Strictness making it worst?

[Jorge Adriano]

Hi, I've just started messing around with strictness.  My goal now is 
understanding when and how to use it.
I began with simple examples like making a strict foldl. 
When trying to sum a list of 60000 elements with lazy foldl I'd get a stack 
space overflow, with a strict foldl I was able to sum a list of 1E8 elements 
:)

Then I thought I'd mess with the fibonnaci function. 
I began with:

> fibac                     :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos)
> fibac n (a,b) 
>               | n == 0    = (a,b)
>               | otherwise = fibac (n-1) (b,a+b)


Then I tried:

> sfibac                     :: IntPos -> (IntPos,IntPos) -> (IntPos,IntPos)
> sfibac n (a,b) 
>                | n == 0    = (a,b)
>                | otherwise = sfibac (n-1)   (b, (+b) $! a)

I tried both functions with ghc 5.00.1
The strict version was not only slower, but I'd get stack overflows for 
numbers that worked with the 1st version!! (like 100000)

I expect sfibac and fibac work this way:
assuming that 
a+b = c
b+c = d
c+d = e

sfibac:
sfibac 4 (a, b) ->
sfibac 3 (b, a+b) ->
sfibac 2 (a+b, b+(a+b)) ->
sfibac 1 (b+(a+b), (a+b)+(b+(a+b))) ->
sfibac 0 ((a+b)+(b+(a+b)), (b+(a+b)) + (a+b)+(b+(a+b)))

now if I had to evaluate the first element I'd get
(a+b)+(b+(a+b)) ->
c+(b+c) ->
c+d ->
e

fibac:
sfibac 4 (a, b) ->
sfibac 3 (b, a+b) ->
sfibac 2 (c, b+c) ->
sfibac 1 (d, c+d) ->
sfibac 0 (e, d+e) 


I'd expect sfibac to be faster, but what I find even more strange is the 
stack space overflow in sfibac! why?


Greetings,
J.A.