Re: [Haskell-cafe] question - which monad to use?

Matthias, Sorry if I was not clear in stating the problem. Your solution works nicely, but I would like to try writing a monad. This is what I came up with: type Failure = String data Computation a = Computation (Either Failure a) [a] instance Monad Computation where (Computation (Left e) h) >>= f = Computation (Left e) h -- do not proceed (Computation (Right a) h) >>= f = let r = f a -- result h' = case r of Left e -> h Right a' -> a':h in Computation r h' return (s,c) = Computation (Right (s,c)) [(s,c)] Basically, I want the >>= operator to call f on the last result, if it is not a failure, and append the new result to the list (if it didn't fail). However, I am getting the following error message: /home/tpapp/doc/research/pricespread/Main.hs:62:58: Couldn't match the rigid variable `b' against the rigid variable `a' `b' is bound by the type signature for `>>=' `a' is bound by the type signature for `>>=' Expected type: [b] Inferred type: [a] In the second argument of `Computation', namely `h' In the definition of `>>=': >>= (Computation (Left e) h) f = Computation (Left e) h I don't know what the problem is. Thanks, Tamas On Mon, Oct 02, 2006 at 03:54:23PM +0200, Matthias Fischmann wrote:

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 From: Tamas K Papp 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

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