[erlang-questions] Bit String Comprehension Question

Kostis Sagonas <>
Tue Sep 9 18:18:58 CEST 2008


David Mercer wrote:
> Why does the bit-string comprehension #133 below work whereas #134 is a 
> syntax error?
> 
> 133> << <<X:N>> || {X,N} <- [{3,5}] >>.
> <<3:5>>
> 
> 134> << Bin || Bin <- [<<3:5>>] >>.  
> * 1: syntax error before: '||'

In bit stream comprehensions, the elements of the resulting binary have 
to have an unambiguously defined size.  This means that they have to be 
a bit stream *segments*, and the parser requires segments to be enclosed 
in << and >>.  So, if you want to write the above, you have to write 
something like:

   << <<Bin/bits>> || Bin <- [<<3:5>>] >>.

which is just a convenient shorthand for the following expression, which 
you can also write:

   << <<Bin:(bit_size(Bin))/bits>> || Bin <- [<<3:5>>] >>.

(This because bit_size(Bin) is the default size for bits type segments.)

If you do not like this default expansion of "take all bits, please" you 
are also allowed to take less:

   << <<Bin:2/bits>> || Bin <- [<<3:5>>] >>.

Kostis




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