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I'm looking to construct a bit string bit by bit, and am wondering how to do so.
I was expecting the syntax to be similar to concatenating two bytes like so:
iex(1)> <<1>> <> <<1>>
<<1, 1>>
So I tried:
iex(2) <<1::1>> <> <<1::1>>
** (ArgumentError) argument error
Is this possible? Thanks in advance.
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By shifting the first byte by 4 bits to the left (first<<4) you'll add 4 trailing zero bits. Second part (second>>4) will shift out to the right 4 LSB bits of your second byte to discard them, so only the 4 MSB bits will remain, then you can just bitwise OR both partial results ( | in python) to combine them.
Bit String Segments By default a Bit String segment represents 8 bits, also known as 1 byte. You can also specify a bit size using either short hand or long form.
A bit string (also called a word) is a sequence of bits of some set length. Usually the length is some small power of 2: 4, 8, 16, 32, 64). A byte is a bit string of length 8. There are 256 different bytes.
I'm not exactly sure if it's a bug or not, but let's explore what's going on and worry about that later.
What is <>? It turns out it's just a macro defined in Kernel.<>/2. What can we do with macros to understand them better? Expand them!
quote(do: <<1::1>> <> <<1::1>>)
|> Macro.expand(__ENV__)
|> Macro.to_string
#=> "<<(<<1::1>>::binary), (<<1::1>>::binary)>>"
We can see that <> desugars to the normal binary syntax. Unfortunately for us it assumes it's arguments are binaries! We have bitstrings - hence the error. How to fix it? We can use the regular binary/bitstring syntax directly:
<< <<1::1>>::bitstring, <<1::1>>::bitstring >>
#=> <<3::size(2)>>
Which works as expected.
EDIT: I followed up on this. This behaviour is expected. <> operator is designed to work on binaries and not bitstrings. The error is rather unpleasant, but it's coming deep from within Erlang.
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