opening a new server socket

[razetime]

Linux sd202148 6.1.0-13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.55-1 (2023-09-29) x86_64 GNU/Linux (devuan)

I just tried the example echo server and it gave me an instantiation error. So i tried the equivalent of the first few lines and got some odd behaviour:

?- socket_server_open('127.0.0.1':Port, ServerSocket).
   ServerSocket = .
?-     format("echo_server: connection opened at ~w:~d~n", [Addr, Port]).
   error(instantiation_error,(is)/2).

I'm not sure what ServerSocket is instantiated to, and I also tried specifying the port, making sure the port wasn't already opened by a different program. it did not seem to work, so I'd like to know if should be configuring anything before this to make it work.

[bakaq]

The SeverSocket is to be used in socket_server_accept/4 to get an actual stream that you can use to get and send data. It is also used in socket_server_close/1 to close a socket. For example:

?- socket_server_open('127.0.0.1':8001, ServerSocket), socket_server_accept(ServerSocket, Client, Stream, []).
   % This will wait here until a connection to the server is made. I acessed the address
   % and port in my browser, and then the variables Client and Stream are instantiated as
   % follows:
   ServerSocket = , Client = '127.0.0.1:59292', Stream = '$stream'(0x55f49f47e498).

In the second query you are making with format/2, the reason there is an instantiation error is because the variables Addr and Port that you are trying to use are not instantiated. At that moment they are just variables, they don't have any value. The format specifier ~w actually can print variables (although I prefer ~q because it prints terms in a way that can be read back into prolog easily), but ~d is just for integers.

?- format("w: ~w~n", [Var]), format("q: ~q~n", [Var]).
w: A
q: A
   % For the case of just a variable these are the same.
   true.
?- Term = 'WeirdAtom+-.%' , format("w: ~w~n", [Term]), format("q: ~q~n", [Term]).
w: WeirdAtom+-.%
q: 'WeirdAtom+-.%'
   % For this case they are different. The ~w version can't be read back because the
   % atom has special characters, starts with an uppercase letter (normally reserved for variables)
   % and even has a % inside (which normally starts comments). The ~q version on the other hand
   % can be read directly into Prolog and it will mean the same thing as before it was formatted:
   % just a very weird atom. 
   Term = 'WeirdAtom+-.%'.
?- format("~d~n", [Var]).
   % Var isn't instantiated to an integer. In fact it isn't instantiated at all, so error.
   error(instantiation_error,(is)/2).
?- format("~d~n", [123]).
123
   % Here it works because it's just an integer.
   true.
?- Var = 123, format("~d~n", [Var]).
123
   % Here it works because the variable is instantiated to an integer by the time it is used.
   Var = 123.

[triska]

In addition to the observations that @bakaq mentioned, this also indicates mistakes with the current socket predicates, I have filed #2221 and #2222 for them, thank you a lot for reporting this!