What are the defaulting rules for IsList? It needs to be backwards compatible.
-- Lennart (iPhone) On Sep 30, 2011, at 19:28, George Giorgidze <giorgi...@gmail.com> wrote: > GHC Users, > > I would like to make to the following two proposals: > * Eliminate the default grouping close from SQL-like comprehensions > * Introduce a GHC extension for list literal overloading > > OK, let me start with the first proposal. > > Currently, the SQL-like comprehension notation (both in its list > comprehension and monad comprehension variants) features the following five > clauses: > > then f > then f by e > then group by e > then group using f > then group by e using f > > The first two clauses are used for specifying transformations of type [a] -> > [a] (or Monad m => m a-> m a for monad comprehensions). The following three > clauses are used for specifying transformations of type [a] -> [[a]] (or > Monad m, Functor f => m a -> m (f a) for monad comprehensions). See [1] for > further details. > > Note that the third clause does not mention which function is used for > grouping. In this case GHC.Exts.groupWith function is used as a default for > list comprehensions and the mgroupWith function from the MonadGroup class is > used as a default for monad comprehensions. > > I would like to suggest to remove the third clause for the following reasons: > * Currently the syntax is asymmetrical. Note that there is the default case > for the 'then group' clause and not for the 'then' clause. > * In the current notation it is not clear which grouping function is used in > the default case > * For many monads including lists it is not clear which function should be > selected as a default (e.g., the groupWith function also does sorting and it > is not clear to me why this should be the default) > * Gets rid of the MonadGroup class. Currently the sole purpose of this class > is to introduce a default grouping function for monad comprehensions. > * Explicit mention of the grouping function would make monad/list > comprehensions much easier to read by making it immediately apparent which > function is used for grouping. > > My second proposal is to introduce the OverloadedLists extension that > overloads list literals. See Section 5.2 in [1] for details. > > Basically the idea is to treat list literals like: > > [1,2,3] > > as > > fromList [1,2,3] > > where > > class IsList l where > type Item l > fromList :: [Item l] -> l > > In the following I give useful instances of the IsList class. > > instance IsList [a] where > type Item [a] = a > fromList = id > > instance (Ord a) => IsList (Set a) where > type Item (Set a) = a > fromList = Set.fromList > > instance (Ord k) => IsList (Map k v) where > type Item (Map k v) = (k,v) > fromList = Map.fromList > > instance IsList (IntMap v) where > type Item (IntMap v) = (Int,v) > fromList = IntMap.fromList > > instance IsList Text where > type Item Text = Char > fromList = Text.pack > > As you can see the extension would allow list literals to be used for sets, > maps and integer maps. In addition the suggested OverloadedLists extension > would subsume OverloadedStrings extension (see the instance for Text, for > example). Having said that, for now, I am not suggesting to remove the > OverloadedStrings extension as it appears to be widely used. > > This extension could also be used for giving data-parallel array literals > instead of the special syntax used currently. > > Unless there is a vocal opposition to the aforementioned two proposals, I > would like to implement them in GHC. Both changes appear to be > straightforward to implement. > > Thanks in advance for your feedback. > > Cheers, George > > [1] http://www-db.informatik.uni-tuebingen.de/files/giorgidze/haskell2011.pdf > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
