Configs

A config (.jmc file) is Jumper cut down to data: values, variables, expressions and conditions. It is a replacement for YAML or JSON that can compute a value from another one, and it never runs anything dangerous: loops and Java are not part of the language.

Writing a config

The top-level variables are the config:

// server.jmc
int port = 25565;
boolean dev = false;
int maxPlayers = dev ? 5 : 100;
dyn motd = ["Welcome", "to the server"];
dyn db = { url: "jdbc:mysql://localhost/mc", pool: { min: 2, max: 10 } };

if (dev) {
    maxPlayers = 1;
}

Its value is the table {port: 25565, dev: false, maxPlayers: 100, motd: [...], db: {...}}, in declaration order. Variables that end up null are left out.

A config may instead end with return <value>;, and then that value is the config:

dyn ports = [25565, 25566];
return { main: ports[0], fallback: ports[1] };

What a config may contain

Allowed	Not allowed
variable declarations (typed or dyn) and assignments	loops
literals: numbers, strings, true, false, null, tables, arrays	functions and lambdas
operators, ?:, + on strings	classes
if / else, switch	import
return	try / throw
comments	Java classes, built-in functions

Types are checked as in scripts: int a = "s"; is an error. Anything not allowed is a syntax error with a line and a column:

Syntax error: A loop is not allowed in a config (.jmc) (line 1, col 1)

Printing a config

The command line runs a .jmc file by printing its value:

java -jar jmp.jar server.jmc
{port: 25565, dev: false, maxPlayers: 100, motd: [Welcome, to the server], db: {url: jdbc:mysql://localhost/mc, pool: {min: 2, max: 10}}}

Reading a config from Java

import me.padej.jumper.interp.Config;
import me.padej.jumper.runtime.JTable;

JTable cfg = (JTable) Config.load(Path.of("server.jmc"));

int port = (Integer) cfg.get("port");                           // 25565
List<?> motd = (List<?>) cfg.get("motd");                       // [Welcome, to the server]
JTable pool = (JTable) ((JTable) cfg.get("db")).get("pool");
int max = (Integer) pool.get("max");                            // 10

Config.load(Path)	read and evaluate a file
Config.parse(String)	evaluate config text

The value is a JTable for a config of variables (or whatever return gives). Numbers come as Integer, Long, Double; strings as String; arrays as a List; tables as JTable (get, has, keys, size, asMap).

A config that breaks the rules throws ParseError (me.padej.jumper.parser); an error while computing a value throws JmpError. Both carry the line.

A config runs with no access to Java at all, whatever policy the host uses for scripts, so loading a config written by someone else is safe.