Added the Interpreter class

src/main/fr/celticinfo/lox/Interpreter.kt

@@ -0,0 +1,111 @@
+package fr.celticinfo.lox
+
+import fr.celticinfo.lox.TokenType.*
+
+class Interpreter: ExprVisitor<Any?>{
+    override fun visitBinary(binary: Binary): Any? {
+        val left = evaluate(binary.left)
+        val right = evaluate(binary.right)
+
+        return when (binary.operator.type) {
+            MINUS -> {
+                checkNumberOperands(binary.operator, left, right)
+                left as Double - right as Double
+            }
+            SLASH -> {
+                checkNumberOperands(binary.operator, left, right)
+                if (right == 0.0) {
+                    throw RuntimeError(binary.operator, "Division by zero")
+                }
+                left as Double / right as Double
+            }
+            STAR -> {
+                checkNumberOperands(binary.operator, left, right)
+                left as Double * right as Double
+            }
+            PLUS -> {
+                if (left is Double && right is Double) {
+                    left + right
+                } else if (left is String && right is String) {
+                    left + right
+                } else {
+                    throw RuntimeError(binary.operator, "Operands must be two numbers or two strings")
+                }
+            }
+            GREATER -> {
+                checkNumberOperands(binary.operator, left, right)
+                left as Double > right as Double
+            }
+            GREATER_EQUAL -> {
+                checkNumberOperands(binary.operator, left, right)
+                left as Double >= right as Double
+            }
+            LESS -> {
+                checkNumberOperands(binary.operator, left, right)
+                (left as Double) < right as Double
+            }
+            LESS_EQUAL -> {
+                checkNumberOperands(binary.operator, left, right)
+                left as Double <= right as Double
+            }
+            BANG_EQUAL -> return !isEqual(left, right)
+            EQUAL_EQUAL -> return isEqual(left, right)
+            else -> null
+        }
+    }
+
+    override fun visitGrouping(grouping: Grouping): Any? {
+        return evaluate(grouping.expression)
+    }
+
+    override fun visitLiteral(literal: Literal): Any? {
+        return literal.value
+    }
+
+    override fun visitUnary(unary: Unary): Any? {
+        val right = evaluate(unary.right)
+
+        return when (unary.operator.type) {
+            MINUS -> {
+                checkNumberOperand(unary.operator, right)
+                -(right as Double)
+            }
+            BANG -> !isTruthy(right)
+            else -> null
+        }
+    }
+
+    private fun isTruthy(obj: Any?): Boolean {
+        return when (obj) {
+            null -> false
+            is Boolean -> obj
+            else -> true
+        }
+    }
+
+    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
+        } else if (left == null) {
+            false
+        } else {
+            left == right
+        }
+    }
+
+    private fun checkNumberOperand(operator: Token, right: Any?) {
+        if (right !is Double) {
+            throw RuntimeError(operator, "Operand must be a number")
+        }
+    }
+
+    private fun checkNumberOperands(operator: Token, left: Any?, right: Any?) {
+        if (left !is Double || right !is Double) {
+            throw RuntimeError(operator, "Operands must be numbers")
+        }
+    }
+}

src/main/fr/celticinfo/lox/RuntimeError.kt

@@ -0,0 +1,5 @@
+package fr.celticinfo.lox
+
+class RuntimeError(val token: Token, message: String) : RuntimeException(message) {
+
+}