diff --git a/src/main/ExprGenerator.ws.kts b/src/main/ExprGenerator.ws.kts
index 314a92a..677965a 100644
--- a/src/main/ExprGenerator.ws.kts
+++ b/src/main/ExprGenerator.ws.kts
@@ -1,3 +1,5 @@
+import java.util.*
+
 val types = listOf(
     "Binary   : Expr left, Token operator, Expr right",
     "Grouping : Expr expression",
@@ -6,10 +8,23 @@ val types = listOf(
 )
 
 println("package fr.celticinfo.lox")
+
 println()
-println("sealed class Expr {")
+println("interface ExprVisitor<R> {")
+for (type in types) {
+    val parts = type.split(":")
+    val name = parts[0].trim()
+    println("    fun visit$name(${name.lowercase(Locale.getDefault())}: $name): R")
+}
 println("}")
 
+println()
+println("""/**
+ * The Expr class represents the different types of expressions that can be parsed by the Parser.
+ */""".trimIndent())
+println("sealed class Expr {")
+println("    abstract fun <R> accept(visitor: ExprVisitor<R>): R")
+println("}")
 for (type in types) {
     val parts = type.split(":")
     val name = parts[0].trim()
@@ -23,5 +38,9 @@ for (type in types) {
         val sep = if (field == fields.last()) "" else ","
         println("    val $name: $type$sep")
     }
-    println(") : Expr()")
-}
\ No newline at end of file
+    println(") : Expr() {")
+    println("    override fun <R> accept(visitor: ExprVisitor<R>): R {")
+    println("        return visitor.visit$name(this)")
+    println("    }")
+    println("}")
+}
diff --git a/src/main/fr/celticinfo/lox/AstPrinter.kt b/src/main/fr/celticinfo/lox/AstPrinter.kt
new file mode 100644
index 0000000..5692f34
--- /dev/null
+++ b/src/main/fr/celticinfo/lox/AstPrinter.kt
@@ -0,0 +1,34 @@
+package fr.celticinfo.lox
+
+class AstPrinter : ExprVisitor<String> {
+    fun print(expr: Expr): String {
+        return expr.accept(this)
+    }
+
+    override fun visitBinary(binary: Binary): String {
+        return parenthesize(binary.operator.lexeme, binary.left, binary.right)
+    }
+
+    override fun visitGrouping(grouping: Grouping): String {
+        return parenthesize("group", grouping.expression)
+    }
+
+    override fun visitLiteral(literal: Literal): String {
+        return literal.value?.toString() ?: "nil"
+    }
+
+    override fun visitUnary(unary: Unary): String {
+        return parenthesize(unary.operator.lexeme, unary.right)
+    }
+
+    private fun parenthesize(name: String, vararg exprs: Expr): String {
+        val builder = StringBuilder()
+        builder.append("(").append(name)
+        for (expr in exprs) {
+            builder.append(" ")
+            builder.append(expr.accept(this))
+        }
+        builder.append(")")
+        return builder.toString()
+    }
+}
\ No newline at end of file
diff --git a/src/main/fr/celticinfo/lox/Expr.kt b/src/main/fr/celticinfo/lox/Expr.kt
index 4a2b22d..a3c623f 100644
--- a/src/main/fr/celticinfo/lox/Expr.kt
+++ b/src/main/fr/celticinfo/lox/Expr.kt
@@ -1,26 +1,48 @@
 package fr.celticinfo.lox
 
+interface ExprVisitor<R> {
+    fun visitBinary(binary: Binary): R
+    fun visitGrouping(grouping: Grouping): R
+    fun visitLiteral(literal: Literal): R
+    fun visitUnary(unary: Unary): R
+
+}
+
 /**
  * The Expr class represents the different types of expressions that can be parsed by the Parser.
  */
 sealed class Expr {
+    abstract fun <R> accept(visitor: ExprVisitor<R>): R
 }
 
 data class Binary(
-    val left: Expr,
-    val operator: Token,
-    val right: Expr
-) : Expr()
+    val left: Expr, val operator: Token, val right: Expr
+) : Expr() {
+    override fun <R> accept(visitor: ExprVisitor<R>): R {
+        return visitor.visitBinary(this)
+    }
+}
 
 data class Grouping(
     val expression: Expr
-) : Expr()
+) : Expr() {
+    override fun <R> accept(visitor: ExprVisitor<R>): R {
+        return visitor.visitGrouping(this)
+    }
+}
 
 data class Literal(
     val value: Any
-) : Expr()
+) : Expr() {
+    override fun <R> accept(visitor: ExprVisitor<R>): R {
+        return visitor.visitLiteral(this)
+    }
+}
 
