# using less stack

**Olaf Chitil
**
olaf@cs.york.ac.uk

*Wed, 20 Mar 2002 19:05:37 +0000*

>* with the cpsfold I get
*>*
*>* f x1 a (\y1 -> f x2 y1 (\y2 -> f x3 y3 (\y3 -> f x4 y4 (\y4 -> y4)
*>*
*>* so x1 and a are available immediately for f to use, and f x1 a is the
*>* outermost expression so will be evaluated first.
*
Yes, however, if f just calls its continuation without forcing the
evaluation of at least its second argument, e.g.
f x y k = k (g x y)
you get
f x1 a (\y1 -> f x2 y1 (\y2 -> f x3 y3 (\y3 -> f x4 y4 (\y4 -> y4))))
=> f x2 (g x1 a) (\y2 -> f x3 y3 (\y3 -> f x4 y4 (\y4 -> y4)))
=> f x3 (g x2 (g x1 a)) (\y3 -> f x4 y4 (\y4 -> y4)))
=> f x4 (g x3 (g x2 (g x1 a))) (\y4 -> y4)
=> g x4 (g x3 (g x2 (g x1 a)))
like the foldr.
>* > larger x y = if x > y then x else y
*>* > cpslarger x y k = if x > y then k x else k y
*
Yes, with this definition of `cpslarger' no stack is used, because the
comparison forces evaluation.
With
cpslarger x y k = k (larger x y)
it does not work.
Still, if your definition of cpslarger is natural for your application,
it is a nice solution of the problem.
Ciao,
Olaf
--
OLAF CHITIL,
Dept. of Computer Science, The University of York, York YO10 5DD, UK.
URL: http://www.cs.york.ac.uk/~olaf/
Tel: +44 1904 434756; Fax: +44 1904 432767