Error Handling

Trap / Evade (try / catch)

The Trap/Evade block catches runtime errors and prevents them from crashing your program.

Trap begins
    Countstone result is 10 / 0;
    Chant("This line never runs.");
Evade (err)
    Chant("Caught: " + err);
end of Trap

Output:

Caught: division by zero - the void consumes all

The error variable (err in the example above) contains the error message as a Runestone (string). You can name it whatever you want.

What gets caught

Trap/Evade catches all runtime errors, including:

• Division by zero
• Index out of bounds
• Type mismatches
• Undefined variable access
• Explicitly thrown errors (via Forfeit)

Execution flow

When an error occurs inside Trap:

1. Execution immediately jumps to the Evade block
2. The error message is bound to the named variable
3. After the Evade block, execution continues normally

Chant("Before trap");

Trap begins
    Chant("Inside trap - before error");
    Countstone bad is 10 / 0;
    Chant("Inside trap - after error (never reached)");
Evade (err)
    Chant("Caught: " + err);
end of Trap

Chant("After trap - program continues");

Output:

Before trap
Inside trap - before error
Caught: division by zero - the void consumes all
After trap - program continues

Forfeit (throw)

Use Forfeit to throw a custom error:

Ritual withdraw(Mutable Countstone balance, Countstone amount) begins
    Quest begins when (amount > balance)
        Forfeit "insufficient gold! Need " + amount + " but only have " + balance;
    end of Quest
    balance is written as balance - amount;
    returneth balance;
end of Ritual

Forfeit takes any expression that evaluates to a value. That value becomes the error message.

Catching custom errors

Custom errors thrown with Forfeit are caught by Trap/Evade just like built-in errors:

Trap begins
    Mutable Countstone gold is 50;
    withdraw(gold, 100);
Evade (err)
    Chant("Transaction failed: " + err);
end of Trap

Output:

Transaction failed: insufficient gold! Need 100 but only have 50

Assert

The Assert built-in function throws an error if a condition is false:

Assert(1 + 1 == 2);              // passes silently
Assert(player.health > 0);       // passes if player is alive
Assert(Falsehood);                // ERROR: assertion failed

Use Assert for debugging and validating assumptions during development.

Patterns

Guard clauses

Use Forfeit at the top of a Ritual to validate inputs early:

Ritual set_level(Mutable Familiar player, Countstone level) begins
    Quest begins when (level < 1)
        Forfeit "level must be at least 1";
    end of Quest
    Quest begins when (level > 100)
        Forfeit "level cannot exceed 100";
    end of Quest
    player.level is written as level;
end of Ritual

Wrapping risky operations

Ritual safe_divide(Countstone a, Countstone b) begins
    Trap begins
        returneth a / b;
    Evade (err)
        Chant("Warning: " + err);
        returneth 0;
    end of Trap
end of Ritual

Error messages

RuneLang’s built-in errors are themed to match the fantasy setting:

Error	Message style
Division by zero	division by zero - the void consumes all
Undefined variable	unknown rune 'x' - it has not been inscribed
Type mismatch	Hex Mismatch (Type): ...
Immutable reassignment	cannot reassign enchanted (immutable) rune 'x'
Index out of bounds	scroll index out of bounds