On Mar 4, 2009, at 7:50 AM, Konrad Hinsen wrote:
> > Rich, > > is there a reason why metadata is explicitly disabled on function > objects? > > I find myself wanting to put metadata on functions frequently. It > seems even more important for functions than for anything else, given > that there is no way to inspect a function object at all, it's a > complete black box. So I looked at the implementation to see if it > would be easy to add metadata handling, and found to my surprise that > function objects already have a metadata pointer but that setting it > is disabled. > > Two situations where I think metadata on functions would be very > useful: > > - To put on a type tag. If functions are to be first-class data > objects, it should be possible to have multimethods dispatching on a > function argument. My concrete use case is stream-generating > functions with different interfaces that I would like to use with a > common interface defined as a multimethod. > > - For implementing automatic derivatives, it would be useful to > attach a derivative function to a mathematical function such as log > or sqrt. > It's not because metadata isn't useful for fns. The problem lies with the fact that fns are values, so if they support metadata they should also support with-meta, i.e. returning a new value with different metadata, vs the destructive metadata of reference types (i.e. alter- meta!). Supporting with-meta for fns requires they be copyable with new metadata (as are all the persistent collections and symbols). I haven't decided how best to support that copyability for fns that are closures while minimizing the overhead for fns that won't have metadata. Rich --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
