String version of term_to_binary

Question

I'm trying to write a simple server that talks to clients via tcp. I have it sending messages around just fine, but now I want it to interpret the messages as Erlang data types. For example, pretend it's HTTP-like (it's not) and that I want to send from the client {get, "/foo.html"} and have the server interpret that as a tuple containing an atom and a list, instead of just a big list or binary.

I will probably end up using term_to_binary and binary_to_term, but debugging text-based protocols is so much easier that I was hoping to find a more list-friendly version. Is there one hiding somewhere?

Answer 1

You can parse a string as an expression (similar to file:consult) via:

% InputString = "...",
{ok, Scanned, _} = erl_scan:string(InputString),
{ok, Exprs} = erl_parse:parse_exprs(Scanned),
{value, ParsedValue, _} = erl_eval:exprs(Exprs, [])

(See http://www.trapexit.org/String_Eval)

You should be able to use io_lib:format to convert an expression to a string using the ~w or ~p format codes, such as io_lib:format("~w", [{get, "/foo.html"}]).

I don't think this will be very fast, so if performance is an issue you should probably not use strings like this.

Also note that this is potentially unsafe since you're evaluating arbitrary expressions -- if you go this route, you should probably do some checks on the intermediate output. I'd suggest looking at the result of erl_parse:parse_exprs to make sure it contains the formats you're interested in (i.e., it's always a tuple of {atom(), list()}) with no embedded function calls. You should be able to do this via pattern matching.