Migration Notes - Exception Handling Support

Version: Next Release

New Reserved Keywords

The following keywords are now reserved and cannot be used as identifiers:

• try
• catch
• finally
• throw
• when (used in exception filters)

Impact: If your code uses any of these as variable names, function names, or type names, you will need to rename them.

Example:

// Before (will now cause parse errors):
try: int = 5;
catch: string = "value";

// After (renamed identifiers):
tryCount: int = 5;
catchValue: string = "value";

New Features

Exception Handling

Fifth now supports C#-style exception handling with try/catch/finally blocks:

main(): int {
    result: int = 0;

    try {
        result = 10;
    } catch {
        result = 1;  // Catch-all handler
    } finally {
        std.print("cleanup");  // Always executes
    }

    return result;
}

Features Supported:

1. Try/Finally - Ensure cleanup code runs
2. Try/Catch - Handle exceptions with catch-all blocks
3. Try/Catch/Finally - Combined exception handling and cleanup
4. Throw Expressions - Use throw in expression contexts (future enhancement)

Current Limitations:

• Typed exception catches (e.g., catch (ex: System.Exception)) require parser support for qualified type names
• Exception throwing requires instantiation syntax enhancements
• Stack trace preservation for rethrow is supported but requires full exception infrastructure

Semantic Validation

The compiler now validates: - TRY001: Catch types must derive from System.Exception - TRY002: Filter expressions must be boolean-convertible - TRY003: Unreachable catch clauses are errors - TRY004: Throw operands must be exception types

Backward Compatibility

• Code not using the new reserved keywords will compile without changes
• No changes to existing control flow semantics
• All existing tests continue to pass