[Haskell-cafe] Re: Lists and monads

Kevin Jardine kevinjardine at gmail.com
Tue Jul 27 14:10:05 EDT 2010

Looks interesting.

I've also come across Data.List.Class:


Has anyone used that?


On Jul 27, 6:02 pm, wren ng thornton <w... at freegeek.org> wrote:
> Kevin Jardine wrote:
> > But as I said, that is just an example. I keep wanting to apply the
> > usual list tools but find that they do not work inside a monad. I find
> > myself wishing that f (m [a]) just automatically returned m f([a])
> Are you looking for these?
>      import Data.Traversable as T
>      T.sequence :: (T.Traversable t, Monad m) => t (m a) -> m (t a)
>      T.mapM :: (T.Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)
>      -- N.B. T.mapM ~ (T.sequence . fmap)
> > without me needing to do anything but I expect that there are reasons
> > why that is not a good idea.
> Not all functors can be distributed over arbitrary monads, so "not a
> good idea" is more like "not always possible" or "here be dragons"
> (fluffy, intriguing, and pretty dragons to be sure; but probably deeper
> than the answer you were looking for).
> In addition to Applicative, the Traversable and Foldable classes should
> be key tools in your toolbox. They take a number of functions typically
> restricted to lists and generalize them to different functors, often
> with Applicatives or Monads involved. The Typeclassopedia should have
> more on them.
> --
> Live well,
> ~wren
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