Am Donnerstag, 11. Dezember 2008 21:11 schrieb Bulat Ziganshin: > Hello Daniel, > > Thursday, December 11, 2008, 11:09:46 PM, you wrote: > > you is almost right. but ghc don't share results of function calls > despite their type. it just assumes that value of any type may use a > lot of memory even if this type is trivial :)
I may misunderstand you here. But if you give a type signature specifying a nice known type, I'm pretty sure ghc _does_ sharing (tried with Int -> Int and Integer -> Integer, g 200 instantaneous), at least with -O2. Without sharing, it would require 2^(n+1) - 1 calls to evaluate g n, that wouldn't be nearly as fast. Without the type signature, it must assume the worst, so it doesn't share. > > example when automatic sharing is very bad idea is: > > main = print (sum[1..10^10] + sum[1..10^10]) Depends on what is shared. Sharing the list would be a very bad idea, sharing sum [1 .. 10^10] would probably be a good idea. > > > Am Donnerstag, 11. Dezember 2008 16:18 schrieb Mattias Bengtsson: > >> 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? > >> > >> > main = print (f 27) > >> > > >> > f 0 = 1 > >> > f n = let f' = f (n-1) > >> > in f' * f' > >> > >> (compiled with ghc --make -O2) > >> > >> Mattias > > > > Not an expert, so I may be wrong. > > The way you wrote your function, you made it clear to the compiler that > > you want sharing, so it shares. > > With > > > > g 0 = 1 > > g n = g (n-1)*g (n-1) > > > > it doesn't, because the type of g is Num t => t -> t, and you might call > > it with whatever weird Num type, for which sharing might be a bad idea > > (okay, for this specific function I don't see how I would define a Num > > type where sharing would be bad). > > If you give g a signature like > > g :: Int ->> Int, > > > the compiler knows that sharing is a good idea and does it (cool thing > > aside: with > > module Main where > > > > f 0 = 1 > > f n = let a = f (n-1) in a*a > > > > main = do > > print (f 27) > > print (g 30) > > > > g 0 = 1 > > g n = g (n-1)*g (n-1) > > > > main still runs instantaneously, but g n takes exponential time at the > > ghci prompt. That's because in main the argument of g is defaulted to > > Integer, so it's shared.) > > _______________________________________________ > > Haskell-Cafe mailing list > > [email protected] > > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
