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Interpreter.cs
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Interpreter.cs
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using static SharpLox.TokenType;
namespace SharpLox;
public class Interpreter : Expr.Visitor<object?>, Stmt.Visitor<object?>
{
public readonly Env Globals;
private Env environment;
private readonly Dictionary<Expr, int> locals;
public Interpreter()
{
Globals = new();
environment = Globals;
locals = new();
Globals.Define("clock", new LoxCallable.Clock());
}
public void Interpret(List<Stmt> statements)
{
try
{
statements.ForEach(Execute);
}
catch (RuntimeError error)
{
Lox.RuntimeError(error);
}
}
public void Resolve(Expr expr, int depth) => locals[expr] = depth;
#region Statement Visitor Implementation
public object? VisitBlockStmt(Stmt.Block stmt)
{
ExecuteBlock(stmt.Statements, new Env(environment));
return null;
}
public object? VisitClassStmt(Stmt.Class stmt)
{
LoxClass? superclass = null;
if (stmt.Superclass is not null)
{
superclass = Evaluate(stmt.Superclass) as LoxClass;
if (superclass is null)
{
throw new RuntimeError(stmt.Superclass.Name, "Superclass must be a class.");
}
}
environment.Define(stmt.Name.Lexeme, null);
if (stmt.Superclass is not null)
{
environment = new Env(environment);
environment.Define("super", superclass);
}
Dictionary<String, LoxFunction> methods = new();
foreach (var method in stmt.Methods)
{
var isInitializer = method.Name.Lexeme == "init";
var function = new LoxFunction(method, environment, isInitializer);
methods[method.Name.Lexeme] = function;
}
var klass = new LoxClass(stmt.Name.Lexeme, superclass, methods);
if (stmt.Superclass is not null)
{
environment = environment.Enclosing!;
}
environment.Assign(stmt.Name, klass);
return null;
}
public object? VisitExpressionStmt(Stmt.Expression stmt)
{
Evaluate(stmt.Expr);
return null;
}
public object? VisitFunctionStmt(Stmt.Function stmt)
{
var function = new LoxFunction(stmt, environment, false);
environment.Define(stmt.Name.Lexeme, function);
return null;
}
public object? VisitIfStmt(Stmt.If stmt)
{
if (Truthy(Evaluate(stmt.Condition)))
{
Execute(stmt.ThenBranch);
}
else if (stmt.ElseBranch is not null)
{
Execute(stmt.ElseBranch);
}
return null;
}
public object? VisitPrintStmt(Stmt.Print stmt)
{
var value = Evaluate(stmt.Expr);
Console.WriteLine(Stringify(value));
return null;
}
public object? VisitReturnStmt(Stmt.Return stmt)
{
var value = stmt.Value is null ? null : Evaluate(stmt.Value);
throw new Return(value);
}
public object? VisitVarStmt(Stmt.Var stmt)
{
object? value = null;
if (stmt.Initializer is not null)
{
value = Evaluate(stmt.Initializer);
}
environment.Define(stmt.Name.Lexeme, value);
return null;
}
public object? VisitWhileStmt(Stmt.While stmt)
{
while (Truthy(Evaluate(stmt.Condition)))
{
Execute(stmt.Body);
}
return null;
}
#endregion
#region Expression Visitor Implementation
public object? VisitAssignExpr(Expr.Assign expr)
{
var value = Evaluate(expr.Value);
environment.Assign(expr.Name, value);
return value;
}
public object? VisitBinaryExpr(Expr.Binary expr)
{
var left = Evaluate(expr.Left);
var right = Evaluate(expr.Right);
switch (expr.Operator.Type)
{
case BANG_EQUAL:
return !object.Equals(left, right);
case EQUAL_EQUAL:
return object.Equals(left, right);
case GREATER:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! > (double)right!;
case GREATER_EQUAL:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! >= (double)right!;
case LESS:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! < (double)right!;
case LESS_EQUAL:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! <= (double)right!;
case MINUS:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! - (double)right!;
case PLUS:
if (left is double && right is double)
{
return (double)left + (double)right;
