On Fri, Dec 4, 2009 at 3:36 AM, Neil Brown <nc...@kent.ac.uk> wrote: > But let's say you have: > > g x y = f x y * f x y > > Now the compiler (i.e. at compile-time) can do some magic. It can spot the > common expression and know the result of f x y must be the same both times, > so it can convert to: > > g x y = let z = f x y in z * z
GHC does *not* do this by default, quite intentionally, even when optimizations are enabled. The reason is because it can cause major changes in the space complexity of a program. Eg. x = sum [1..10^6] + product [1..10^6] x' = let l = [1..10^6] in sum l + product l x runs in constant space, but x' keeps the whole list in memory. The CSE here has actually wasted both time and space, since it is harder to save [1..10^6] than to recompute it! (Memory vs. arithmetic ops) So GHC leaves it to the user to specify sharing. If you want an expression shared, let bind it and reuse. Luke _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