 data class Unary(
-    val operator: Token,
-    val right: Expr
-) : Expr()
\ No newline at end of file
+    val operator: Token, val right: Expr
+) : Expr() {
+    override fun <R> accept(visitor: ExprVisitor<R>): R {
+        return visitor.visitUnary(this)
+    }
+}
\ No newline at end of file
diff --git a/src/main/fr/celticinfo/loxext/RpnPrinter.kt b/src/main/fr/celticinfo/loxext/RpnPrinter.kt
new file mode 100644
index 0000000..525e49f
--- /dev/null
+++ b/src/main/fr/celticinfo/loxext/RpnPrinter.kt
@@ -0,0 +1,35 @@
+package fr.celticinfo.loxext
+
+import fr.celticinfo.lox.*
+
+class RpnPrinter : ExprVisitor<String> {
+    fun print(expr: Expr): String {
+        return expr.accept(this)
+    }
+
+    override fun visitBinary(binary: Binary): String {
+        return stack(binary.operator.lexeme, binary.left, binary.right)
+    }
+
+    override fun visitGrouping(grouping: Grouping): String {
+        return stack("", grouping.expression)
+    }
+
+    override fun visitLiteral(literal: Literal): String {
+        return literal.value?.toString() ?: "nil"
+    }
+
+    override fun visitUnary(unary: Unary): String {
+        return stack(unary.operator.lexeme, unary.right)
+    }
+
+    private fun stack(name: String, vararg exprs: Expr): String {
+        val builder = StringBuilder()
+        for (expr in exprs) {
+            builder.append(expr.accept(this))
+            if (name.isNotEmpty()) builder.append(" ")
+        }
+        builder.append(name)
+        return builder.toString()
+    }
+}
diff --git a/src/test/fr/celticinfo/lox/AstPrinterTest.kt b/src/test/fr/celticinfo/lox/AstPrinterTest.kt
new file mode 100644
index 0000000..34439bc
--- /dev/null
+++ b/src/test/fr/celticinfo/lox/AstPrinterTest.kt
@@ -0,0 +1,50 @@
+package fr.celticinfo.lox
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class AstPrinterTest {
+    private val astPrinter = AstPrinter()
+
+    @Test
+    fun print() {
+        val expression: Expr = Binary(
+            Unary(
+                Token(TokenType.MINUS, "-", null, 1), Literal(123)
+            ), Token(TokenType.STAR, "*", null, 1), Grouping(
+                Literal(45.67)
+            )
+        )
+        assertEquals("(* (- 123) (group 45.67))", astPrinter.print(expression))
+    }
+
+    @Test
+    fun visitBinary() {
+        val expression: Expr = Binary(
+            Literal(1), Token(TokenType.PLUS, "+", null, 1), Literal(2)
+        )
+        assertEquals("(+ 1 2)", astPrinter.print(expression))
+    }
+
+    @Test
+    fun visitGrouping() {
+        val expression: Expr = Grouping(
+            Literal(123)
+        )
+        assertEquals("(group 123)", astPrinter.print(expression))
+    }
+
+    @Test
+    fun visitLiteral() {
+        val expression: Expr = Literal(123)
+        assertEquals("123", astPrinter.print(expression))
+    }
+
+    @Test
+    fun visitUnary() {
+        val expression: Expr = Unary(
+            Token(TokenType.MINUS, "-", null, 1), Literal(123)
+        )
+        assertEquals("(- 123)", astPrinter.print(expression))
+    }
+}
\ No newline at end of file
diff --git a/src/test/fr/celticinfo/loxext/RpnPrinterTest.kt b/src/test/fr/celticinfo/loxext/RpnPrinterTest.kt
new file mode 100644
index 0000000..c0c3cde
--- /dev/null
+++ b/src/test/fr/celticinfo/loxext/RpnPrinterTest.kt
@@ -0,0 +1,70 @@
+package fr.celticinfo.loxext
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+import fr.celticinfo.lox.*
+
+class RpnPrinterTest {
+    private val rpnPrinter = RpnPrinter()
+
+    @Test
+    fun print() {
+        val expression: Expr = Binary(
+            Unary(
+                Token(TokenType.MINUS, "-", null, 1), Literal(123)
+            ), Token(TokenType.STAR, "*", null, 1), Grouping(
+                Literal(45.67)
+            )
+        )
+        assertEquals("123 - 45.67 *", rpnPrinter.print(expression))
+    }
+
+    @Test
+    fun testComplexExpr() {
+        val expression: Expr = Binary(
+            Grouping(
+                Binary(
+                    Literal(1), Token(TokenType.PLUS, "+", null, 1), Literal(2)
+                )
+            ),
+            Token(TokenType.STAR, "*", null, 1),
+            Grouping(
+                Binary(
+                    Literal(4), Token(TokenType.MINUS, "-", null, 1), Literal(3)
+                )
+            )
+        )
+        assertEquals("1 2 + 4 3 - *", rpnPrinter.print(expression))
+    }
+
+    @Test
+    fun visitBinary() {
+        val expression: Expr = Binary(
+            Literal(1), Token(TokenType.PLUS, "+", null, 1), Literal(2)
+        )
+        assertEquals("1 2 +", rpnPrinter.print(expression))
+    }
+
+    @Test
+    fun visitGrouping() {
+        val expression: Expr = Grouping(
+            Literal(123)
+        )
+        assertEquals("123", rpnPrinter.print(expression))
+    }
+
+    @Test
+    fun visitLiteral() {
+        val expression: Expr = Literal(123)
+        assertEquals("123", rpnPrinter.print(expression))
+    }
+
+    @Test
+    fun visitUnary() {
+        val expression: Expr = Unary(
+            Token(TokenType.MINUS, "-", null, 1), Literal(123)
+        )
+        assertEquals("123 -", rpnPrinter.print(expression))
+    }
+}
\ No newline at end of file