009 cross module mock

// Test: Cross-module mocking
//
// This test verifies mocking events from IMPORTED modules.
//
// The syntax `~std.fs:read_lines = failed "mock error"` means:
// "For this test, provide this implementation for the std.fs:read_lines event"
//
// This is NOT just for testing - Source blocks with impl declarations enable:
// - Compile-time code synthesis
// - AI-generated implementations
// - Remote code fetching
// - Interactive compilation
//
// The compiler becomes a development partner, not just a batch processor.

~import "$std/testing"
~import "$std/fs"

// A flow that depends on an external module event
~event process_config { path: []const u8 }
| ok { line_count: usize }
| failed { reason: []const u8 }

~process_config = std.fs:read_lines(path: path)
| lines l |> ok { line_count: l.len }
| failed msg |> failed { reason: msg }

// ============================================
// THE TEST - Cross-module mock
// ============================================

~test(Cross-module mock with failed branch) {
    // Mock the external module's event with the 'failed' branch
    // This returns a simple string ([]const u8), which works without array literals
    ~std.fs:read_lines = failed "mocked file read error"

    ~process_config(path: "/this/file/does/not/exist.txt")
    | ok |> assert.fail()
    | failed result |> assert(result.reason.len > 0)
}

// ============================================
// FUTURE: Array literal test (feature gap)
// ============================================
//
// Once Koru has array literal syntax, add this test:
//
// ~test(Cross-module mock should return 3 lines) {
//     ~std.fs:read_lines = lines ["line1", "line2", "line3"]
//
//     ~process_config(path: "/fake/path")
//     | ok result |> assert(result.line_count == 3)
//     | failed |> assert.fail()
// }
//
// FEATURE GAP: Koru needs native array literals like ["a", "b"]
// WORKAROUND: Use &.{ "a", "b" } Zig escape when available