Are you sure extensible records are needed, I used to really want them until I developed some idioms which seem to replace most needs for them. (1) Haskell Records just plain old haskell records can be used in an extensible fashion quite easily, just provide an alternate to the constructor which fills uninitialized fields with sane values. for example data Employee = Emp {empName :: String, empNumber :: Int} now, rather than creating them like john = Emp {empName = "John Meacham", empNumber = 7} provide a default value employee = Emp {empName = "", empNumber = 0} -- I tend to use the type -- name uncapitalized and use that instead of the constructor everywhere like so john = employee {empName = "John Meacham", empNumber = 7} now, to extend it, just add a new field to both the type and your constructor data Employee = Emp {empName :: String, empNumber :: Int, empIsDisgruntled :: Bool} employee = Emp {empName = "", empNumber = 0, empIsDisgruntled = False} now notice all your old code still works perfectly, the new fields just take on their default value john = employee {empName = "John Meacham", empNumber = 7} (2) Function Dispatch often times when you want extensible data types, you really dont even need data types at all, manual deforestation could lead to a nicely extensible framework. take the following type Width = Int data Doc = DocString String | DocVert [Doc] | DocHoriz [Doc] renderDoc :: Doc -> Width -> String renderDoc = {- complicated code -} now, you want to add the possibility of an image to your document but that would require modiying everywhere that Doc is deconstructed to account for that possibility. well, why not get rid of the Doc type alltogether? type Doc = Width -> String a document is now considered _anything_ that can render itself, no need for extensible types as any function of the appropriate type can be used as a document plus your code is easier to maintain. even if your entire structure cannot be boiled down to a function type, just parts of your structure can be. anywhere you want an extensible disjunction is a canidate. (3) Strafunski http://www.cs.vu.nl/Strafunski/ generic programming also makes this easy, (although it does require some commonly implemented extensions). since strafunski provides all your term traversal functionality, which is exactly where current datatypes fall down when you try to extend them, you can usually modify your data types all you want and use your 'generic' functions implemented with strafunski on them. see the web page for much better examples than I can come up with. sorry if this post is way off base, but perhaps someone will find it useful. (this should go in the Wiki. poor Wiki.) John On Wed, Nov 06, 2002 at 09:37:45AM +0100, Nicolas Oury wrote:
I need something that can allow to use records without declaring their type first and that can be extended by creating a new records with the same contents plus one or less one. --
John Meacham - California Institute of Technology, Alum. - john@foo.net ---------------------------------------------------------------------------