Mattias Bengtsson wrote:
The program below computes (f 27) almost instantly but if i replace the definition of (f n) below with (f n = f (n - 1) * f (n -1)) then it takes around 12s to terminate. I realize this is because the original version caches results and only has to calculate, for example, (f 25) once instead of (i guess) four times. There is probably a good reason why this isn't caught by the compiler. But I'm interested in why. Anyone care to explain?
GHC does "opportunistic CSE", when optimizations are enabled. See http://www.haskell.org/haskellwiki/GHC:FAQ#Does_GHC_do_common_subexpression_... (http://tinyurl.com/33q93a) I've found it very hard to predict whether this will happen or not, from a given source code, because the optimizer will transform the program a lot and the "opportunistic CSE" rule may apply to one of the transformed versions. It's best to make sharing explicit when you need it, as you did below.
main = print (f 27)
f 0 = 1 f n = let f' = f (n-1) in f' * f'
Bertram