From 9645d499c017db67655b8f1bc5aa4d5c74786b61 Mon Sep 17 00:00:00 2001
From: nibral <nibral@users.noreply.github.com>
Date: Wed, 23 Jan 2019 21:10:47 +0900
Subject: [PATCH] 3.6: Evaluate if statements

---
 evaluator/evaluator.go      | 29 +++++++++++++++++++++++++++++
 evaluator/evaluator_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 994680e..eb2750d 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -19,6 +19,8 @@ func Eval(node ast.Node) object.Object {
 		return evalStatements(node.Statements)
 	case *ast.ExpressionStatement:
 		return Eval(node.Expression)
+	case *ast.BlockStatement:
+		return evalStatements(node.Statements)
 
 	// expressions
 	case *ast.IntegerLiteral:
@@ -32,6 +34,8 @@ func Eval(node ast.Node) object.Object {
 		left := Eval(node.Left)
 		right := Eval(node.Right)
 		return evalInfixExpression(node.Operator, left, right)
+	case *ast.IfExpression:
+		return evalIfExpression(node)
 
 	}
 
@@ -126,3 +130,28 @@ func nativeBoolToBooleanObject(input bool) *object.Boolean {
 	}
 	return FALSE
 }
+
+func evalIfExpression(ie *ast.IfExpression) object.Object {
+	condition := Eval(ie.Condition)
+
+	if isTruthy(condition) {
+		return Eval(ie.Consequence)
+	} else if ie.Alternative != nil {
+		return Eval(ie.Alternative)
+	} else {
+		return NULL
+	}
+}
+
+func isTruthy(obj object.Object) bool {
+	switch obj {
+	case NULL:
+		return false
+	case TRUE:
+		return true
+	case FALSE:
+		return false
+	default:
+		return true
+	}
+}
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index 31bb465..4ecf4b3 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -122,3 +122,36 @@ func TestBangOperator(t *testing.T) {
 		testBooleanObject(t, evaluated, tt.expected)
 	}
 }
+
+func TestIfElseExpressions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{"if (true) { 10 }", 10},
+		{"if (false) { 10 }", nil},
+		{"if (1) { 10 }", 10},
+		{"if (1 < 2) { 10 }", 10},
+		{"if (1 > 2) { 10 }", nil},
+		{"if (1 > 2) { 10 } else { 20 }", 20},
+		{"if (1 < 2) { 10 } else { 20 }", 10},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		integer, ok := tt.expected.(int)
+		if ok {
+			testIntegerObject(t, evaluated, int64(integer))
+		} else {
+			testNullObject(t, evaluated)
+		}
+	}
+}
+
+func testNullObject(t *testing.T, obj object.Object) bool {
+	if obj != NULL {
+		t.Errorf("object is not NULL. got=%T (%+v)", obj, obj)
+		return false
+	}
+	return true
+}