Hi all, I have a question about optimization of shared values in local definitions. I frequently use this scheme: fun a b c d x = r where q = costly computation depending only on a, b, c, and d r = depends only on q and x g1 = fun 1 2 3 4 g2 = fun 5 4 2 7 (etc.) When I compute (using ghc -O) things like map g1 [1 .. 1000] the common q is evaluated only once, which is very nice. But the problem is that in some strange cases this kind of optimization is not applied, and the same q is evaluated 1000 times. Curiously, this happens if I add just: module Main where to one of my programs. Optimization is also lost if in the same program I use two partially applied functions: map g1 [1 .. 1000] map g2 [1 .. 1000] And in some cases optimization is only applied if the local definition is "easy" enough. For example: --------------------------------------------------- fun1 :: Int -> Int -> Int fun1 a x = q*x where q = {-# SCC "easy" #-} a+1+a^2 -- OK fun2 :: Int -> Int -> Int fun2 a x = q*x where q = {-# SCC "hard" #-} a+1+a^2 +a^3+(2*a) -- NO fun3 :: Int -> Int -> Int fun3 a x = r where q = local a r = q*x local u = {-# SCC "local easy" #-} u+1 -- OK fun4 :: Int -> Int -> Int fun4 a x = r where q = local a r = q*x local u = {-# SCC "local hard" #-} u+1+u^2 -- NO test h = print $ sum $ map h [1 .. 100] main = do test (fun1 3) test (fun2 3) test (fun3 3) test (fun4 3) -------------------------------------------------- COST CENTRE MODULE no. entries MAIN MAIN 1 0 main Main 154 101 fun4 Main 166 300 local hard Main 167 100 NO fun3 Main 163 100 fun2 Main 161 100 hard Main 162 100 NO fun1 Main 157 100 test Main 155 4 CAF Main 148 6 fun4 Main 168 0 main Main 158 0 fun3 Main 164 1 local easy Main 165 1 OK fun1 Main 159 0 easy Main 160 1 OK test Main 156 0 CAF System.IO 103 1 CAF GHC.Handle 101 3 ------------------------------------------------------------ Where can I find information about this topic? I have made some searches but probably using wrong keywords. Should I use some individual optimization flags? Thanks, Alberto