006 api evolution

// Test 918f: Optional Branches - API Evolution
//
// Verifies:
// 1. Event evolved by adding NEW optional branch
// 2. Old handler (|? catch-all) silently catches new branch
// 3. Adding optional branches never breaks existing handlers
// 4. Contrast: adding REQUIRED branches would break compilation (good!)

~import "$std/io"

// VERSION 2: added ?debug branch (new in v2)
~event process { value: u32 }
| success { result: u32 }        // REQUIRED (both versions)
| ?warning { msg: []const u8 }   // OPTIONAL (both versions)
| ?debug { info: []const u8 }    // OPTIONAL (NEW in v2)

~proc process {
    if (value < 5) {
        return .{ .debug = .{ .info = "Small value" } };
    }
    if (value > 100) {
        return .{ .warning = .{ .msg = "Large value" } };
    }
    return .{ .success = .{ .result = value * 2 } };
}

// Handler written for v1 — doesn't know about ?debug
// |? silently catches the new branch
~process(value: 10)
| success _ |> std.io:print.ln("SUCCESS")
|? |> std.io:print.ln("OPTIONAL")

~process(value: 150)
| success _ |> std.io:print.ln("SUCCESS")
|? |> std.io:print.ln("OPTIONAL")

// value=2 → returns ?debug (new in v2) — caught by |?
~process(value: 2)
| success _ |> std.io:print.ln("SUCCESS")
|? |> std.io:print.ln("OPTIONAL")