[原创]erlang二进制语法点滴

Erlang/OTP 17 [erts-6.0] [source] [smp:2:2] [async-threads:10]
[kernel-poll:false]

Eshell V6.0  (abort with ^G)
1>
<<100,200,300,400>>.
<<100,200,44,144>>
2>
<<100,200,300:16,400:16>>.
<<100,200,1,44,1,144>>
说明了一个segment默认是8位,高于8位的部分被截断

同理
1>
A = <<15768105>>.
<<")">>
2> io:format("~.16B",
[15768105]).
F09A29ok

16进制29是10进制的41,由此可以看出<<15768105>>其实等于<<41>>

但是
1>
<<41>> = <<15768105>>.
<<")">>
2>
<<15768105>> = <<41>>.
** exception error: no match
of right hand side value <<")">>
3>  <<1:1>>
= <<3:1>>.
<<1:1>>
4> <<3:1>> =
<<1:1>>.
** exception error: no match of right hand side value
<<1:1>>
5> <<5:2>> = <<13:2>>.
**
exception error: no match of right hand side value <<1:2>>
6>
A = <<15768105>>.
<<")">>
7> B =
<<41>>.
<<")">>
8> A =
B.
<<")">>

这些该如何解释呢?

-------------------------------------------------

&
基本形式:
Value:Size/TypeSpecifierList
TypeSpecifierList包括
Type(integer,
float, binary, bitstring),
Signedness(signed, unsigned),
Endianness(big,
little, native),
Unit(1-256)
e.g.

X:4/little-signed-integer-unit:8
<<X:4/big-signed-float-unit:16>>
= <<100.5/float>>.

&
默认值
类型默认为integer
Size的默认项:
integer默认8位,float默认64位
binary或bitstring处于尾部时默认匹配全部
Unit的默认项:
integer和float和bitstring为1,
binary为8
Signedness默认为unsigned
Endianness默认为big

&
词法注意
B=<<1>>需要写成B =
<<1>>
<<X+1:8>>需要写成<<(X+1):8>>
匹配时Value和Size必须是常量或已绑定的变量
下面的形式会报N
is unbound错,
foo(N, <<X:N,T/binary>>) ->
  
{X,T}.
正确的写法:
foo(N, Bin) ->
  
<<X:N,T/binary>> = Bin,
   {X,T}.

&
Unit
最好不要改binary和bitstring的Unit

&
语法糖
<<"hello">>是<<$h,$e,$l,$l,$o>>的语法糖

&
常用写法
取尾部的写法:
foo(<<A:8,Rest/binary>>)
->
foo(<<A:8,Rest/bitstring>>)
->
高效拼binary的写法:
triples_to_bin(T) ->
   
triples_to_bin(T, <<>>).

triples_to_bin([{X,Y,Z} | T], Acc)
->
    triples_to_bin(T,
<<Acc/binary,X:32,Y:32,Z:32>>);   % inefficient before
R12B
triples_to_bin([], Acc) ->
    Acc.

参照:http://www.erlang.org/doc/programming_examples/bit_syntax.html

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