(= l ())
  (is-persisted l)
  (= s (table))
  (is-persisted s)

  (= (s 3) 4)
  (push 34 l)

  (save-state) ; redundant; see save-thread* below
  (quit)

  ; New session
  (load-state)

  s ; => #hash((3 . 4))
  l ; => (34)

  (= load-registry* () save-registry* ())

  (def load-state()
    (prn "*** Loading state")
    (each loadfn load-registry*
      (loadfn)))

  (def save-state()
    (each savefn save-registry*
      (savefn)))

  (def save-thread()
    (while t
      (sleep 10)
      (save-state)))
  (= save-thread* (new-thread save-thread))

  (mac is-persisted(var)
    (withs (save-function-name (symize "save-" var)
            load-function-name (symize "load-" var))
      `(do
        (def ,save-function-name()
          (errsafe:rmfile "snapshot.tmp") ;; XXX not thread-safe
          (fwrite "snapshot.tmp" ,var)
          (errsafe:mvfile "snapshot.tmp" (snapshot-name ,var)))
        (def ,load-function-name()
          (when (file-exists (snapshot-name ,var))
            (prn "Loading " ',var)
            (fread (snapshot-name ,var) ,var)))
        (push ,load-function-name load-registry*)
        (push ,save-function-name save-registry*))))

  (mac snapshot-name(var)
    `(+ "snapshot." ,(stringify var)))

  (mac fwrite(filename form)
    (let f (uniq)
      `(w/outfile ,f ,filename
          (if (isa ,form 'table)
            (write-nested-table ,form ,f)
            (write ,form ,f)))))

  (mac fread(filename form)
    (let f (uniq)
      `(w/infile ,f ,filename
          (if (isa ,form 'table)
            (= ,form (read-nested-table ,f))
            (= ,form (read ,f))))))

  ;; http://arclanguage.org/item?id=10677
  (def list-len(l)
    (if (acons l)
      (len l)
      0))
  (def alist? (l)
    (if (isa l 'cons)
      (all 2 (map list-len l))))

  (def converting-tablists(l)
    (if (alist? l)
      (listtab2 l)
      l))

  (def listtab2(al)
    (let h (table)
      (map (fn ((k v)) (= (h k) (converting-tablists v)))
           al)
      h))

  (def read-nested-table((o i (stdin)) (o eof))
    (let e (read i eof)
      (if (alist e) (listtab2 e) e)))

  (def tablist2(h)
    (if (isa h 'table)
      (accum a (maptable (fn (k v) (a (list k (tablist2 v)))) h))
      h))

  (def write-nested-table(h (o o (stdout)))
    (write (tablist2 h) o))

> My current approach is more efficient in I/O: registries for load and save functions, and a thread that periodically saves everything.

I think my improved solution (http://arclanguage.org/item?id=10696, just below) is actually the most I/O effective. Yours will rewrite the file(s) every 10 seconds, regarless of if the content has actually been modified. But I like the global thread idea.

In 'is-persisted:

  (fwrite "snapshot.tmp" ,var)
  (errsafe:mvfile "snapshot.tmp" (snapshot-name ,var)))

This brings me to: your code is difficult to read. You're defining, IMO, way too many clones/variants of existing Arc functions ('tablist2, 'listtab2, 'fwrite, etc...). Maybe they're actually needed for your case, but maybe take the time to dive into Arc source, and see if something existing is not fitting your needs. If you really need to define them, please include a short docstring or a comment to explain why '...2 is needed.

'alist?: ahhh I'm like you, I like the '?' convention for predicates, but Arc doesn't follow it, and use the ambiguous 'a... convention instead. IMO, it's better to follow the convention, even if bad, because otherwise, again, it makes the code inconsistent, difficult to read by others. 'alist? is a name too close to the existing 'alist and this is quite confusing. fallintothis' suggestion, 'an-alist is a good one IMO. Still, I laughed reading it, because well, that's where the 'a... convention brings us. 'an-a... LOL.

Lisp is so awesome to let you use "special" characters in identifiers, and predicates are not often used, and '?' is like '=' it's easily parsable, I can't understand the use of 'a... But this is maybe just personal taste. The ...[-]p convention of CL is worse than '?' but at least less ambiguous. Why is 'afn not [isa _ 'fn]? What about the anaphoric stuff, which also use 'a... etc. And of course, english-centric convention, which is worse than latin-centric when you can choose between the two.

In general: interesting idea and implementation. Still, I prefer either 'diskvar/'disktable because it's in vanilla Arc, either my 'db/'ptable because the code is, well I wrote it so I can't objectively judge but, easier, shorter to read, and it doesn't use macros. And 'db/'ptable is transparent, where your code is like 'disktable: half-transparent (still need to 'todisk/'save-state).

I like your alternative; weird that our comments kinda crossed (sorry I missed it before). I don't yet understand how it avoids unnecessary writes. I didn't appreciate how foundational sref is.

'ptable/'db? Because it only calls 'save-table when 'sref is called. 'sref is called when you modify/delete/create an element in the table.

  arc> (= sometbl!somekey 42)   ; 'sref is called, so is 'save-table (not immediately if using 'db)

> http://arclanguage.org/item?id=10677 / read-nested-table and write-nested-table

OK, I re-read it and I can understand now. Thanks. An unified interface is a good idea.

Every sref calls the corresponding buffer-exec savefn. The first call to buffer-exec in an interval spawns a thread to save after the interval.

I think I just had my head stuck in the 'iterative' way and had to twist a little to return to the event-driven approach.

  (= buffered-execs* (table))

  (def buffer-exec (f (o delay 1))
    (unless buffered-execs*.f
      (= buffered-execs*.f 
         (thread (sleep delay) (wipe buffered-execs*.f) (f)))))

  (= dbs* ())

  (def db (fname (o delay 0.5))  ; file "synced" every 0.5 sec
    (withs (tbl (safe-load-table fname)
    	    savefn (fn () (atomic:save-table tbl fname)))
      (push (list tbl (fn () (buffer-exec savefn delay))) dbs*)
      tbl))

  (let _sref sref
    (def sref (com val ind)
      (do1 (_sref com val ind)
        (awhen (and (isa com 'table) (alref dbs* com)) (it))))
  )