Matthias Fischmann fis at wiwi.hu-berlin.de
Mon Oct 2 09:54:23 EDT 2006

```
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:
> From: Tamas K Papp <tpapp at Princeton.EDU>
> Date: Sun, 1 Oct 2006 18:00:43 -0400
>
> 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
> _______________________________________________

--
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