hi, i don't fully understand your problem, but perhaps you could use iterate to produce a list or type [Result a], ie, of all computation steps, and then use this function to extract either result or error from the list:
type Failmessage = Int data Result a = Root a | Failure Failmessage deriving (Show) f :: [Result a] -> Either a (Int, [Result a]) f cs = f [] cs where f (Root r:_) [] = Left r f l [Failure i] = Right (i, reverse l) f l (x:xs) = f (x:l) xs cs = [Root 1.2, Root 1.4, Root 1.38, Root 1.39121] cs' = [Root 1.2, Root 1.4, Root 1.38, Failure 1] -- f cs ==> Left 1.39121 -- f cs' ==> Right (1,[Root 1.2,Root 1.4,Root 1.38]) (although this way you probably have the list still floating around somewhere if you process the error returned by f, so f should probably just drop the traversed part of the list.) hth, matthias On Sun, Oct 01, 2006 at 06:00:43PM -0400, Tamas K Papp wrote: > To: Haskell Cafe <haskell-cafe@haskell.org> > From: Tamas K Papp <[EMAIL PROTECTED]> > Date: Sun, 1 Oct 2006 18:00:43 -0400 > Subject: [Haskell-cafe] question - which monad to use? > > Hi, > > I have a computation where a function is always applied to the > previous result. However, this function may not return a value (it > involves finding a root numerically, and there may be no zero on the > interval). The whole problem has a parameter c0, and the function is > also parametrized by the number of steps that have been taken > previously. > > To make things concrete, > > type Failmessage = Int -- this might be something more complex > data Result a = Root a | Failure Failmessage -- guess I could use Either too > > f :: Double -> Int -> Double 0 -> Result Double > f c0 0 _ = c0 > f c0 j x = {- computation using x, parameters calculated from c0 and j -} > > Then > > c1 = f c0 0 c0 > c2 = f c0 1 c1 > c3 = f c0 2 c2 > ... > > up to cn. > > I would like to > > 1) stop the computation when a Failure occurs, and store that failure > > 2) keep track of intermediate results up to the point of failure, ie > have a list [c1,c2,c3,...] at the end, which would go to cn in the > ideal case of no failure. > > I think that a monad would be the cleanest way to do this. I think I > could try writing one (it would be a good exercise, I haven't written > a monad before). I would like to know if there is a predefined one > which would work. > > Thank you, > > Tamas > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -- Institute of Information Systems, Humboldt-Universitaet zu Berlin web: http://www.wiwi.hu-berlin.de/~fis/ e-mail: [EMAIL PROTECTED] tel: +49 30 2093-5742 fax: +49 30 2093-5741 office: Spandauer Strasse 1, R.324, 10178 Berlin, Germany pgp: AD67 CF64 7BB4 3B9A 6F25 0996 4D73 F1FD 8D32 9BAA
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