Chris Kuklewicz wrote:
Tamas K Papp wrote:
Hi,
I am a newbie learning Haskell. I have used languages with functional features before (R, Scheme) but not purely functional ones without side-effects.
Most of the programming I do is numerical (I am an economist). I would like to know how to implement the iterative algorithm below in Haskell.
f is an a->a function, and there is a stopping rule goOn(a,anext) :: a a -> Bool which determines when to stop. The algorithm looks like this (in imperative pseudocode):
a = ainit
while (true) { anext <- f(a) if (goOn(a,anext)) a <- anext else stop and return anext }
For example, f can be a contraction mapping and goOn a test based on the metric. I don't know how to do this in a purely functional language, especially if the object a is large and I would like it to be garbage collected if the iteration goes on.
Thank you,
Tamas _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
iterUntil :: (a -> a -> Bool) -> (a -> a) -> a -> a iterUntil goOn f aInit = let loop a = let a' = f a in if goOn a a' then loop a' -- tail recursive (so "a" will be collected) else a' in loop aInit
In Haskell you can do this iterUntil :: (a -> a -> Bool) -> (a -> a) -> a -> a iterUntil goOn f a | goOn a anext = iterUntil goOn f anext | otherwise = anext where anext = f a