diff --git a/src/main/fr/celticinfo/lox/Interpreter.kt b/src/main/fr/celticinfo/lox/Interpreter.kt
index 6091077..77207f6 100644
--- a/src/main/fr/celticinfo/lox/Interpreter.kt
+++ b/src/main/fr/celticinfo/lox/Interpreter.kt
@@ -3,6 +3,22 @@ package fr.celticinfo.lox
 import fr.celticinfo.lox.TokenType.*
 
 class Interpreter: ExprVisitor<Any?>{
+
+    fun interpret(expr: Expr): Any? {
+        return try {
+            val value = evaluate(expr)
+            println(stringify(value))
+            value
+        } catch (error: RuntimeError) {
+            Lox.runtimeError(error)
+            null
+        }
+    }
+
+    private fun evaluate(expr: Expr): Any? {
+        return expr.accept(this)
+    }
+
     override fun visitBinary(binary: Binary): Any? {
         val left = evaluate(binary.left)
         val right = evaluate(binary.right)
@@ -83,10 +99,6 @@ class Interpreter: ExprVisitor<Any?>{
         }
     }
 
-    private fun evaluate(expr: Expr): Any? {
-        return expr.accept(this)
-    }
-
     private fun isEqual(left: Any?, right: Any?): Boolean {
         return if (left == null && right == null) {
             true
@@ -108,4 +120,18 @@ class Interpreter: ExprVisitor<Any?>{
             throw RuntimeError(operator, "Operands must be numbers")
         }
     }
+
+    private fun stringify(obj: Any?): String {
+        return when (obj) {
+            null -> "nil"
+            is Double -> {
+                var text = obj.toString()
+                if (text.endsWith(".0")) {
+                    text = text.substring(0, text.length - 2)
+                }
+                text
+            }
+            else -> obj.toString()
+        }
+    }
 }
\ No newline at end of file
diff --git a/src/main/fr/celticinfo/lox/Lox.kt b/src/main/fr/celticinfo/lox/Lox.kt
index 63d3719..b102902 100644
--- a/src/main/fr/celticinfo/lox/Lox.kt
+++ b/src/main/fr/celticinfo/lox/Lox.kt
@@ -3,11 +3,9 @@ package fr.celticinfo.lox
 import kotlin.system.exitProcess
 
 fun main(args: Array<String>) {
-    val lox = Lox()
-
     when (args.size) {
-        0 -> lox.runPrompt()
-        1 -> lox.runFile(args.first())
+        0 -> Lox.runPrompt()
+        1 -> Lox.runFile(args.first())
         else -> {println("Usage: jlox [script]") ; exitProcess(64)}
     }
 }
@@ -15,8 +13,10 @@ fun main(args: Array<String>) {
 /**
  * Lox is the main class that runs the Lox interpreter.
  */
-class Lox {
+object Lox {
     private var hadError = false
+    private var hadRuntimeError = false
+    private val interpreter = Interpreter()
 
     fun runPrompt() {
         while (true) {
@@ -33,6 +33,7 @@ class Lox {
         run(String(bytes))
         // Indicate an error in the exit code.
         if (hadError) exitProcess(65)
+        if (hadRuntimeError) exitProcess(70)
     }
 
     private fun run(source: String) {
@@ -44,24 +45,28 @@ class Lox {
         // Stop if there was a syntax error.
         if (hadError) return
 
-        println(AstPrinter().print(expression!!))
+        interpreter.interpret(expression!!)
     }
 
-    companion object {
-        fun error(line: Int, s: String) {
-            report(line, "", s)
-        }
+    fun error(line: Int, s: String) {
+        report(line, "", s)
+    }
 
-        fun error(token: Token, message: String) {
-            if (token.type == TokenType.EOF) {
-                report(token.line, " at end", message)
-            } else {
-                report(token.line, " at '${token.lexeme}'", message)
-            }
+    fun error(token: Token, message: String) {
+        if (token.type == TokenType.EOF) {
+            report(token.line, " at end", message)
+        } else {
+            report(token.line, " at '${token.lexeme}'", message)
         }
+    }
 
-        private fun report(line: Int, where: String, message: String) {
-            System.err.println("[line $line] Error$where: $message")
-        }
+    fun runtimeError(error: RuntimeError) {
+        System.err.println("${error.message}\n[line ${error.token.line}]")
+        hadRuntimeError = true
+    }
+
+    private fun report(line: Int, where: String, message: String) {
+        System.err.println("[line $line] Error$where: $message")
+        hadError = true
     }
 }
\ No newline at end of file
diff --git a/src/main/fr/celticinfo/lox/Parser.kt b/src/main/fr/celticinfo/lox/Parser.kt
index c8abc4c..6f4b09d 100644
--- a/src/main/fr/celticinfo/lox/Parser.kt
+++ b/src/main/fr/celticinfo/lox/Parser.kt
@@ -11,7 +11,13 @@ class Parser(private val tokens: List<Token>) {
 
     fun parse(): Expr? {
         return try {
-            expression()
+            val result = expression()
+
+            if (!isAtEnd()) {
+                throw error(peek(), "Expect end of expression.")
+            }
+
+            result
         } catch (error: ParseError) {
             null
         }
diff --git a/src/test/fr/celticinfo/lox/InterpreterTest.kt b/src/test/fr/celticinfo/lox/InterpreterTest.kt
new file mode 100644
index 0000000..811d8e8
--- /dev/null
+++ b/src/test/fr/celticinfo/lox/InterpreterTest.kt
@@ -0,0 +1,46 @@
+package fr.celticinfo.lox
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.Assertions.*
+
+class InterpreterTest {
+
+    @Test
+    fun `validate interpreter`() {
+        val code = """
+            (1 + 2 * 3 - 5) / 2
+        """.trimIndent()
+        val scanner = Scanner(code)
+        val tokens = scanner.scanTokens()
+        val parser = Parser(tokens)
+        val expr = parser.parse()
+        val value = Interpreter().interpret(expr!!)
+        assertEquals(1.0, value)
+    }
+
+    @Test
+    fun `Division by zero should raise error`() {
+        val code = """
+            1 / 0
+        """.trimIndent()
+        val scanner = Scanner(code)
+        val tokens = scanner.scanTokens()
+        val parser = Parser(tokens)
+        val expr = parser.parse()
+        val value = Interpreter().interpret(expr!!)
+        assertNull(value)
+    }
+
+    @Test
+    fun `Invalid type raise error`() {
+        val code = """
+            1 + false
+        """.trimIndent()
+        val scanner = Scanner(code)
+        val tokens = scanner.scanTokens()
+        val parser = Parser(tokens)
+        val expr = parser.parse()
+        val value = Interpreter().interpret(expr!!)
+        assertNull(value)
+    }
+}
\ No newline at end of file
diff --git a/src/test/fr/celticinfo/lox/ParserTest.kt b/src/test/fr/celticinfo/lox/ParserTest.kt
index e663801..d5c70f3 100644
--- a/src/test/fr/celticinfo/lox/ParserTest.kt
+++ b/src/test/fr/celticinfo/lox/ParserTest.kt
@@ -29,4 +29,16 @@ class ParserTest {
         val expr = parser.parse()
         assertNull(expr)
     }
+
+    @Test
+    fun `invalid expression generates parsing error`() {
+        val code = """
+            1(2
+        """.trimIndent()
+        val scanner = Scanner(code)
+        val tokens = scanner.scanTokens()
+        val parser = Parser(tokens)
+        val expr = parser.parse()
+        assertNull(expr)
+    }
 }
\ No newline at end of file