[erlang-questions] Binaries and Core Erlang

Richard Carlsson <>
Sun Jan 13 21:46:04 CET 2013


I think the short answer is that the Core Erlang spec was not updated 
when bitstrings were added.

     /Richard

On 2013-01-13 18:00, Aggelos Giantsios wrote:
> Hello everybody!!
>
> Let's assume we have the following code:
>
> -module(foo).
> bar() -> <<1:3>>.
>
> In R15B03, calling foo:bar() evaluates to <<1:3>> as we should expect.
>
> Should you compile the module to Core Erlang the resulting code is
>
> 'foo'/0 =
>      fun () ->
> #{#<1>(3,1,'integer',['unsigned'|['big']])}#
>
> and by using the core_scan and core_parse modules the resulting AST in
> record format of foo is
>
> {c_fun,[],[],
>         {c_binary,[],
>                   [{c_bitstr,[],
>                              {c_literal,[],1},
>                              {c_literal,[],3},
>                              {c_literal,[],1},
>                              {c_literal,[],integer},
>                              {c_literal,[],[unsigned,big]}}]}}
>
> Having said that, let's try to evaluate the above #c_fun{}.
> #c_fun{} has no parameters so we use an empty set of bindings and we
> evaluate #c_binary{}.
> #c_binary{} evaluates to the concatenation of the [#c_bitstr{}]. We have
> one #c_bitstr{} so the resulting valu is the value of #c_bitstr{}.
> #c_bitstr{} evaluates to <<1:3/unsigned-big-integer-unit:1>> that equals
> to <<1:3>> making this the overall value of the #c_fun{}, which is
> consistent with what we got from the interpreter.
>
> However, in the Core Erlang 1.0.3 language specs, it explicitly says
> that the number of bits in the resulting value of a #c_binary{} must be
> divisible by 8 making it a valid Erlang binary. In this case, the
> #c_binary{} evaluates to the bitstring <<1:3>> which is not a valid
> binary. So, in theory, we should have raised an exception at this point.
> But this is not what happens when we evaluate foo:bar/0 in the interpreter.
>
> So is there an inconsistency with the Core Erlang specs or am I missing
> something?
>
> Thanks,
> Aggelos
>
>
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