Adding quote built-in. Fixed car/cdr which now function properly. Added tests for car/cdr/quote

Author: Jeff Aigner

lisp.go

@@ -157,11 +157,11 @@ func begin(args []Object) Object {
 }
 
 func car(args []Object) Object {
-	return args[0]
+	return args[0].(List)[0]
 }
 
 func cdr(args []Object) Object {
-	return args[1:]
+	return args[0].(List)[1:]
 }
 
 func getStandardEnv() Env {
@@ -196,6 +196,9 @@ func (e *Env) eval(x Object) Object {
 		return e.mapping[val]
 	} else if _, is_list := x.(List); !is_list {
 		return x
+	} else if l := x.(List); l[0] == Symbol("quote") {
+		exp := l[1]
+		return exp
 	} else if l := x.(List); l[0] == Symbol("define") {
 		val := e.eval(l[2])
 		e.mapping[l[1].(Symbol)] = val

lisp_test.go

@@ -34,13 +34,16 @@ func TestEval(t *testing.T) {
 		{"(fib 8)", Number(34)},
 		{"(fib 9)", Number(55)},
 		{"(fib 10)", Number(89)},
+		{"(car (quote (5 4 3 2 1)))", Number(5)},
+		{"(second (quote (1 2 3 4 5)))", Number(2)},
 	}
 	e := getStandardEnv()
 
 	// define some functions
 	e.eval(parse("(define circle-area (lambda (r) (* pi (* r r))))"))
 	e.eval(parse("(define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1))))))"))
 	e.eval(parse("(define fib (lambda (n) (if (<= n 1) 1 (+ (fib (- n 1)) (fib (- n 2)))))))"))
+	e.eval(parse("(define second (lambda (x) (car (cdr x))))"))
 
 	for _, c := range cases {
 		got := e.eval(parse(c.in))