Question

Short version: is it safe to use ets:foldl to delete every ETS record as one is iterating through them?

Suppose an ETS table is accumulating information and now it’s time to process it all. A record is read from the table, used in some way, then deleted. (Also, assume the table is private, so no concurrency issues.)

In another language, with a similar data structure, you might use a for…each loop, processing every record and then deleting it from the hash/dict/map/whatever. However, the ets module does not have foreach as e.g. lists does.

But this might work:

1> ets:new(ex, [named_table]).
ex
2> ets:insert(ex, {alice, "high"}).
true
3> ets:insert(ex, {bob, "medium"}).
true
4> ets:insert(ex, {charlie, "low"}).
true
5> ets:foldl(fun({Name, Adjective}, DontCare) ->
      io:format("~p has a ~p opinion of you~n", [Name, Adjective]),
      ets:delete(ex, Name),
      DontCare
   end, notused, ex).
bob has a "medium" opinion of you
alice has a "high" opinion of you
charlie has a "low" opinion of you
notused
6> ets:info(ex).
[...
 {size,0},
 ...]
7> ets:lookup(ex, bob).
[]

Is this the preferred approach? Is it at least correct and bug-free?

I have a general concern about modifying a data structure while processing it, however the ets:foldl documentation implies that ETS is pretty comfortable with you modifying records inside foldl. Since I am essentially wiping the table clean, I want to be sure.

I am using Erlang R14B with a set table however I’d like to know if there are any caveats with any Erlang version, or any type of table as well. Thanks!

Answer 1

Your approach is safe. The reason it is safe is that ets:foldl/3 internally use ets:first/1, ets:next/2 and ets:safe_fixtable/2. These have the guarantee you want, namely that you can kill elements and still get the full traverse. See the CONCURRENCY section of erl -man ets.

For your removal of all elements from the table, there is a simpler one-liner however:

ets:match_delete(ex, '_').

although it doesn’t work should you want to do the IO-formatting for each row in which case your approach with foldl is probably easier.