A colleague of mine pointed out that ghc wasn't performing as he expected when optimising some code. I wonder if anyone could offer any insight as to why its not noting this common subexpression: main = print $ newton 4 24 newton a 0 = a newton a n = ((newton a (n-1))^2 + a)/(2*(newton a (n-1))) Compiled with 'ghc -O3 --make perf.hs', results in: real 0m5.544s user 0m5.492s sys 0m0.008s However if we factor out the repeated call to the newton method: main = print $ newton2 4 24 newton2 a 0 = a newton2 a n = (x^2 + a)/(2*x) where x = newton2 a (n-1) real 0m0.004s user 0m0.004s sys 0m0.004s It looks to me like Referential transparency should make this a sound optimisation to apply, but ghc isn't doing it even on -O3. regards, Richard