diff --git a/pkg/parser/parser_test.go b/pkg/parser/parser_test.go
index 0eafba7..9403229 100644
--- a/pkg/parser/parser_test.go
+++ b/pkg/parser/parser_test.go
@@ -12,7 +12,7 @@ func TestLetStatements(t *testing.T) {
 	tests := []struct {
 		input              string
 		expectedIdentifier string
-		expectedValue      any
+		expectedValue      interface{}
 	}{
 		{"let x = 5;", "x", 5},
 		{"let y = true;", "y", true},
@@ -42,32 +42,6 @@ func TestLetStatements(t *testing.T) {
 	}
 }
 
-func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
-	if s.TokenLiteral() != "let" {
-		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())
-		return false
-	}
-
-	letStmt, ok := s.(*ast.LetStatement)
-	if !ok {
-		t.Errorf("s not *ast.LetStatement. got=%T", s)
-		return false
-	}
-
-	if letStmt.Name.Value != name {
-		t.Errorf("letStmt.Name.Value not '%s'. got=%s", name, letStmt.Name.Value)
-		return false
-	}
-
-	if letStmt.Name.TokenLiteral() != name {
-		t.Errorf("letStmt.Name.TokenLiteral() not '%s'. got=%s",
-			name, letStmt.Name.TokenLiteral())
-		return false
-	}
-
-	return true
-}
-
 func TestReturnStatements(t *testing.T) {
 	tests := []struct {
 		input         string
@@ -104,12 +78,14 @@ func TestReturnStatements(t *testing.T) {
 	}
 }
 
-func TestIdentifierExpressions(t *testing.T) {
+func TestIdentifierExpression(t *testing.T) {
 	input := "foobar;"
+
 	l := lexer.New(input)
 	p := New(l)
 	program := p.ParseProgram()
 	checkParserErrors(t, p)
+
 	if len(program.Statements) != 1 {
 		t.Fatalf("program has not enough statements. got=%d",
 			len(program.Statements))
@@ -168,10 +144,12 @@ func TestParsingPrefixExpressions(t *testing.T) {
 	prefixTests := []struct {
 		input    string
 		operator string
-		value    any
+		value    interface{}
 	}{
 		{"!5;", "!", 5},
 		{"-15;", "-", 15},
+		{"!foobar;", "!", "foobar"},
+		{"-foobar;", "-", "foobar"},
 		{"!true;", "!", true},
 		{"!false;", "!", false},
 	}
@@ -207,14 +185,12 @@ func TestParsingPrefixExpressions(t *testing.T) {
 	}
 }
 
-// parser/parser_test.go
-
 func TestParsingInfixExpressions(t *testing.T) {
 	infixTests := []struct {
 		input      string
-		leftValue  any
+		leftValue  interface{}
 		operator   string
-		rightValue any
+		rightValue interface{}
 	}{
 		{"5 + 5;", 5, "+", 5},
 		{"5 - 5;", 5, "-", 5},
@@ -224,6 +200,14 @@ func TestParsingInfixExpressions(t *testing.T) {
 		{"5 < 5;", 5, "<", 5},
 		{"5 == 5;", 5, "==", 5},
 		{"5 != 5;", 5, "!=", 5},
+		{"foobar + barfoo;", "foobar", "+", "barfoo"},
+		{"foobar - barfoo;", "foobar", "-", "barfoo"},
+		{"foobar * barfoo;", "foobar", "*", "barfoo"},
+		{"foobar / barfoo;", "foobar", "/", "barfoo"},
+		{"foobar > barfoo;", "foobar", ">", "barfoo"},
+		{"foobar < barfoo;", "foobar", "<", "barfoo"},
+		{"foobar == barfoo;", "foobar", "==", "barfoo"},
+		{"foobar != barfoo;", "foobar", "!=", "barfoo"},
 		{"true == true", true, "==", true},
 		{"true != false", true, "!=", false},
 		{"false == false", false, "==", false},
@@ -246,7 +230,8 @@ func TestParsingInfixExpressions(t *testing.T) {
 				program.Statements[0])
 		}
 
-		if !testInfixExpression(t, stmt.Expression, tt.leftValue, tt.operator, tt.rightValue) {
+		if !testInfixExpression(t, stmt.Expression, tt.leftValue,
+			tt.operator, tt.rightValue) {
 			return
 		}
 	}
@@ -333,6 +318,10 @@ func TestOperatorPrecedenceParsing(t *testing.T) {
 			"2 / (5 + 5)",
 			"(2 / (5 + 5))",
 		},
+		{
+			"(5 + 5) * 2 * (5 + 5)",
+			"(((5 + 5) * 2) * (5 + 5))",
+		},
 		{
 			"-(5 + 5)",
 			"(-(5 + 5))",
@@ -483,8 +472,7 @@ func TestIfElseExpression(t *testing.T) {
 
