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Parser.cs
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Parser.cs
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using static SharpLox.TokenType;
namespace SharpLox;
public class Parser
{
private class ParseError : Exception { }
private readonly List<Token> tokens;
private int currentPos = 0;
private bool AtEnd => Current.Type == EOF;
private Token Current => tokens[currentPos];
private Token Previous => tokens[currentPos - 1];
public Parser(List<Token> tokens)
{
this.tokens = tokens;
}
public List<Stmt> Parse()
{
var statements = new List<Stmt>();
while (!AtEnd)
{
var decl = Declaration();
if (decl is not null) statements.Add(decl);
}
return statements;
}
#region Rules
private Stmt? Declaration()
{
try
{
if (Match(CLASS)) return ClassDeclaration();
if (Match(FUN)) return FunctionDeclaration("function");
if (Match(VAR)) return VarDeclaration();
return Statement();
}
catch (ParseError)
{
Synchronize();
return null;
}
}
private Stmt ClassDeclaration()
{
var name = Consume(IDENTIFIER, "Expect class name.");
Expr.Variable? superclass = null;
if (Match(LESS))
{
Consume(IDENTIFIER, "Expect superclass name.");
superclass = new Expr.Variable(Previous);
}
Consume(LEFT_BRACE, "Expect '{' before class body.");
List<Stmt.Function> methods = new();
while (!Check(RIGHT_BRACE) && !AtEnd)
{
methods.Add(FunctionDeclaration("method"));
}
Consume(RIGHT_BRACE, "Expect '}' after class body.");
return new Stmt.Class(name, superclass, methods);
}
private Stmt.Function FunctionDeclaration(string kind)
{
var name = Consume(IDENTIFIER, $"Expect {kind} name.");
Consume(LEFT_PAREN, $"Expect '(' after {kind} name.");
List<Token> parameters = new();
if (!Check(RIGHT_PAREN))
{
parameters.Add(Consume(IDENTIFIER, "Expect parameter name."));
while (Match(COMMA))
{
if (parameters.Count >= 255) Error(Current, "Can't have more than 255 parameters.");
parameters.Add(Consume(IDENTIFIER, "Expect parameter name."));
}
}
Consume(RIGHT_PAREN, "Expect ')' after parameters.");
Consume(LEFT_BRACE, $"Expect '{{' before {kind} body.");
var body = Block();
return new Stmt.Function(name, parameters, body);
}
private Stmt VarDeclaration()
{
Token name = Consume(IDENTIFIER, "Expect variable name.");
Expr? initializer = null;
if (Match(EQUAL))
{
initializer = Expression();
}
Consume(SEMICOLON, "Expect ';' after variable declaration.");
return new Stmt.Var(name, initializer);
}
private Stmt Statement()
{
if (Match(FOR)) return ForStatement();
if (Match(IF)) return IfStatement();
if (Match(PRINT)) return PrintStatement();
if (Match(RETURN)) return ReturnStatement();
if (Match(WHILE)) return WhileStatement();
if (Match(LEFT_BRACE)) return new Stmt.Block(Block());
return ExpressionStatement();
}
private Stmt ForStatement()
{
Consume(LEFT_PAREN, "Expect '(' after 'for'.");
var initializer = Match(SEMICOLON) ? null :
Match(VAR) ? VarDeclaration() : ExpressionStatement();
var condition = Check(SEMICOLON) ? null : Expression();
Consume(SEMICOLON, "Expect ';' after loop condition.");
var increment = Check(RIGHT_PAREN) ? null : Expression();
Consume(RIGHT_PAREN, "Expect ')' after for clauses.");
var body = Statement();
if (increment is not null)
{
var incStmt = new Stmt.Expression(increment);
body = new Stmt.Block(new List<Stmt> { body, incStmt });
}
condition ??= new Expr.Literal(true);
body = new Stmt.While(condition, body);
if (initializer is not null)
{
body = new Stmt.Block(new List<Stmt> { initializer, body });
}
return body;
}
private Stmt IfStatement()
{
Consume(LEFT_PAREN, "Expect '(' after 'if'.");
var condition = Expression();
Consume(RIGHT_PAREN, "Expect ')' after if condition.");
var thenBranch = Statement();
var elseBranch = Match(ELSE) ? Statement() : null;
return new Stmt.If(condition, thenBranch, elseBranch);
}
private Stmt PrintStatement()
{
var value = Expression();
Consume(SEMICOLON, "Expect ';' after value.");
return new Stmt.Print(value);
}
private Stmt ReturnStatement()
{
var keyword = Previous;
var value = Check(SEMICOLON) ? null : Expression();
Consume(SEMICOLON, "Expect ';' after return value.");
return new Stmt.Return(keyword, value);
}
private Stmt WhileStatement()
{
