I was compiling ghc-6.13.20100831 from source (*) and then compiling repa-examples with that, and the generated executable says (when called with +RTS -N2): Most RTS options are disabled. Link with -rtsopts to enable them. Where? How? When? (Did I make some error earlier?) (*) that's the latest available src snapshot? (Sadly, it does not contain containers-0.4) How come there are binaries without sources at: http://www.haskell.org/ghc/dist/current/dist/ While "cabal install repa-examples" works nicely, it is still way too easy to break the system, since cabal install will happily (and silently) destroy ghc's libraries. (e.g. happens for me after installing syb) unfortunately, this means I cannot easily test ghc HEAD and/or containers-0.4 on my sources. J.W.