 	exp, ok := stmt.Expression.(*ast.IfExpression)
 	if !ok {
-		t.Fatalf("stmt.Expression is not ast.IfExpression. got=%T",
-			stmt.Expression)
+		t.Fatalf("stmt.Expression is not ast.IfExpression. got=%T", stmt.Expression)
 	}
 
 	if !testInfixExpression(t, exp.Condition, "x", "<", "y") {
@@ -498,7 +486,7 @@ func TestIfElseExpression(t *testing.T) {
 
 	consequence, ok := exp.Consequence.Statements[0].(*ast.ExpressionStatement)
 	if !ok {
-		t.Fatalf("Statements[0] of consequence is not ast.ExpressionStatement. got=%T",
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T",
 			exp.Consequence.Statements[0])
 	}
 
@@ -507,15 +495,16 @@ func TestIfElseExpression(t *testing.T) {
 	}
 
 	if len(exp.Alternative.Statements) != 1 {
-		t.Errorf("alternative is not 1 statement. got=%d\n",
+		t.Errorf("exp.Alternative.Statements does not contain 1 statements. got=%d\n",
 			len(exp.Alternative.Statements))
 	}
 
 	alternative, ok := exp.Alternative.Statements[0].(*ast.ExpressionStatement)
 	if !ok {
-		t.Fatalf("Statements[0] of alternative is not ast.ExpressionStatement. got=%T",
+		t.Fatalf("Statements[0] is not ast.ExpressionStatement. got=%T",
 			exp.Alternative.Statements[0])
 	}
+
 	if !testIdentifier(t, alternative.Expression, "y") {
 		return
 	}
@@ -636,6 +625,60 @@ func TestCallExpressionParsing(t *testing.T) {
 	testInfixExpression(t, exp.Arguments[2], 4, "+", 5)
 }
 
+func TestCallExpressionParameterParsing(t *testing.T) {
+	tests := []struct {
+		input         string
+		expectedIdent string
+		expectedArgs  []string
+	}{
+		{
+			input:         "add();",
+			expectedIdent: "add",
+			expectedArgs:  []string{},
+		},
+		{
+			input:         "add(1);",
+			expectedIdent: "add",
+			expectedArgs:  []string{"1"},
+		},
+		{
+			input:         "add(1, 2 * 3, 4 + 5);",
+			expectedIdent: "add",
+			expectedArgs:  []string{"1", "(2 * 3)", "(4 + 5)"},
+		},
+	}
+
+	for _, tt := range tests {
+		l := lexer.New(tt.input)
+		p := New(l)
+		program := p.ParseProgram()
+		checkParserErrors(t, p)
+
+		stmt := program.Statements[0].(*ast.ExpressionStatement)
+		exp, ok := stmt.Expression.(*ast.CallExpression)
+		if !ok {
+			t.Fatalf("stmt.Expression is not ast.CallExpression. got=%T",
+				stmt.Expression)
+		}
+
+		if !testIdentifier(t, exp.Function, tt.expectedIdent) {
+			return
+		}
+
+		if len(exp.Arguments) != len(tt.expectedArgs) {
+			t.Fatalf("wrong number of arguments. want=%d, got=%d",
+				len(tt.expectedArgs), len(exp.Arguments))
+		}
+
+		for i, arg := range tt.expectedArgs {
+			if exp.Arguments[i].String() != arg {
+				t.Errorf("argument %d wrong. want=%q, got=%q", i,
+					arg, exp.Arguments[i].String())
+			}
+		}
+	}
+}
+
 func TestStringLiteralExpression(t *testing.T) {
 	input := `"hello world";`
 
@@ -655,6 +698,25 @@ func TestStringLiteralExpression(t *testing.T) {
 	}
 }
 
+func TestParsingEmptyArrayLiterals(t *testing.T) {
+	input := "[]"
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	stmt, ok := program.Statements[0].(*ast.ExpressionStatement)
+	array, ok := stmt.Expression.(*ast.ArrayLiteral)
+	if !ok {
+		t.Fatalf("exp not ast.ArrayLiteral. got=%T", stmt.Expression)
+	}
+
+	if len(array.Elements) != 0 {
+		t.Errorf("len(array.Elements) not 0. got=%d", len(array.Elements))
+	}
+}
+
 func TestParsingArrayLiterals(t *testing.T) {
 	input := "[1, 2 * 2, 3 + 3]"
 
