<< and >> symbols in Erlang

Question

First of all, I'm an Erlang rookie here. I need to interface with a MySQL database and I found the erlang-mysql-driver. I'm trying that out, and am a little confused by some of the syntax.

I can get a row of data from the database with this (greatly oversimplified for brevity here):

Result = mysql:fetch(P1, ["SELECT column1, column2 FROM table1 WHERE column2='", Key, "'"]),  
case Result of  
    {data, Data} ->  
        case mysql:get_result_rows(Data) of  
            [] -> not_found;  
            Res ->  
              %% Now 'Res' has the row

So now here is an example of what `Res' has:

[[<<"value from column1">>, <<"value from column2">>]]

I get that it's a list of records. In this case, the query returned 1 row of 2 columns.

My question is:
What do the << and >> symbols mean? And what is the best (Erlang-recommended) syntax for turning a list like this into a records which I have defined like:

-record(  
    my_record,  
    {   
      column1 = ""  
      ,column2 = ""  
    }  
 ).     

Answer 1

Just a small note: the results are not bit string comprehensions per see, they are just bit strings. However you can use bit string comprehensions to produce a sequence of bit strings (which is described above with the generators and that), much like lists and lists comprehensions.

you can use erlang:binary_to_list/1 and erlang:list_to_binary/1 to convert between binary and strings (lists).

The reason the mysql driver returns bit strings is probably because they are much faster to manipulate.

Answer 2

In your specific example, you can do the conversion by matching on the returned column values, and then creating a new record like this:

case mysql:get_result_rows(Data) of
  [] ->
    not_found;  
  [[Col1, Col2]] ->  
    #my_record{column1 = Col1, column2 = Col2}
end