On Fri, Oct 9, 2009 at 10:45 AM, Henning Thielemann < lemm...@henning-thielemann.de> wrote:
> > On Fri, 9 Oct 2009, Martijn van Steenbergen wrote: > > Hallo café, >> >> Can anyone think of datatypes that are Foldable but not Traversable? > > Data.Set.Set is a good example. The values contained in the Set are critical to the well-formedness of the data structure. Foldables let you iterate over a container while preserving structure, Traversables allow you to swap out the contents, but preserve the existing structure. Since you can do that you get fmap for free. > If not, what is the purpose of having a separate Foldable class? >> > > Recently I wondered, why Foldable is superclass of Traversable, since I > have examples where 'traverse' makes sense, but 'fold' not. > > http://www.haskell.org/pipermail/libraries/2009-February/011361.html > The superclass is there simply because there is a free implementation of foldMap (and fmap) given a Traversable instance, and it is useful to have access to all of those methods without building up a big long dictionary list, or having to clone them to make Traversable-specific versions) -Edward > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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