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

Tests/KiteCoreTests/TailCallTests.swift

@@ -0,0 +1,46 @@
+import XCTest
+@testable import KiteCore
+
+final class TailCallTests: XCTestCase {
+    private func evalShow(_ src: String) throws -> String {
+        displayString(try Interpreter().evalString(src))
+    }
+
+    func testDeepNamedLetLoop() throws {
+        let program = """
+        (define (count-up n)
+          (let loop ((i 0))
+            (if (= i n) i (loop (+ i 1)))))
+        (count-up 1000000)
+        """
+        XCTAssertEqual(try evalShow(program), "1000000")
+    }
+
+    func testDeepTailRecursiveAccumulator() throws {
+        let program = """
+        (define (sum-to n acc)
+          (if (= n 0) acc (sum-to (- n 1) (+ acc 1))))
+        (sum-to 1000000 0)
+        """
+        XCTAssertEqual(try evalShow(program), "1000000")
+    }
+
+    func testMutualTailRecursion() throws {
+        let program = """
+        (define (even? n) (if (= n 0) #t (odd? (- n 1))))
+        (define (odd? n) (if (= n 0) #f (even? (- n 1))))
+        (even? 500000)
+        """
+        XCTAssertEqual(try evalShow(program), "#t")
+    }
+
+    func testTailPositionInCond() throws {
+        let program = """
+        (define (loop i n)
+          (cond ((= i n) 'done)
+                (else (loop (+ i 1) n))))
+        (loop 0 500000)
+        """
+        XCTAssertEqual(try evalShow(program), "done")
+    }
+}