Re: [Haskell-cafe] Polymorphic variants

On Thursday 02 August 2007, Jon Harrop wrote:

On Thursday 26 July 2007 00:07:23 Josef Svenningsson wrote:

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On 7/26/07, Jon Harrop wrote:

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Does Haskell have anything similar to OCaml's polymorphic variants?

No as such, but it's possible to simulate them. As always Oleg was the one to demonstrate how: http://okmij.org/ftp/Haskell/generics.html

Thank you.

Is there any interest in adding polymorphic variants to Haskell? I think it is fair to say that OCaml has shown them to be an extremely useful construct (more common than objects in OCaml, for example).

I think this topic goes along with the oft-discussed issue of what should be done about Haskell's record system. Several of the papers in that area use row polymorphism for typing extensible records, and the same thing can be used for typing polymorphic variants. Unfortunately, I guess it's hard to get everyone nailed down on what exactly the right solution is (and I guess there are also people who aren't interested in scrapping the existing record system), so not much has been done outside proposals. To my knowledge, no one has gone as far as implementing the proposals yet (which is a requirement for getting into Haskell', if that matters), although Hugs reportedly has a record system extension that inspired the newer proposals to some extent (I haven't used it myself). -- Dan P.S.: Some papers: http://www.cse.ogi.edu/PacSoft/publications/2000/extensiblerecords.pdf http://research.microsoft.com/users/daan/download/papers/scopedlabels.pdf The second actually discusses all the operations on variants and whatnot. The first just mentions that it's a related topic.