ACL

A config language

Spec

Tokens

• Identifiers: name, init_path, eth0 [A-Za-z_][A-Za-z0-9_]*
• Types: int, float, bool, string, ref
• Literals: integers (123), floats (1.5), bools (true/false), strings ("text")
• Symbols: { } [ ] ( ) ; = , + - * / % ! < > & | ^ $ . :
• Comments: // single-line and /* block */

Semicolons are required for field statements.

---

Top-level blocks

Named blocks group fields and sub-blocks:

System {
    string name = "Atlas";
    int boot_delay = 200;
}

Block name is an identifier. Blocks may be repeated (e.g. multiple interface blocks).

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Fields

Two forms:

• Typed field:

  type name = expr;
  

  Example: string init_path = "/sbin/init";

• Inferred type:

  name = expr;
  

  Example: debug = true;

Arrays: type[] name = { expr, expr, ... }; Maps (optional): { "key": expr, ... } (useful later).

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Sub-blocks (repeatable / named)

Blocks inside blocks; can be labeled with a string:

Network {
    interface "eth0" {
        bool dhcp = true;
        string gateway = "192.168.1.1";
    }

    interface "wlan0" {
        bool dhcp = false;
        string ip = "192.168.1.100";
    }
}

interface "eth0" is a repeated/named entry; the label is the quoted string.

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References

• Global: $Path.to.field Example: $Network.interface["eth0"].gateway
• Local: $.field (same block)
• Parent: ^field (parent block)

Use ["name"] index form to look up repeated/named blocks.

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Casting & expressions

• Cast: (type)expr - e.g. (int)(1.5 * 256)

• Usual C-like operators and precedence:

  • Unary: ! - + (type)expr
  • Mul/Div/Mod: * / %
  • Add/Sub: + - (also string concatenation for +)
  • Comparison: < > <= >=
  • Equality: == !=
  • Logical: && ||
  • Ternary: cond ? a : b (optional)

Examples:

int max_procs = (int)(1.5 * 256);
string welcome = "Hi " + name;
bool ok = ($System.debug && (port > 1024));

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Minimal syntax rules (summary)

• Blocks: Name { ... }
• Field: [type] ident = expr;
• Array: type[] ident = { e, e, ... };
• Sub-block label: ident "label" { ... }
• $ for global refs; $. local; ^ parent.
• Semicolons required; braces required for blocks.
• Strings double-quoted with C-like escapes.

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Compact example (what this looks like in practice)

System {
    string name = "Atlas";
    int boot_delay = 200;
    bool debug = true;
}

Network {
    interface "eth0" {
        bool dhcp = true;
        string gateway = "192.168.1.1";
    }

    interface "wlan0" {
        bool dhcp = false;
        string ip = "192.168.1.100";
        string gateway = $Network.interface["eth0"].gateway;
    }
}

Services {
    service "getty" {
        string exec = "/sbin/getty";
        int ttys = $System.boot_delay > 0 ? 3 : 1;
    }
}

Modules {
    string[] load = { "virtio", "e1000" };
}