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

[Tamas K Papp]

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 <haskell-cafe at haskell.org>
> > From: Tamas K Papp <tpapp at Princeton.EDU>
> > 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
> -- 
> Institute of Information Systems, Humboldt-Universitaet zu Berlin
> 
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