On Tuesday 13 February 2007 22:32, Bernie Pope wrote:
Creighton Hogg wrote: [...]
So for example in the case of, facTail 1 n' = n' facTail n n' = facTail (n-1) (n*n')
The problem with this example is that it will build up an expression of the form:
(n1 * n2 * n3 .....) [...]
This is not true if one takes strictness analysis into account: facTail is strict in both arguments, and any decent strictness analyser will detect this, e.g. GHC with -O. Strict arguments can be evaluated before the function call, so in the example above no stack space will be consumed and the above will basically be a good old loop. For the brave: Use "ghc -v4 -O" on the example above to see what really happens. GHC is even clever enough to factor out (un-)boxing (at least for Int and friends), so there is a rather tight loop with unboxed Ints. Cheers, S.