[Haskell-cafe] Memoization-question

[Bertram Felgenhauer]

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_elimination.3F
(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