[CS4215] Lazy Evaluation for Source §2 in Evaluator

README.md

@@ -14,6 +14,7 @@ The evaluators in this section all follow the general style of SICP JS Chapter 4
 * [`src/evaluators/source-2-lazy.js`](https://github.com/source-academy/source-programs/blob/master/src/evaluators/source-2-lazy.js): lazy evaluator for Source §2, described in SICP JS 4.2
 * [`src/evaluators/source-2-non-det.js`](https://github.com/source-academy/source-programs/blob/master/src/evaluators/source-2-non-det.js): evaluator for Source §2 with non-determinism, described in SICP JS 4.3
 * [`src/evaluators/typed-source.js`](https://github.com/source-academy/source-programs/blob/master/src/evaluators/typed-source.js): evaluator for Typed Source (typed version of a Source §1 sublanguage)
+* [`src/evaluators/source-4-1-lazy.js`](https://github.com/source-academy/source-programs/blob/master/src/evaluators/source-4-1-lazy.js): lazy evaluator for Source §2.
 
 ## Steppers
 

src/evaluators/__tests__/source-4-1-lazy.test1.js

@@ -0,0 +1,5 @@
+parse_and_eval("function try_me(a, b) {\
+    return a === 0 ? 1 : b; \
+    }\
+    try_me(0, head(null));");
+// 1

src/evaluators/__tests__/source-4-1-lazy.test2.js

@@ -0,0 +1,5 @@
+parse_and_eval("function try_me(a, b) {\
+    return a;}\
+    try_me(0, head(null));");
+
+// 0

src/evaluators/__tests__/source-4-1-lazy.test3.js

@@ -0,0 +1,3 @@
+parse_and_eval("const a = 1 + a;");
+
+// undefined

src/evaluators/__tests__/source-4-1-lazy.test4.js

@@ -0,0 +1,5 @@
+parse_and_eval("const a = non_existing;\
+                const non_existing = 11;\
+                a;");
+
+// 11

src/evaluators/__tests__/source-4-1-lazy.test5.js

@@ -0,0 +1,6 @@
+parse_and_eval("function factorial(x) {\
+    return n === 1\
+    ? 1\
+    : factorial(n - 1) * n;}");
+
+// undefined

src/evaluators/__tests__/source-4-1-lazy.test6.js

@@ -0,0 +1,9 @@
+parse_and_eval("function infinite_list(n)\
+                {\
+                return pair(n, infinite_list(n+1))\
+                ;}\
+                const a = head(tail(tail(tail(tail(infinite_list(1))))));\
+                a;"
+              );
+
+// 5