[erlang-questions] bit operations/matching

Pieter Erlang <>
Thu May 8 20:43:21 CEST 2008


thx a lot...works now!
Pieter

On 5/8/08, Kostis Sagonas <> wrote:
>
> Pieter Erlang wrote:
>
>> Hi,
>>
>> Can someone explain why line 2 fails (and line 3 not)?
>> (also reproducible in other ways)
>>
>>
>> Erlang (BEAM) emulator version 5.6.2 [async-threads:0]
>>
>> Eshell V5.6.2 (abort with ^G)
>> 1> B = <<0:1000>>.
>> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>> 0,...>>
>> 2> <<_:80,Val:2,_/binary>> = B.
>> ** exception error: no match of right hand side value
>> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>> 0,...>>
>>
>
> Till a point, Erlang supported only binaries == sequences of bytes.
> Till then, the type qualifier that was to be used for matching against a
>  sequence of bytes was "binary".
>
> Starting with R12, this type qualifier has been properly renamed "bytes"
> (but "binary" has been maintained for backwards compatibility reasons) so
> that programmers like Pieter Erlang are less likely to be confused by
> matching failures:
>
> -------------------------------------------------------------------------
> Eshell V5.6.3  (abort with ^G)
> 1> B = <<0:1000>>.
> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>  0,...>>
> 2> <<_:80,Val:2,_/binary>> = B.
> ** exception error: no match of right hand side value
> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>                                                        0,...>>
> 3> <<_:80,Val:2,_/bytes>> = B.
> ** exception error: no match of right hand side value
> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>                                                        0,...>>
> -------------------------------------------------------------------------
>
> I would strongly recommend that programs using "binary" get changed to
> "bytes".
>
> If one really wants bit level pattern matching, the qualifier "bits" has
> been added for that purpose:
>
> -------------------------------------------------------------------------
> 4> <<_:80,Val:2,_/bits>> = B.
> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>  0,...>>
> 5> <<_:13,Val:69,_/bits>> = B.
> <<0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
>  0,...>>
> -------------------------------------------------------------------------
>
>
> Kostis
>
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