Re: [Haskell-cafe] Data.Tree.Zipper in the standard libraries

Using 'monad' here makes it easier to make a mistake with the code, as it permits new kinds of unexpected failure. This is Haskell, we should use Maybe. And users that want it can lift Maybe a -> m a -- Don (-1) for new uses of fail in place of Nothing. kr.angelov:

The monads design is used in Data.Map i.e.

lookup :: (Monad m, Ord k) => k -> Map k a -> m a

and I think that this will be more consistent.

On 5/23/08, Ross Paterson wrote:

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On Fri, May 23, 2008 at 09:03:29AM +0200, Krasimir Angelov wrote:

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Alternatively I can use monad with failure. In other words, there are two possibilities:

1. Use error ".." and types like: TreeLoc a -> TreeLoc a 2. Use monad and type like: Monad m => TreeLoc a -> m (TreeLoc a)

I'd suggest that TreeLoc a -> Maybe (TreeLoc a) is better than the second version. The problem with using Monad(fail) is that it hides possible runtime errors among testable conditions. With 1. you can at least search the code for occurrences of the dangerous function. With the Monad version you need to consider the type of each use to know whether it is dangerous. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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