[erlang-questions] bit syntax

[Erik Søe Sørensen]

2012/11/30 Dmitry Klionsky <dm.klionsky@REDACTED>

> Hi!
>
> All below comes from http://www.erlang.org/doc/**programming_examples/bit_
> **syntax.html<http://www.erlang.org/doc/programming_examples/bit_syntax.html>
>
> ...
> Value:Size
> ...
> The Size part of the segment multiplied by the unit in the
> TypeSpecifierList (described below) gives the number of bits for the
> segment. In construction, Size is any
> expression that evaluates to an integer. In matching, Size must be a
> constant expression or a variable.
> ...
>
>
>
> And (2*8) is a legal Size:
>
> 7> <<Length, Value:(2*8)>> = X.
> <<2,1,0>>
>
> This works because (2*8) really evaluates at compile time and it equals to
>
> <<Length, Value:16>> = X.
>
>
>
>
> 12> <<Length, Value:(Length*8)>> = X.
> * 1: illegal bit size
>
> The expression (Length*8) is supported when constructing a binary. This is
> definitely a pattern matching, so as stated above: the Size must be a
> constant expression or a variable. Not an expression to be evaluated at
> runtime.
>
> As a workaround you can do this
>
> > <<Length, Rest/binary>> = X.
> > ValueLen = Length*8.
> > <<Value:ValueLen>> = Rest.


Or this:
 2> <<Length,Value:Length/unit:8>> = X.
<<2,1,0>>
3> {Length,Value}.
{2,256}

Multiplication of a length by a constant is a special case which is
supported through the 'unit' qualifier.

>
>
> 76> << [49, 50, 51, 52] >>.
> ** exception error: bad argument
>
> 77> X = "1234".
> "1234"
>
> 78> <<X>>.
> ** exception error: bad argument
>
> The only way you can do this is
>
> list_to_binary([49, 50, 51, 52]).
> and
> list_to_binary("1234").
>

It is a common source of confusion. The types of the items in bit syntax
are always determined at compile-time, not at runtime.
And for items without any qualifiers, the type is "8-bit integer" (i.e., a
byte).
The thing is that <<"ABC">> is *syntactic sugar* for <<65,66,67>>; this
(literal strings) is a special case, and it is a *syntactic* special case.
The runtime makes no decisions based on type, in particular, it does not
dynamically treat <<X>> differently depending on whether X is bound to an
integer or list value.[1]

/Erik

[1] Well, it does - it generates a type exception in the case of
non-integers, which I suppose counts as a difference... but not in the way
you intended.

BR,
> Dmitry
>
>
>
> On 11/30/2012 09:20 AM, 7stud wrote:
>
>> Hi,
>>
>> I have a couple of questions about the bit syntax.
>>
>> 1) I can read the first byte of a binary to get the length of the next
>> value:
>>
>> 1> X = <<16, 256:16>>.
>> <<16,1,0>>
>>
>> 2> <<Length, Value:Length>> = X.
>> <<16,1,0>>
>>
>> 3> Length.
>> 16
>>
>> 4> Value.
>> 256
>>
>>
>>
>> And (2*8) is a legal Size:
>>
>> 5> f().
>> ok
>>
>> 6> X = <<2, 256:16>>.
>> <<2,1,0>>
>>
>> 7> <<Length, Value:(2*8)>> = X.
>> <<2,1,0>>
>>
>> 8> Length.
>> 2
>>
>> 9> Value.
>> 256
>>
>>
>> So why does Size = Length*8 fail when Length=2?
>>
>> 10> f(Length).
>> ok
>>
>> 11> f(Value).
>> ok
>>
>> 12> <<Length, Value:(Length*8)>> = X.
>> * 1: illegal bit size
>>
>>
>>
>>
>> 2)  What is going on here:
>>
>>
>> 73> f().
>> ok
>>
>> 74> <<"1234">>.
>> <<"1234">>
>>
>> 75> "1234" == [49, 50, 51, 52].
>> true
>>
>> 76> << [49, 50, 51, 52] >>.
>> ** exception error: bad argument
>>
>> 77> X = "1234".
>> "1234"
>>
>> 78> <<X>>.
>> ** exception error: bad argument
>>
>> In line 74, I can use a string while constructing a binary, but in 76 I
>> can't use the equivalent list.  And in line 77, when I bind the string to a
>> variable, I can't use the variable to construct a binary.
>>
>> Thanks.
>> ______________________________**_________________
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>>
>
>
> --
> Best regards,
> Dmitry Klionsky
>
>
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