one minor problem: Building extensible-exceptions-0.1.0.0... Control/Exception/Extensible.hs:2:13: cannot parse LANGUAGE pragma ghc 6.6.1 does not know about DeriveDataTypeable - I just removed that line. Software used: ghc-6.6.1 alex 2.2 happy 1.17 Cabal 1.4.0.1 (not relevant, I believe) gcc 4.1.2 (may be relevant because of cpp) linux 2.6.24.4 I'm not sure what Magicloud's problem is, but it's related to alex; the contents of .../alex-2.3/AlexTemplate-ghc (whereever alex was installed) would be interesting -- does it contain any lines starting with # ? This template is generated from templates/GenericTemplate.hs while building alex. Another idea is that {-# OPTIONS -fglasgow-exts -cpp #-} is somehow missing from the generated utils/genprimopcode/Lexer.hs file in the ghc tree - but I can't really see how that would happen. Bertram