}
if (left is string && right is string)
{
return (string)left + (string)right;
}
throw new RuntimeError(expr.Operator, "Operands must be two numbers or two strings.");
case SLASH:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! / (double)right!;
case STAR:
CheckNumberOperands(expr.Operator, left, right);
return (double)left! * (double)right!;
default: return null;
};
}
public object? VisitCallExpr(Expr.Call expr)
{
var function = Evaluate(expr.Callee) as LoxCallable;
var arguments = expr.Arguments.ConvertAll(Evaluate);
if (function is null)
{
throw new RuntimeError(expr.Paren, "Can only call function and classes.");
}
if (arguments.Count != function.Arity)
{
var message = $"Expected {function.Arity} arguments but got {arguments.Count}.";
throw new RuntimeError(expr.Paren, message);
}
return function.Call(this, arguments);
}
public object? VisitGetExpr(Expr.Get expr)
{
var obj = Evaluate(expr.Object);
if (obj is LoxInstance instance)
{
return instance.Get(expr.Name);
}
throw new RuntimeError(expr.Name, "Only instances have properties.");
}
public object? VisitGroupingExpr(Expr.Grouping expr)
{
return Evaluate(expr.Expression);
}
public object? VisitLiteralExpr(Expr.Literal expr)
{
return expr.Value;
}
public object? VisitLogicalExpr(Expr.Logical expr)
{
var left = Evaluate(expr.Left);
if (expr.Operator.Type == OR)
{
if (Truthy(left)) return left;
}
else
{
if (!Truthy(left)) return left;
}
return Evaluate(expr.Right);
}
public object? VisitSetExpr(Expr.Set expr)
{
var obj = Evaluate(expr.Object);
if (obj is not LoxInstance instance)
{
throw new RuntimeError(expr.Name, "Only instances have fields.");
}
var value = Evaluate(expr.Value);
instance.Set(expr.Name, value);
return value;
}
public object? VisitSuperExpr(Expr.Super expr)
{
// extremely yolo with nullability by this point
var distance = locals.GetValueOrDefault(expr);
var superclass = environment.GetAt(distance, "super") as LoxClass;
var instance = environment.GetAt(distance - 1, "this") as LoxInstance;
var method = superclass?.FindMethod(expr.Method.Lexeme);
if (method is null)
{
throw new RuntimeError(expr.Method, $"Undefined property '{expr.Method.Lexeme}'.");
}
return method.Bind(instance!);
}
public object? VisitThisExpr(Expr.This expr)
{
return LookUpVariable(expr.Keyword, expr);
}
public object? VisitUnaryExpr(Expr.Unary expr)
{
var right = Evaluate(expr.Right);
switch (expr.Operator.Type)
{
case BANG:
return !Truthy(right);
case MINUS:
CheckNumberOperand(expr.Operator, right);
return -(double)right!;
default:
return null;
};
}
public object? VisitVariableExpr(Expr.Variable expr)
{
return LookUpVariable(expr.Name, expr);
}
#endregion
private object? Evaluate(Expr expr)
{
return expr.Accept(this);
}
private void Execute(Stmt stmt)
{
stmt.Accept(this);
}
public void ExecuteBlock(List<Stmt> statements, Env blockEnv)
{
var previous = environment;
try
{
environment = blockEnv;
statements.ForEach(Execute);
}
finally
{
environment = previous;
}
}
private object? LookUpVariable(Token name, Expr expr)
{
if (locals.TryGetValue(expr, out var distance))
{
return environment.GetAt(distance, name.Lexeme);
}
return Globals.Get(name);
}
private static void CheckNumberOperand(Token op, object? operand)
{
if (operand is not double)
{
throw new RuntimeError(op, "Operand must be a number.");
}
}
private static void CheckNumberOperands(Token op, object? left, object? right)
{
if (left is not double || right is not double)
{
throw new RuntimeError(op, "Operands must be numbers.");
}
}
private static string? Stringify(object? o)
{
if (o is null) return "nil";
if (o is double n)
{
var text = n.ToString();
if (text.EndsWith(".0"))
{
return text[0..(text.Length - 2)];
}
return text;
}
return o.ToString();
}
private static bool Truthy(object? o) => o switch
{
null => false,
bool b => b,
_ => true
};
}