[Chapter6&Parsing] - Implementing the first part of the parser

run.sh

@@ -7,6 +7,6 @@ javac -d out src/com/tool/*.java
 # Run Lox interpreter
 java -cp out com.lox.Lox
 # Run Lox ast print printer
-java -cp out com.lox.AstPrinter
+# java -cp out com.lox.AstPrinter
 # Run Lox tool
-java -cp out com.tool.GenerateAst
+# java -cp out com.tool.GenerateAst

src/com/lox/AstPrinter.java

@@ -37,16 +37,16 @@ class AstPrinter implements Expr.Visitor<String> {
 
     return builder.toString();
   }
-  public static void main(String[] args) {
-    Expr expression = new Expr.Binary(
-        new Expr.Unary(
-            new Token(TokenType.MINUS, "-", null, 1),
-            new Expr.Literal(123)),
-        new Token(TokenType.STAR, "*", null, 1),
-        new Expr.Grouping(
-            new Expr.Literal(45.67)));
+  // public static void main(String[] args) {
+  //   Expr expression = new Expr.Binary(
+  //       new Expr.Unary(
+  //           new Token(TokenType.MINUS, "-", null, 1),
+  //           new Expr.Literal(123)),
+  //       new Token(TokenType.STAR, "*", null, 1),
+  //       new Expr.Grouping(
+  //           new Expr.Literal(45.67)));
 
-    System.out.println(new AstPrinter().print(expression));
-  }
+  //   System.out.println(new AstPrinter().print(expression));
+  // }
 
 }

src/com/lox/Lox.java

@@ -24,11 +24,13 @@ public class Lox {
   private static void run(String source) {
     Scanner scanner = new Scanner(source);
     List<Token> tokens = scanner.scanTokens();
+    Parser parser = new Parser(tokens);
+    Expr expression = parser.parse();
 
-    // For now, just print the tokens.
-    for (Token token : tokens) {
-      System.out.println(token);
-    }
+    // Stop if there was a syntax error.
+    if (hadError) return;
+
+    System.out.println(new AstPrinter().print(expression));
   }
   static void error(int line, String message) {
     report(line, "", message);
@@ -41,6 +43,14 @@ public class Lox {
     hadError = true;
   }
 
+  static void error(Token token, String message) {
+    if (token.type == TokenType.EOF) {
+      report(token.line, " at end", message);
+    } else {
+      report(token.line, " at '" + token.lexeme + "'", message);
+    }
+  }
+
 
   private static void runFile(String path) throws IOException {
     byte[] bytes = Files.readAllBytes(Paths.get(path));

src/com/lox/Parser.java

@@ -0,0 +1,154 @@
+package com.lox;
+
+import java.util.List;
+
+import static com.lox.TokenType.*;
+
+class Parser {
+  private static class ParseError extends RuntimeException {}
+  private final List<Token> tokens;
+  private int current = 0;
+
+  Parser(List<Token> tokens) {
+    this.tokens = tokens;
+  }
+  Expr parse() {
+    try {
+      return expression();
+    } catch (ParseError error) {        
+      return null;
+    }
+  }
+  private Expr expression(){
+    return equality();
+  }
+  private Expr equality() {
+    Expr expr = comparison();
+
+    while (match(BANG_EQUAL, EQUAL_EQUAL)) {
+      
+      Token operator = previous();
+      Expr right = comparison();
+      expr = new Expr.Binary(expr, operator, right);
+    }
+
+    return expr;
+  }
+  private Expr comparison(){
+    Expr expr = term();
+
+    while(match(GREATER, GREATER_EQUAL, LESS, LESS_EQUAL)){
+        Token operator = previous();
+        Expr right = term();
+        expr = new Expr.Binary(expr, operator, right);
+    }
+    return expr;
+  }
+
+  private Expr term() {
+    Expr expr = factor();
+
+    while (match(MINUS, PLUS)) {
+      Token operator = previous();
+      Expr right = factor();
+      expr = new Expr.Binary(expr, operator, right);
+    }
+
+    return expr;
+  }
+  private Expr factor() {
+    Expr expr = unary();
+
+    while (match(SLASH, STAR)) {
+      Token operator = previous();
+      Expr right = unary();
+      expr = new Expr.Binary(expr, operator, right);
+    }
+
+    return expr;
+  }
+  private Expr unary() {
+    if (match(BANG, MINUS)) {
+      Token operator = previous();
+      Expr right = unary();
+      return new Expr.Unary(operator, right);
+    }
+
+    return primary();
+  }
+  
+  private Expr primary() {
+    if (match(FALSE)) return new Expr.Literal(false);
+    if (match(TRUE)) return new Expr.Literal(true);
+    if (match(NIL)) return new Expr.Literal(null);
+
+    if (match(NUMBER, STRING)) {
+      return new Expr.Literal(previous().literal);
+    }
+
+    if (match(LEFT_PAREN)) {
+      Expr expr = expression();
+      consume(RIGHT_PAREN, "Expect ')' after expression.");
+      return new Expr.Grouping(expr);
+    }
+    throw error(peek(), "Expect expression.");
+  }
+
+  private boolean match(TokenType... types) {
+    for(TokenType type : types) {
+        if(check(type)){
+            advance();
+            return true;
+        }        
+    }
+    return false;
+  }
+  private Token consume(TokenType type, String message) {
+    if (check(type)) return advance();
+
+    throw error(peek(), message);
+  }
+  private boolean check(TokenType type) {
+    if(isAtEnd()) return false;
+    return peek().type == type;    
+  }
+  private Token advance() {
+    if(!isAtEnd()) current++;
+    return previous();
+  }
+  private boolean isAtEnd(){
+    return peek().type == EOF;
+  }
+  private Token peek(){
+    return tokens.get(current);
+  }
+  private Token previous(){
+    return tokens.get(current - 1);
+  }
+  private ParseError error(Token token, String message) {
+    Lox.error(token, message);
+    return new ParseError();
+  }
+  private void synchronize() {
+    advance();
+
+    while (!isAtEnd()) {
+      if (previous().type == SEMICOLON) return;
+
+      switch (peek().type) {
+        case CLASS:
+        case FUN:
+        case VAR:
+        case FOR:
+        case IF:
+        case WHILE:
+        case PRINT:
+        case RETURN:
+          return;
+      }
+
+      advance();
+    }
+  }
+
+}