Erlang Language Data Types Bit Syntax: Defaults


Example

Clarification of Erlang doc on Bit Syntax:

4.4 Defaults

[Beginning omitted: <<3.14>> isn't even legal syntax.]

The default Size depends on the type. For integer it is 8. For float it is 64. For binary it is the actual size of the specified binary:

    1> Bin = << 17/integer, 3.2/float, <<97, 98, 99>>/binary >>. 
    <<17,64,9,153,153,153,153,153,154,97,98,99>>
      ^ |<-------------------------->|<------>|
      |            float=64           binary=24
  integer=8


    2> size(Bin). % Returns the number of bytes:
    12            % 8 bits + 64 bits + 3*8 bits = 96 bits => 96/8 = 12 bytes

In matching, a binary segment without a Size is only allowed at the end of the pattern, and the default Size is the rest of the binary on the right hand side of the match:

25> Bin = <<97, 98, 99>>. 
<<"abc">>

26> << X/integer, Rest/binary >> = Bin. 
<<"abc">>

27> X. 
97

28> Rest. 
<<"bc">>

All other segments with type binary in a pattern must specify a Size:

12> Bin = <<97, 98, 99, 100>>.         
<<"abcd">>

13> << B:1/binary, X/integer, Rest/binary >> = Bin. %'unit' defaults to 8 for  
<<"abcd">>                    %binary type, total segment size is Size * unit  

14> B.
<<"a">>

15> X.
98

16> Rest.
<<"cd">>

17> << B2/binary, X2/integer, Rest2/binary >> = Bin. 
* 1: a binary field without size is only allowed at the end of a binary pattern