defrecord improvements

Problems:

• completeness and consistency: need to be able to read and write records like other Clojure types
• dynamism: need to create defrecords without class-in-hand. Performantly, of course
• convenience: want map-like construction instead of positional slotwise construction

Open questions:

• modularity friendliness
  • multimethods are a poor fit
    • need to have a map that we can clear entries from later
  • keep a weak ref to the entire block of code that instantiates objects
    • tested this at the REPL and it works fine
  • instead of a multimethod for records:
    • record keeps a ConcurrentHashMap of keywords => (weak-ref instantiating-fn)
    • every update to the map walks the queue and clears dead entries
    • similar to the way Clojure handles keywords today
• print/read syntax
  • for now, use #=(record ...)
  • caveat: forces people to allow arbitrary execution in order to deserialize records
  • Rich: is there some way to open record instantiation without opening arbitrary execution?
• fn/macro/both?
  • both
  • need macro for metadata from reader
• one fn, or fn-per-record?
  • one fn
  • otherwise you get namespace issues and name pollution

Note: the resolutions of these questions imply two significant changes to Stu's ancient patch

• weakref map instead of multimethod
• macro in addition to fn form

Tickets:

• print/read syntax for defrecords
• record multimethod

Some (non-contributed) code that demonstrates people's need for this:

• cemerick's defrecord slot defaults
• David McNeil's enhanced clojure records