Looking at prelude/PrelRules.hs has reminded me of an old conundrum: if I switch from Int to Word, should I expect any performance differences? A while ago, I needed lots of fairly small positive numbers, together with a small number of flags for each, so I thought I'd switch from Int to Word, and map the flags to bits. But the performance dropped so drastically that I went back to Int, slightly complicating the bitmaps. I didn't expect that, and I can't see any builtin rules for Int that would have no Word counterpart. Here is a trivial example with drastic difference between T = Int and T = Word (~2.5x here): main = print $ foldl' (+) 0 [1..100000000::T] What am I missing here? Also, in the real code I ended up seeing things like this in the -ddump-simpl output for the bit-fiddling code: GHC.Prim.word2Int# (GHC.Prim.and# (GHC.Prim.int2Word# wild13_XbE) (GHC.Prim.int2Word# y#_a4EZ)) Is that likely to cost me a lot or are these conversions cheap? Claus