How To Convert An Elixir Binary To A String?

问题:

So I'm trying to convert a binary to a string. This code:

t = [{<<71,0,69,0,84,0>>}]
String.from_char_list(t)

But I'm getting this when I try this conversion:

** (ArgumentError) argument error
    (stdlib) :unicode.characters_to_binary([{<<70, 0, 73, 0, 78, 0>>}])
    (elixir) lib/string.ex:1161: String.from_char_list/1

I'm assuming the <<70, 0, etc. is likely a list of graphemes (it's the return from an API call and the API is not quite documented) but do I need to specify the encoding somehow?

I know I'm likely missing something obvious (maybe that's not the right function to use?) but I can't seem to figure out what to do here.

---

EDIT:

For what it's worth, the binary above is the return value of an Erlang ODBC call. After a little more digging I found that the binary in question is actually a "Unicode binary encoded as UTF16 little endian" (see here: http://www.erlang.org/doc/apps/odbc/odbc.pdf pg. 9 re: SQL_WVARCHAR) Doesn't really change the issue but it does add some context.

回答1:

There's a couple of things here:

1.) You have a list with a tuple containing one element, a binary. You can probably just extract the binary and have your string. Passing the current data structure to to_string is not going to work.

2.) The binary you used in your example contains 0, an unprintable character. In the shell, this will not be printed properly as a string, due to the fact that Elixir can't tell the difference between just a binary, and a binary representing a string, when the binary representing a string contains unprintable characters.

3.) You can use pattern matching to convert a binary to a particular type. For instance:

iex> raw = <<71,32,69,32,84,32>>
...> Enum.join(for <<c::utf8 <- raw>>, do: <<c::utf8>>)
"G E T "
...> <<c::utf8, _::binary>> = raw
"G"

Also, if you are getting binary data from a network connection, you probably want to use :erlang.iolist_to_binary, since the data will be an iolist, not a charlist. The difference is that iolists can contain binaries, nested lists, as well as just be a list of integers. Charlists are always just a flat list of integers. If you call to_string, on an iolist, it will fail.

回答2:

I made a function to convert binary to string

def raw_binary_to_string(raw) do
   codepoints = String.codepoints(raw)  
      val = Enum.reduce(codepoints, 
                        fn(w, result) ->  
                            cond do 
                                String.valid?(w) -> 
                                    result <> w 
                                true ->
                                    << parsed :: 8>> = w 
                                    result <>   << parsed :: utf8 >>
                            end
                        end)

  end

Executed on iex console

iex(6)>raw=<<65, 241, 111, 32, 100, 101, 32, 70, 97, 99, 116, 117, 114, 97, 99, 105, 111, 110, 32, 65, 99, 116, 117, 97, 108>>
iex(6)>raw_binary_to_string(raw)
iex(6)>"Año de Facturacion Actual"

回答3:

Not sure if OP has since solved his problem, but in relation to his remark about his binary being utf16-le: for specifically that encoding, I found that the quickest (and to those more experienced with Elixir, probably-hacky) way was to use Enum.reduce:

# coercing it into utf8 gives us ["D", <<0>>, "e", <<0>>, "v", <<0>>, "a", <<0>>, "s", <<0>>, "t", <<0>>, "a", <<0>>, "t", <<0>>, "o", <<0>>, "r", <<0>>]
<<68, 0, 101, 0, 118, 0, 97, 0, 115, 0, 116, 0, 97, 0, 116, 0, 111, 0, 114, 0>>  
|> String.codepoints()
|> Enum.reduce("", fn(codepoint, result) ->
                     << parsed :: 8>> = codepoint
                     if parsed == 0, do: result, else: result <> <<parsed>>
                   end)

# "Devastator"
|> IO.puts()

Assumptions:

• utf16-le encoding

• the codepoints are backwards-compatible with utf8 i.e. they use only 1 byte

Since I'm still learning Elixir, it took me a while to get to this solution. I looked into other libraries people made, even using something like iconv at a bash level.

回答4:

The last point definitely does change the issue, and explains it. Elixir uses binaries as strings but assumes and demands that they are UTF8 encoded, not UTF16.

回答5:

In reference to http://erlang.org/pipermail/erlang-questions/2010-December/054885.html

You can use :unicode.characters_to_list(binary_string, {:utf16, :little}) to verify result and store too

IEX eval

iex(1)> y                                                
<<115, 0, 121, 0, 115, 0>>
iex(2)> :unicode.characters_to_list(y, {:utf16, :little})
'sys'

Note : Value printed as sys for <<115, 0, 121, 0, 115, 0>>

回答6:

You can use Comprehensions

    defmodule TestModule do
      def convert(binary) do
        for c <- binary, into: "", do: <<c>>
      end
    end
    TestModule.convert([71,32,69,32,84,32]) |> IO.puts