@@ -701,42 +763,6 @@ func TestParsingIndexExpressions(t *testing.T) {
 	}
 }
 
-func TestParsingHashLiteralsStringKeys(t *testing.T) {
-	input := `{"one": 1, "two": 2, "three": 3}`
-
-	l := lexer.New(input)
-	p := New(l)
-	program := p.ParseProgram()
-	checkParserErrors(t, p)
-
-	stmt := program.Statements[0].(*ast.ExpressionStatement)
-	hash, ok := stmt.Expression.(*ast.HashLiteral)
-	if !ok {
-		t.Fatalf("exp is not ast.HashLiteral. got=%T", stmt.Expression)
-	}
-
-	if len(hash.Pairs) != 3 {
-		t.Errorf("hash.Pairs has wrong length. got=%d", len(hash.Pairs))
-	}
-
-	expected := map[string]int64{
-		"one":   1,
-		"two":   2,
-		"three": 3,
-	}
-
-	for key, value := range hash.Pairs {
-		literal, ok := key.(*ast.StringLiteral)
-		if !ok {
-			t.Errorf("key is not ast.StringLiteral. got=%T", key)
-		}
-
-		expectedValue := expected[literal.String()]
-
-		testIntegerLiteral(t, value, expectedValue)
-	}
-}
-
 func TestParsingEmptyHashLiteral(t *testing.T) {
 	input := "{}"
 
@@ -756,6 +782,114 @@ func TestParsingEmptyHashLiteral(t *testing.T) {
 	}
 }
 
+func TestParsingHashLiteralsStringKeys(t *testing.T) {
+	input := `{"one": 1, "two": 2, "three": 3}`
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	stmt := program.Statements[0].(*ast.ExpressionStatement)
+	hash, ok := stmt.Expression.(*ast.HashLiteral)
+	if !ok {
+		t.Fatalf("exp is not ast.HashLiteral. got=%T", stmt.Expression)
+	}
+
+	expected := map[string]int64{
+		"one":   1,
+		"two":   2,
+		"three": 3,
+	}
+
+	if len(hash.Pairs) != len(expected) {
+		t.Errorf("hash.Pairs has wrong length. got=%d", len(hash.Pairs))
+	}
+
+	for key, value := range hash.Pairs {
+		literal, ok := key.(*ast.StringLiteral)
+		if !ok {
+			t.Errorf("key is not ast.StringLiteral. got=%T", key)
+			continue
+		}
+
+		expectedValue := expected[literal.String()]
+		testIntegerLiteral(t, value, expectedValue)
+	}
+}
+
+func TestParsingHashLiteralsBooleanKeys(t *testing.T) {
+	input := `{true: 1, false: 2}`
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	stmt := program.Statements[0].(*ast.ExpressionStatement)
+	hash, ok := stmt.Expression.(*ast.HashLiteral)
+	if !ok {
+		t.Fatalf("exp is not ast.HashLiteral. got=%T", stmt.Expression)
+	}
+
+	expected := map[string]int64{
+		"true":  1,
+		"false": 2,
+	}
+
+	if len(hash.Pairs) != len(expected) {
+		t.Errorf("hash.Pairs has wrong length. got=%d", len(hash.Pairs))
+	}
+
+	for key, value := range hash.Pairs {
+		boolean, ok := key.(*ast.Boolean)
+		if !ok {
+			t.Errorf("key is not ast.BooleanLiteral. got=%T", key)
+			continue
+		}
+
+		expectedValue := expected[boolean.String()]
+		testIntegerLiteral(t, value, expectedValue)
+	}
+}
+
+func TestParsingHashLiteralsIntegerKeys(t *testing.T) {
+	input := `{1: 1, 2: 2, 3: 3}`
+
+	l := lexer.New(input)
+	p := New(l)
+	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
+	stmt := program.Statements[0].(*ast.ExpressionStatement)
+	hash, ok := stmt.Expression.(*ast.HashLiteral)
+	if !ok {
+		t.Fatalf("exp is not ast.HashLiteral. got=%T", stmt.Expression)
+	}
+
+	expected := map[string]int64{
+		"1": 1,
+		"2": 2,
+		"3": 3,
+	}
+
+	if len(hash.Pairs) != len(expected) {
+		t.Errorf("hash.Pairs has wrong length. got=%d", len(hash.Pairs))
+	}
+
+	for key, value := range hash.Pairs {
+		integer, ok := key.(*ast.IntegerLiteral)
+		if !ok {
+			t.Errorf("key is not ast.IntegerLiteral. got=%T", key)
+			continue
+		}
+
+		expectedValue := expected[integer.String()]
+
+		testIntegerLiteral(t, value, expectedValue)
+	}
+}
+
 func TestParsingHashLiteralsWithExpressions(t *testing.T) {
 	input := `{"one": 0 + 1, "two": 10 - 8, "three": 15 / 5}`
 
@@ -803,6 +937,97 @@ func TestParsingHashLiteralsWithExpressions(t *testing.T) {
 	}
 }
 
