[erlang-questions] Why does fun2ms produce this particular result?

Mazen Harake <>
Wed May 4 09:03:44 CEST 2011

Well, yes.

1) Quoting documentation:

"ETS match specifications are there to produce a return value. Usually the
MatchBody contains one single ConditionExpression which defines the return
value without having any side effects. Calls with side effects are not
allowed in the ETS context.

When tracing there is no return value to produce, the match specification
either matches or doesn't. The effect when the expression matches is a trace
message rather then a returned term. The ActionTerm's are executed as in an
imperative language, i.e. for their side effects. Functions with side
effects are also allowed when tracing.

In ETS the match head is a tuple() (or a single match variable) while it is
a list (or a single match variable) when tracing."
So in other words (in the context of dbg): yes, it is a list because you can
have several "actions" in it. But in the context of ets there is usually
only 1 namely what you want to return, if you have more than 1 then the last
one will be returned.

2) This is correct.

I would say the opposite; I see it as semantics of both are similar enough
to use a "mini language" with exceptions rather than 2 different
specifications all together, but I guess that is just a matter of personal


On 4 May 2011 05:18, Daniel Dormont <> wrote:

> Thanks, let me summarize to see if this is correct:
> 1) In general (though not in the case of ETS/Mnesia), the MatchBody might
> contain actions to be performed before returning the final result. This is
> why it's a list.
> 2) Actions may be specified in the form of a tuple, the first element of
> which describes the action to be performed. Therefore, if you want to have
> your "action" be a value which is itself a tuple, you have to wrap it in
> another tuple.
> Is this right? If so, I guess I understand what the rules are, but I'm
> still not really clear on why, since it seems like dbg and ets are different
> enough that you'd never really use the same kind of MatchSpec for both, so
> there isn't much benefit in having the structure be compatible.
> dan
> On Mon, May 2, 2011 at 2:35 AM, Mazen Harake <>wrote:
>> Check out chapter 2 in my dbg tutorial, it describes matchspecs and how to
>> use them. You will also understand what it is that fun2ms actually
>> translates to.
>> http://www.erlang-factory.com/conference/testingtutorialworkshop2010/speakers/MazenHarake
>> Dbg matchspecs aren't exactly like the mnesia ones, there is a small
>> difference but the concept is exactly the same.
>> /M
>> On 1 May 2011 21:06, Daniel Dormont <> wrote:
>>> I'm trying to get the hang of match specs. Here's a pretty simple one but
>>> it has a feature I'm not getting. It's intended for use with Mnesia:
>>> 1> rd(person, {name, address, email, phone}).
>>> person
>>> 2> ets:fun2ms(fun(#person{name=Name, email=Email}) -> {Name, Email}
>>> end).
>>> [{#person{name = '$1',address = '_',email = '$2',
>>>           phone = '_'},
>>>   [],
>>>   [{{'$1','$2'}}]}]
>>> My first question was going to be why the whole thing is a single-element
>>> list, but I think I figured that out: it's because there could be multiple
>>> clauses of the match as a whole. The part I don't get is in the result:
>>> [{{'$1','$2'}}]
>>> 1) Why is it a list at all, since a particular head in the function can
>>> only have one result?
>>> 2) Why does the tuple have to be wrapped in another tuple? ie why isn't
>>> it just {'$1','$2'}
>>> For now I'm happy to just trust that fun2ms works, but I'd like to
>>> understand it a little better.
>>> thanks,
>>> Dan
>>> _______________________________________________
>>> erlang-questions mailing list
>>> http://erlang.org/mailman/listinfo/erlang-questions
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