Add token peeking to lexer

lexer.cpp

@@ -32,7 +32,7 @@ int Lexer::getNextChar() {
   return ch;
 }
 
-Result<Token> Lexer::Lex() {
+Result<Token> Lexer::LexImpl() {
   int ch = getNextChar();
   pos_t row = row_, col = col_;
 

test.cpp

@@ -29,6 +29,24 @@ TEST(Lexer, HelloWorld) {
   ASSERT_EQ(lexer.Lex().get(), xos::Token(xos::Token::eof, 2, 20));
 }
 
+TEST(Lexer, Peek) {
+  std::stringstream ss(kHelloWorldStr);
+  xos::Lexer lexer(ss);
+
+  xos::Token expected(xos::Token::identifier, 1, 1, "main");
+
+  // We can peek as much as we want and see the same token.
+  ASSERT_EQ(lexer.Peek().get(), expected);
+  ASSERT_EQ(lexer.Peek().get(), expected);
+  ASSERT_EQ(lexer.Peek().get(), expected);
+  ASSERT_EQ(lexer.Lex().get(), expected);
+
+  xos::Token lparen(xos::Token::lparen, 1, 5);
+  ASSERT_EQ(lexer.Peek().get(), lparen);
+  ASSERT_EQ(lexer.Peek().get(), lparen);
+  ASSERT_EQ(lexer.Peek().get(), lparen);
+}
+
 TEST(Lexer, UnfinishedArrowToken) {
   std::stringstream ss("=");
   xos::Lexer lexer(ss);

xos.h

@@ -40,6 +40,7 @@ class Token {
     number,
   };
 
+  Token() = default;
   Token(Kind kind, pos_t row, pos_t col, std::string val)
       : kind_(kind), start_{row, col}, val_(val) {}
   Token(Kind kind, pos_t row, pos_t col) : kind_(kind), start_{row, col} {}
@@ -128,16 +129,37 @@ class Result {
 class Lexer {
  public:
   Lexer(std::istream &input);
-  Result<Token> Lex();
+
+  Result<Token> Lex() {
+    if (has_lookahead_token_) {
+      has_lookahead_token_ = false;
+      return lookahead_token_;
+    }
+    return LexImpl();
+  }
+
+  Result<Token> Peek() {
+    if (!has_lookahead_token_) {
+      auto next = LexImpl();
+      if (next.hasError()) return next;
+
+      has_lookahead_token_ = true;
+      lookahead_token_ = next.get();
+    }
+    return lookahead_token_;
+  }
 
  private:
   int getNextChar();
+  Result<Token> LexImpl();
 
   std::istream &input_;
   pos_t row_ = 1, col_ = 0;
 
   bool has_lookahead_ = false;
   int lookahead_;
+  bool has_lookahead_token_ = false;
+  Token lookahead_token_;
 };
 
 namespace ast {