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Re^5: Doing "it" only onceby BrowserUk (Patriarch) |
on Sep 22, 2005 at 17:32 UTC ( [id://494218]=note: print w/replies, xml ) | Need Help?? |
Hmm. I guess I stand corrected, but maybe you would explain the following:
When compiled without optimisations, the program runs out of memory when trying to sum the integers from 1 to 10,000,000. So, as you advised, I compile with optimisations. Now it performs the calculation in around 3 seconds. The first time. It then takes around 2 seconds to calculated the sum of 1 .. 5,000,000. Obviously, the intermediate value was not memoized. But ... Ask it to calculate 1 .. 10,000,000 a second time, and it now takes no time at all! One interpretation of this empirical evidence is that without optimisations enabled, it runs out of memory trying to retain all the intermediate return values. With the optimisations enabled, it only retains those values that are returned to the caller, and not the intermediate values.. The truth is (probably) somewhat more along the lines that the optimiser replaces the recursive algorithm with an iterative loop, thereby doing away with any recursive calls, and therefore, also doing away with the memoization of the intermediate function returns--as there are none. This would be a more convincing argument if I knew how to get more accurate timestamping, but none the less, both the empirical evidence and my (shallow) understanding of GHC internals, support the statement I made--every single function in Haskell is automatically memoized by the compiler--though I should have added a note to the effect that this only works for function calls that are not optimised away. If you have the time/inclination to explain this further I would be happy to have my understanding clarified. If you prefer to this in by email or elsewhere, use the address on my homenode. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.
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