test(eval): add tail-call and call/cc tests

Tests/KiteCoreTests/CallCCTests.swift

@@ -0,0 +1,47 @@
+import XCTest
+@testable import KiteCore
+
+final class CallCCTests: XCTestCase {
+    private func evalShow(_ src: String) throws -> String {
+        displayString(try Interpreter().evalString(src))
+    }
+
+    func testEarlyExit() throws {
+        let program = """
+        (define (product items)
+          (call/cc
+            (lambda (break)
+              (let loop ((xs items) (acc 1))
+                (cond ((null? xs) acc)
+                      ((= (car xs) 0) (break 0))
+                      (else (loop (cdr xs) (* acc (car xs)))))))))
+        (list (product '(1 2 3 4)) (product '(1 2 0 4)))
+        """
+        XCTAssertEqual(try evalShow(program), "(24 0)")
+    }
+
+    func testContinuationValueWhenNotInvoked() throws {
+        XCTAssertEqual(try evalShow("(+ 1 (call/cc (lambda (k) 10)))"), "11")
+    }
+
+    func testReentrantContinuationLoop() throws {
+        let program = """
+        (let ((sum 0) (i 0) (k #f))
+          (call/cc (lambda (c) (set! k c)))
+          (set! i (+ i 1))
+          (set! sum (+ sum i))
+          (if (< i 5) (k #f) sum))
+        """
+        XCTAssertEqual(try evalShow(program), "15")
+    }
+
+    func testMultiShotContinuationAccumulates() throws {
+        let program = """
+        (let ((results '()) (k #f) (n 0))
+          (set! n (+ 1 (call/cc (lambda (c) (set! k c) 0))))
+          (set! results (cons n results))
+          (if (< n 4) (k n) results))
+        """
+        XCTAssertEqual(try evalShow(program), "(4 3 2 1)")
+    }
+}