On 5 October 2010 15:41, George Giorgidze <giorgi...@gmail.com> wrote: > One can also look at how recently introduced 'order by' and 'group by' > constructs generalise to monad comprehensions. If that works, one could > implement even more "stylish" monad comprehension notation.
They do: see the comments by MichaelAdams at http://haskell.org/haskellwiki/Simonpj/Talk:ListComp. Last I checked, the code there was slightly buggy but correct in spirit. What *doesn't* generalise is the zip comprehensions extension: [(x, y) | x <- xs | y <- ys] == zip xs ys The required operator :: m a -> m b -> m (a, b) is that of the ZipList applicative functor, not that of the standard applicative functor for lists. Probably to generalise this you need a new typeclass like this one (copied from my own library): class Functor z => Zippable z where -- Naturality: -- fmap (first f) (zip_ as bs) == zip_ (fmap f as) bs -- fmap (second f) (zip_ as bs) == zip_ as (fmap f bs) -- -- Information preservation: -- fmap fst (zip_ as bs) == as -- fmap snd (zip_ as bs) == bs zip_ :: z a -> z b -> z (a, b) zip_ = zipWith_ (,) zipWith_ :: (a -> b -> c) -> z a -> z b -> z c zipWith_ f as bs = fmap (uncurry f) (zip_ as bs) It probably needs some extra laws to say how it interacts with the Monad operators. > * Do you think that it would be hard to integrate this extension into > current GHC codebase? Pretty easy IMHO. The list comprehensions are already half-set up for this job, and you should be able to reuse lots of the code that handles the monad notation desugaring. > * Have you already thought about how to generalise 'order by' and 'group > by' to monad comprehensions? See above. > * Have you already thought about how to address the original objections to > the monad comprehension notation? I thought it was rejected because it caused newbies to get confusing type error messages: they expected *list* error messages but got errors mentioning a scary *Monad* thing. Personally I'm not sure how to solve that, but if it's only available as an extension this won't cause a problem. Cheers, Max _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
