[Haskell-cafe] Is it possible to read existential types?
Is is possible to read/show an existentially typed list? Would it be possible if, instead of using read/show, I defined some variant that relies on the typeof/typeable infrastructure? Is there a way to read in a typename for use in coercing a read? -Alex- _ S. Alexander Jacobson mailto:[EMAIL PROTECTED] tel:917-770-6565 http://alexjacobson.com ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Is it possible to read existential types?
S. Alexander Jacobson [EMAIL PROTECTED] wrote in article [EMAIL PROTECTED] in gmane.comp.lang.haskell.cafe: Is is possible to read/show an existentially typed list? Would it be possible if, instead of using read/show, I defined some variant that relies on the typeof/typeable infrastructure? Is there a way to read in a typename for use in coercing a read? Dylan Thurston and I encountered this problem a while ago. Show is easy: just use a type like exists a. Show a = Read is hard, because one needs to get the Read dictionary from somewhere, and the dynamic typing facility in Haskell does not include type-class constraint reduction at runtime. The best solution we came up with was to replace the existentially typed list with a list of string-string pairs, where the first string names the type and the second string names the value. The call to read is only done at lookup time, not at file-reading time. -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig Be it declared and enacted by this present Parliament / That the People of England / are / a Commonwealth and free State / without any King or House of Lords.-- An Act declaring England to be a Commonwealth 1649-05-19 | 355 years | 2004-05-19http://tinyurl.com/2dqnh ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data constructors
hi, i have thought about things like that, but the qualification Type.Constructor does not seem particularly useful. you can achieve the same by using _, e.g data A = A_X | A_Y data B = B_X | B_Y alternatively (at least for non-recursilve datatypes) anonymous sums (ala TREX's records) could work pretty well, but they are not in Haskell. details about those can probably be found in Ben Gaster's thesis. -iavor Mark Carroll wrote: I keep running into annoyance in having to name data constructors differently if they're for different types if they're in the same module or something. I wish that something like some Type.Constructor syntax worked in order to disambiguate. Or, better still, I have that problem with function names too (e.g. Data.List.union, Data.Set.union, IIRC) and it occurs to me that a lot of this can be resolved automatically because the types only make sense with one of the choices. I'm not really proposing any changes; more, I'm wondering what others' thinking is about this sort of thing - what annoys them, how they get around it, etc. -- Mark ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
