Re: [Haskell-cafe] abstract extensible types?
On 2008 Nov 12, at 5:38, Alberto G. Corona wrote: Is there any abstract container that permits the addition of new types of data? I know how to simulate the extension of Algebraic datatypes, but this does not permit the addition of data with new types in the same container and recover them in a type-safe way. Did I reinvent the Weel? I found something, that permits this for any Typeable datatype. For example I think you want http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Dynamic.html . -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] abstract extensible types?
http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf This lets you create sets of types to store in a container, with the static guarantee that only members of the set of types are included. The types can contain other elements of the set within them recursively. To extract value from the container, you can provide observation algebras that work on subsets of these values. It's a great solution to this problem and avoids the anything with Typeable issue that you get using Dynamic, which adds lots of run-time failure cases if something you didn't expect makes it into your data. -- ryan 2008/11/12 Alberto G. Corona [EMAIL PROTECTED]: Is there any abstract container that permits the addition of new types of data? I know how to simulate the extension of Algebraic datatypes, but this does not permit the addition of data with new types in the same container and recover them in a type-safe way. Did I reinvent the Weel? I found something, that permits this for any Typeable datatype. For example x=5 list= [put x, put hello] [t1,t2 ]= list x' = get t1 :: Int s = get t2 :: String print (x' +1) -- 2 print s-- hello x''= get t2 :: Int--get: casting from String to type Int The code: data Abstract= forall a. Typeable a = T !String a class FromToAbstract x where put :: x - Abstract get :: Abstract - x unsafeGet :: Abstract - x -- get(put x)== x instance Typeable x = FromToAbstract x where put x= T (typeString x) x get (Abstract type1 a)= if type2 == type1 then v else error (get: casting ++ from type ++type1++ to type ++type2) where v= unsafeCoerce a :: x type2= typeString v unsafeGet (Abstract type1 a)= unsafeCoerce a typeString !x= tyConString $ typeRepTyCon $ typeOf x instance Typeable T where typeOf _= mkTyConApp (mkTyCon Abstract) [] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] abstract extensible types?
Is there any abstract container that permits the addition of new types of data? I know how to simulate the extension of Algebraic datatypes, but this does not permit the addition of data with new types in the same container and recover them in a type-safe way. Did I reinvent the Weel? I found something, that permits this for any Typeable datatype. For example x=5 list= [put x, put hello] [t1,t2 ]= list x' = get t1 :: Int s = get t2 :: String print (x' +1) -- 2 print s-- hello x''= get t2 :: Int--get: casting from String to type Int The code: data Abstract= forall a. Typeable a = T !String a class FromToAbstract x where put :: x - Abstract get :: Abstract - x unsafeGet :: Abstract - x -- get(put x)== x instance Typeable x = FromToAbstract x where put x= T (typeString x) x get (Abstract type1 a)= if type2 == type1 then v else error (get: casting ++ from type ++type1++ to type ++type2) where v= unsafeCoerce a :: x type2= typeString v unsafeGet (Abstract type1 a)= unsafeCoerce a typeString !x= tyConString $ typeRepTyCon $ typeOf x instance Typeable T where typeOf _= mkTyConApp (mkTyCon Abstract) [] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
