Implement strings and partial implementation of arrays (missing index expr eval)

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/evaluator/evaluator_test.go

@@ -186,6 +186,10 @@ if (10 > 1) {
 			"foobar",
 			"identifier not found: foobar",
 		},
+		{
+			`"Hello" - "World"`,
+			"unknown operator: STRING - STRING",
+		},
 	}
 
 	for _, tt := range tests {
@@ -264,6 +268,86 @@ func TestFunctionApplication(t *testing.T) {
 	}
 }
 
+func TestStringLiteral(t *testing.T) {
+	input := `"Hello World!"`
+
+	evaluated := testEval(input)
+	str, ok := evaluated.(*object.String)
+	if !ok {
+		t.Fatalf("object is not String. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if str.Value != "Hello World!" {
+		t.Errorf("String has wrong value. got=%q", str.Value)
+	}
+}
+
+func TestStringConcatenation(t *testing.T) {
+	input := `"Hello" + " " + "World!"`
+
+	evaluated := testEval(input)
+	str, ok := evaluated.(*object.String)
+	if !ok {
+		t.Fatalf("object is not String. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if str.Value != "Hello World!" {
+		t.Errorf("String has wrong value. got=%q", str.Value)
+	}
+}
+
+func TestBuiltinFunctions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected any
+	}{
+		{`len("")`, 0},
+		{`len("four")`, 4},
+		{`len("hello world")`, 11},
+		{`len(1)`, "argument to `len` not supported, got INTEGER"},
+		{`len("one", "two")`, "wrong number of arguments. got=2, want=1"},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+
+		switch expected := tt.expected.(type) {
+		case int:
+			testIntegerObject(t, evaluated, int64(expected))
+		case string:
+			errObj, ok := evaluated.(*object.Error)
+			if !ok {
+				t.Errorf("object is not Error. got=%T (%+v)",
+					evaluated, evaluated)
+				continue
+			}
+			if errObj.Message != expected {
+				t.Errorf("wrong error message. expected=%q, got=%q",
+					expected, errObj.Message)
+			}
+		}
+	}
+}
+
+func TestArrayLiterals(t *testing.T) {
+	input := "[1, 2 * 2, 3 + 3]"
+
+	evaluated := testEval(input)
+	result, ok := evaluated.(*object.Array)
+	if !ok {
+		t.Fatalf("object is not Array. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if len(result.Elements) != 3 {
+		t.Fatalf("array has wrong num of elements. got=%d",
+			len(result.Elements))
+	}
+
+	testIntegerObject(t, result.Elements[0], 1)
+	testIntegerObject(t, result.Elements[1], 4)
+	testIntegerObject(t, result.Elements[2], 6)
+}
+
 func testNullObject(t *testing.T, obj object.Object) bool {
 	if obj != _NULL {
 		t.Errorf("object is not NULL. got=%T (%+v)", obj, obj)