+func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
+	if s.TokenLiteral() != "let" {
+		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())
+		return false
+	}
+
+	letStmt, ok := s.(*ast.LetStatement)
+	if !ok {
+		t.Errorf("s not *ast.LetStatement. got=%T", s)
+		return false
+	}
+
+	if letStmt.Name.Value != name {
+		t.Errorf("letStmt.Name.Value not '%s'. got=%s", name, letStmt.Name.Value)
+		return false
+	}
+
+	if letStmt.Name.TokenLiteral() != name {
+		t.Errorf("letStmt.Name.TokenLiteral() not '%s'. got=%s",
+			name, letStmt.Name.TokenLiteral())
+		return false
+	}
+
+	return true
+}
+
+func testInfixExpression(t *testing.T, exp ast.Expression, left interface{},
+	operator string, right interface{}) bool {
+
+	opExp, ok := exp.(*ast.InfixExpression)
+	if !ok {
+		t.Errorf("exp is not ast.InfixExpression. got=%T(%s)", exp, exp)
+		return false
+	}
+
+	if !testLiteralExpression(t, opExp.Left, left) {
+		return false
+	}
+
+	if opExp.Operator != operator {
+		t.Errorf("exp.Operator is not '%s'. got=%q", operator, opExp.Operator)
+		return false
+	}
+
+	if !testLiteralExpression(t, opExp.Right, right) {
+		return false
+	}
+
+	return true
+}
+
+func testLiteralExpression(
+	t *testing.T,
+	exp ast.Expression,
+	expected interface{},
+) bool {
+	switch v := expected.(type) {
+	case int:
+		return testIntegerLiteral(t, exp, int64(v))
+	case int64:
+		return testIntegerLiteral(t, exp, v)
+	case string:
+		return testIdentifier(t, exp, v)
+	case bool:
+		return testBooleanLiteral(t, exp, v)
+	}
+	t.Errorf("type of exp not handled. got=%T", exp)
+	return false
+}
+
+func testIntegerLiteral(t *testing.T, il ast.Expression, value int64) bool {
+	integ, ok := il.(*ast.IntegerLiteral)
+	if !ok {
+		t.Errorf("il not *ast.IntegerLiteral. got=%T", il)
+		return false
+	}
+
+	if integ.Value != value {
+		t.Errorf("integ.Value not %d. got=%d", value, integ.Value)
+		return false
+	}
+
+	if integ.TokenLiteral() != fmt.Sprintf("%d", value) {
+		t.Errorf("integ.TokenLiteral not %d. got=%s", value,
+			integ.TokenLiteral())
+		return false
+	}
+
+	return true
+}
+
 func testIdentifier(t *testing.T, exp ast.Expression, value string) bool {
 	ident, ok := exp.(*ast.Identifier)
 	if !ok {
@@ -845,71 +1070,6 @@ func testBooleanLiteral(t *testing.T, exp ast.Expression, value bool) bool {
 	return true
 }
 
-func testLiteralExpression(
-	t *testing.T,
-	exp ast.Expression,
-	expected any,
-) bool {
-	switch v := expected.(type) {
-	case int:
-		return testIntegerLiteral(t, exp, int64(v))
-	case int64:
-		return testIntegerLiteral(t, exp, v)
-	case string:
-		return testIdentifier(t, exp, v)
-	case bool:
-		return testBooleanLiteral(t, exp, v)
-	}
-	t.Errorf("type of exp not handled. got=%T", exp)
-	return false
-}
-
-func testInfixExpression(t *testing.T, exp ast.Expression, left any,
-	operator string, right any) bool {
-
-	opExp, ok := exp.(*ast.InfixExpression)
-	if !ok {
-		t.Errorf("exp is not ast.InfixExpression. got=%T(%s)", exp, exp)
-		return false
-	}
-
-	if !testLiteralExpression(t, opExp.Left, left) {
-		return false
-	}
-
-	if opExp.Operator != operator {
-		t.Errorf("exp.Operator is not '%s'. got=%q", operator, opExp.Operator)
-		return false
-	}
-
-	if !testLiteralExpression(t, opExp.Right, right) {
-		return false
-	}
-
-	return true
-}
-
-func testIntegerLiteral(t *testing.T, il ast.Expression, value int64) bool {
-	integ, ok := il.(*ast.IntegerLiteral)
-	if !ok {
-		t.Errorf("il not *ast.IntegerLiteral. got=%T", il)
-		return false
-	}
-
-	if integ.Value != value {
-		t.Errorf("integ.Value not %d. got=%d", value, integ.Value)
-		return false
-	}
-
-	if integ.TokenLiteral() != fmt.Sprintf("%d", value) {
-		t.Errorf("integ.TokenLiteral not %d. got=%s", value,
-			integ.TokenLiteral())
-		return false
-	}
-
-	return true
-}
-
 func checkParserErrors(t *testing.T, p *Parser) {
 	errors := p.Errors()
 	if len(errors) == 0 {