Is it possible to write an dot-product function that GHC will compile to efficient code without using unsafeAt? I've tried and can't. I'm looking at generic IArray functions pragma-specialized to UArray Int Double. With unsafeAt the -ddump-simpl output looks optimal to me (aside from not unrolling or pipelining**):

loop i s | i > ubSI = s | otherwise = let s' = s + (small `unsafeAt` i) * (big `unsafeAt` (off + i)) in seq s' $ loop (i + 1) s'

Isn't that rather the wrong question? I'd be more interested in "How do I get ghc to generate efficient code for dotprod?" dotprod a b = sum (map e (indices a)) where e ix = a!ix * b!ix Especially since ghc does a pretty damn good job for this version {-# SPECIALISE dotprod :: UArray Int Double -> UArray Int Double -> Double #-} dotprod a b = foldr (+) 0 (map e (indices a)) where e ix = a!ix * b!ix (It's a pity that it can only eliminate the list for foldr, not the foldl used in sum). I'm not at all familiar with the intermediate code, so I don't know what in the output from the above is particularly inefficient. It does seem to do a lot of deconstructing and I've no idea how to get round that, but I'd much prefer to see a definition like dotprod in the Haskell code and all the mysteries and contortions done in ghc. Cheers, Jón PS It's a national holiday in the UK, so you might not hear anything from the ghc guys for a bit. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)