Consume(LEFT_PAREN, "Expect '(' after 'while'.");
var condition = Expression();
Consume(RIGHT_PAREN, "Expect ')' after while condition.");
var body = Statement();
return new Stmt.While(condition, body);
}
private Stmt ExpressionStatement()
{
var expr = Expression();
Consume(SEMICOLON, "Expect ';' after expression.");
return new Stmt.Expression(expr);
}
private List<Stmt> Block()
{
List<Stmt> statements = new();
while (!Check(RIGHT_BRACE) && !AtEnd)
{
var decl = Declaration();
if (decl is not null) statements.Add(decl);
}
Consume(RIGHT_BRACE, "Expect '}' after block.");
return statements;
}
private Expr Expression()
{
return Assignment();
}
private Expr Assignment()
{
var expr = LogicalOr();
if (Match(EQUAL))
{
var equals = Previous;
var value = Assignment();
if (expr is Expr.Variable variable)
{
return new Expr.Assign(variable.Name, value);
}
else if (expr is Expr.Get get)
{
return new Expr.Set(get.Object, get.Name, value);
}
Error(equals, "Invalid assignment target.");
}
return expr;
}
private Expr LogicalOr()
{
var expr = LogicalAnd();
while (Match(OR))
{
var op = Previous;
var right = LogicalAnd();
expr = new Expr.Logical(expr, op, right);
}
return expr;
}
private Expr LogicalAnd()
{
var expr = Equality();
while (Match(AND))
{
var op = Previous;
var right = Equality();
expr = new Expr.Logical(expr, op, right);
}
return expr;
}
private Expr Equality()
{
Expr expr = Comparison();
while (Match(BANG_EQUAL, EQUAL_EQUAL))
{
var op = Previous;
var right = Comparison();
expr = new Expr.Binary(expr, op, right);
}
return expr;
}
private Expr Comparison()
{
Expr expr = Term();
while (Match(GREATER, GREATER_EQUAL, LESS, LESS_EQUAL))
{
var op = Previous;
var right = Term();
expr = new Expr.Binary(expr, op, right);
}
return expr;
}
private Expr Term()
{
Expr expr = Factor();
while (Match(MINUS, PLUS))
{
var op = Previous;
var right = Factor();
expr = new Expr.Binary(expr, op, right);
}
return expr;
}
private Expr Factor()
{
Expr expr = Unary();
while (Match(SLASH, STAR))
{
var op = Previous;
var right = Unary();
expr = new Expr.Binary(expr, op, right);
}
return expr;
}
private Expr Unary()
{
if (Match(BANG, MINUS))
{
var op = Previous;
Expr right = Unary();
return new Expr.Unary(op, right);
}
return Call();
}
private Expr Call()
{
var expr = Primary();
while (true)
{
if (Match(LEFT_PAREN))
{
expr = FinishCall(expr);
}
else if (Match(DOT))
{
var name = Consume(IDENTIFIER, "Expect property name after '.'.");
expr = new Expr.Get(expr, name);
}
else
{
break;
}
}
return expr;
}
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(SUPER))
{
var keyword = Previous;
Consume(DOT, "Expect '.' after 'super'.");
var method = Consume(IDENTIFIER, "Expect superclass method name.");
return new Expr.Super(keyword, method);
}
if (Match(THIS))
{
return new Expr.This(Previous);
}
if (Match(IDENTIFIER))
{
return new Expr.Variable(Previous);
}
if (Match(LEFT_PAREN))
{
Expr expr = Expression();
Consume(RIGHT_PAREN, "Expect ')' after expression.");
return new Expr.Grouping(expr);
}
throw Error(Current, "Expect expression.");
}
private Expr FinishCall(Expr callee)
{
List<Expr> arguments = new();
if (!Check(RIGHT_PAREN))
{
do
{
if (arguments.Count >= 255)
{
Error(Current, "Can't have more than 255 arguments.");
}
arguments.Add(Expression());
} while (Match(COMMA));
}
var paren = Consume(RIGHT_PAREN, "Expect ')' after arguments.");
return new Expr.Call(callee, paren, arguments);
}
#endregion
#region Helpers
private Token Advance()
{
if (!AtEnd) currentPos++;
return Previous;
}
private bool Check(TokenType type)
{
if (AtEnd) return false;
return Current.Type == type;
}
private Token Consume(TokenType type, string message)
{
if (Check(type)) return Advance();
throw Error(Current, message);
}
private ParseError Error(Token token, String message)
{
Lox.Error(token, message);
return new ParseError();
}
private bool Match(params TokenType[] types)
{
foreach (TokenType type in types)
{
if (Check(type))
{
Advance();
return true;
}
}
return false;
}
private void Synchronize()
{
Advance();
while (!AtEnd)
{
if (Previous.Type == SEMICOLON) return;
switch (Current.Type)
{
case CLASS:
case FUN:
case VAR:
case FOR:
case IF:
case WHILE:
case PRINT:
case RETURN:
return;
}
Advance();
}
}
#endregion
}