Created Parser class

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

@@ -38,9 +38,13 @@ class Lox {
     private fun run(source: String) {
         val scanner = Scanner(source)
         val tokens = scanner.scanTokens()
-        for (token in tokens) {
-            println(token)
-        }
+        val parser = Parser(tokens)
+        val expression = parser.parse()
+
+        // Stop if there was a syntax error.
+        if (hadError) return
+
+        println(AstPrinter().print(expression!!))
     }
 
     companion object {
@@ -48,6 +52,14 @@ class Lox {
             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)
+            }
+        }
+
         private fun report(line: Int, where: String, message: String) {
             System.err.println("[line $line] Error$where: $message")
         }

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

@@ -0,0 +1,151 @@
+package fr.celticinfo.lox
+
+import fr.celticinfo.lox.TokenType.*
+
+/**
+ * The Parser class is responsible for parsing the tokens produced by the Scanner into an abstract syntax tree (AST).
+ * It is a recursive descent parser that uses a series of methods to parse different parts of the grammar.
+ */
+class Parser(private val tokens: List<Token>) {
+    private var current = 0
+
+    fun parse(): Expr? {
+        return try {
+            expression()
+        } catch (error: ParseError) {
+            null
+        }
+    }
+
+    private fun expression(): Expr {
+        return equality()
+    }
+
+    private fun equality(): Expr {
+        var expr = comparison()
+
+        while (match(BANG_EQUAL, EQUAL_EQUAL)) {
+            val operator = previous()
+            val right = comparison()
+            expr = Binary(expr, operator, right)
+        }
+
+        return expr
+    }
+
+    private fun comparison(): Expr {
+        var expr = term()
+
+        while (match(GREATER, GREATER_EQUAL, LESS, LESS_EQUAL)) {
+            val operator = previous()
+            val right = term()
+            expr = Binary(expr, operator, right)
+        }
+
+        return expr
+    }
+
+    private fun term(): Expr {
+        var expr = factor()
+
+        while (match(MINUS, PLUS)) {
+            val operator = previous()
+            val right = factor()
+            expr = Binary(expr, operator, right)
+        }
+
+        return expr
+    }
+
+    private fun factor(): Expr {
+        var expr = unary()
+
+        while (match(SLASH, STAR)) {
+            val operator = previous()
+            val right = unary()
+            expr = Binary(expr, operator, right)
+        }
+
+        return expr
+    }
+
+    private fun unary(): Expr {
+        if (match(BANG, MINUS)) {
+            val operator = previous()
+            val right = unary()
+            return Unary(operator, right)
+        }
+
+        return primary()
+    }
+
+    private fun match(vararg types: TokenType): Boolean {
+        for (type in types) {
+            if (check(type)) {
+                advance()
+                return true
+            }
+        }
+        return false
+    }
+
+    private fun primary(): Expr {
+        when {
+            match(FALSE) -> return Literal(false)
+            match(TRUE) -> return Literal(true)
+            match(NIL) -> return Literal(null)
+            match(NUMBER, STRING) -> return Literal(previous().literal)
+            match(LEFT_PAREN) -> {
+                val expr = expression()
+                consume(RIGHT_PAREN, "Expect ')' after expression.")
+                return Grouping(expr)
+            }
+            else -> throw error(peek(), "Expect expression.")
+        }
+    }
+
+    private fun check(type: TokenType): Boolean {
+        if (isAtEnd()) return false
+        return peek().type == type
+    }
+
+    private fun advance(): Token {
+        if (!isAtEnd()) current++
+        return previous()
+    }
+
+    private fun isAtEnd(): Boolean {
+        return peek().type == EOF
+    }
+
+    private fun peek(): Token {
+        return tokens[current]
+    }
+
+    private fun previous(): Token {
+        return tokens[current - 1]
+    }
+
+    private fun consume(type: TokenType, message: String): Token {
+        if (check(type)) return advance()
+        throw error(peek(), message)
+    }
+
+    private fun error(token: Token, message: String): ParseError {
+        Lox.error(token, message)
+        return ParseError()
+    }
+
+    private fun synchronize() {
+        advance()
+        while (!isAtEnd()) {
+            if (previous().type == SEMICOLON) return
+            when (peek().type) {
+                CLASS, FUN, VAR, FOR, IF, WHILE, PRINT, RETURN -> return
+                else -> advance()
+            }
+        }
+    }
+
+    class ParseError: RuntimeException()
+}

src/test/fr/celticinfo/lox/ParserTest.kt

@@ -0,0 +1,32 @@
+package fr.celticinfo.lox
+
+import fr.celticinfo.loxext.RpnPrinter
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.Assertions.*
+
+class ParserTest {
+    @Test
+    fun `validate parser`() {
+        val code = """
+            1 + 2 * 3 - 4 / 5
+        """.trimIndent()
+        val scanner = Scanner(code)
+        val tokens = scanner.scanTokens()
+        val parser = Parser(tokens)
+        val expr = parser.parse()
+        assertNotNull(expr)
+        assertEquals("1.0 2.0 3.0 * + 4.0 5.0 / -", RpnPrinter().print(expr!!))
+    }
+
+    @Test
+    fun `invalid code 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)
+    }
+}