You might use the Prelude function until: until :: (a -> Bool) -> (a -> a) -> a -> a until (> 3) (+ 2) 0 = 4 or for your purpose: until (\ a -> not (goOn(a, f(a))) f ainit http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A... http://www.haskell.org/onlinereport/prelude-index.html http://www.haskell.org/onlinereport/standard-prelude.html#$vuntil HTH Christian Tamas K Papp schrieb:
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