I like both George's proposals. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of George Giorgidze | Sent: 30 September 2011 18:28 | To: glasgow-haskell-users@haskell.org | Subject: Two Proposals | | 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