defrecord improvements

rh says:

Please don't work on any other aspect of this until the problem section is good.

Problems:

• completeness and consistency: need to be able to read and write records like other Clojure types
  • better - it's a problem that moving from maps to records compromises read/print
• dynamism: need to create defrecords without class-in-hand. Performantly, of course
  • rh - this smells like a desire, not a real problem for people
    • thus, low priority unless it falls out of something else
• convenience: want map-like construction instead of positional slotwise construction
  • but why do we want map-like construction?
    • what are the problems with positional construction?
    • can they only be solved by map-like construction?
• People often, but not always, write their own factory fns
  • not being standardized means people don't know what to expect
  • why are they doing that?
    • survey cases
      • optional fields
      • derived/private fields
      • factory fn gets pulled in from ns while class doesn't

Some history:

The record multimethod was almost ready to go when Rich raised the GC issue. What happens when somebody creates a ton of record classes over time? GC can collect records that are not longer in use, but doesn't clean up the old multimethod functions.

Open Issues

• print/read syntax
  • record factory function needs to enable print/read syntax for defrecords
  • for now, use #=(record ...) per Rich
    • rh - that was 'for then', we should do better now
  • caveat: forces people to allow arbitrary execution in order to deserialize records
  • is there some way to open record instantiation without opening arbitrary execution?
    • e.g. special reader syntax for records?
    • rh - I'm open to that as part of a complete design in this area
• GC friendliness
  • original proposed multimethod approach is a poor fit
    • would need some way to atomically remove old multimethod fns
    • and need to use two weak references (to class and to factory fn)
    • if you already need a weak map, and multimethods are in play just to map names -> factory fns, might as well skip multimethods and have just a weak map
  • record factory fn maintains map keyed by weak ref to class, value weak ref to class-specific factory fn
  • updates to factory fn also purge dead entries from the map (a la Clojure keywords)
• what about deftypes?
  • first guess: you are on your own

Additional Reading

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

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