[Haskell-cafe] extensible data types in Haskell?

David Walker princedpw at gmail.com
Tue Jul 8 08:25:11 EDT 2008

thanks for everyone's help -- it seems the simplest solution is to use
the Typeable class, existential types and type-safe cast.


On Sun, Jul 6, 2008 at 9:18 PM, Don Stewart <dons at galois.com> wrote:
> princedpw:
>> Hi all,
>> SML conveniently contains the type "exn" which is an instance of an
>> "extensible data type".  In other words, unlike normal data types that
>> are "closed" (can't admit new constructors once defined), SML's exn
>> type is "open," allowing programmers to keep adding new alternatives
>> as often as they choose.  Like normal datatypes, the elimination form
>> for an extensible data type is a case statement (or match function).
>> Friends have told me that Haskell doesn't have extensible data types.
>> However, it seems fairly straightforward to code them up using type
>> classes....though the solution I'm thinking of has a little bit of
>> boilerplate I'd like to scrap (you have to define a new type
>> declaration *and* an instance of a type class with a "match" method)
>> and matching occurs through a string comparison (which can lead to
>> silly programmer errors if there is accidentally a typo in the
>> string).
> You should probably use Typeable here, for the type matching, rather
> than a custom matcher. class Typeable a => Extensible a, this leads to a
> fairly straighforward extensible data type, where the open instance
> definition lets you add variants on the fly.
>> Anyway, it's possible with some thought I could come up with a better
>> solution, but before worrying about it, I figured I'd ask if anybody
>> else already has a package that does this.  It seems like a pretty
>> natural feature to want to have.
> There's a number of ways to do this, including fully statically via type
> classes and existential types, or via the Dynamic type.
> Googling for "expression problem Haskell" will turn up some things. Some
> implementions of open data types in use can be found in xmonad, and the
> extensible exceptions proposal here,
> -- Don

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