[erlang-patches] Allow limited pattern matching on magic binaries

Sverker Eriksson <>
Mon Apr 11 12:22:38 CEST 2011

Thank you for your explanation, but your patch was not rejected because 
of lack of understanding. (Hopefully the opposite)

I understand the need to match NIF resources. But the simple solution of 
just comparing the (magic) binary data is just not enough as it may 
cause very strange behavior that violates the functional properties of 
the language.

For example, the binary content of NIF resources are allowed to be 
mutated. Your binary comparison will expose such a mutation and violate 
the single assignment property. A variable referring a NIF resource may 
suddenly compare different to what it did earlier.

Another (too) simple solution could be to compare the pointers *to* the 
magic binaries instead of their content. That would on the other hand 
introduce a comparison that is based on identity (rather than equality), 
which we otherwise don't have in the language.

I think the right way to go, is to introduce a new callback for resource 
types, that allows the NIF resource designer to implement how 
comparisons should be done.

/Sverker, Erlang/OTP Ericsson

Evax Software wrote:
> Ok, this patch has just been rejected.
> Maybe I was unclear as to the goal of the patch, I'll try to explain it
> a bit better.
> NIF resources are implemented as magic binaries.
> Magic binaries are special binaries holding private data but presenting
> an apparent length of zero.
> I understand that this is a good thing as the private data is not
> exposed directly to the Erlang side, but this has a special side effect
> for NIF resource users: any resource will match any other resource (even
> from a different resource type, as well of course as the empty binary.
> This is somehow dangerous as it prevents us from catching some errors
> from the Erlang side, where for example assigning a resource to a
> variable already bound to a resource will NOT cause an error.
> Moreover not being able to distinguish NIF resources from Erlang causes
> some extra work. Imagine a NIF project implementing some kind of
> socket-like mechanism (erlzmq2 or libev-erlang for example), and imagine
> a process handling several of these sockets in a receive loop (as in
> active mode), then one must wrap resources in a tuple with a ref to be
> able to match on them.
> The patch implements special cases for magic binaries in the eq and cmp
> operations, causing the private data to be compared, and magic binaries
> with identical private data to match.
> So, and maybe the patch name was badly chosen, there is no real
> sub-binary pattern matching happening on private data here, it's just a
> simple and straightforward way to tell resources apart, still without
> exposing the private data to the Erlang side.
> Evax
> Le jeudi 17 mars 2011 à 11:59 +0100, Henrik Nord a écrit :
>> On 03/16/2011 09:41 PM, Evax Software wrote:
>>> Hi,
>>> The proposed patch allows limited pattern matching on magic binaries.
>>> It tries to be the as non-intrusive as possible.
>>> The goal of the patch is to allow matching on NIF resources, but it
>>> shouldn't cause problems to other magic binary beam users.
>>> git fetch :evax/otp.git pattern_match_on_magic_binaries
>>> Evax
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>> Hello
>> I have included this in 'pu' and will let it cook for a while.
>> /Henrik Nord, Erlang/OTP
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