finn-lang/Interpreter.cs

using System;
using System.Collections.Immutable;

using AST = Finn.AST;

namespace Finn;

public class Interpreter : AST.IExprVisitor<object>
{
	public class RuntimeError : Exception
	{
		public readonly Token Token;

		internal RuntimeError(Token token, String message) : base(message)
		{
			Token = token;
		}
	}

	private static void checkTypesEqual(object a, object b)
	{
		var aType = a.GetType();
		var bType = b.GetType();
		if (aType != bType)
		{
			throw new Exception("Type mismatch: {aType} != {bType}.");
		}
	}

	private class Record
	{
		public static Record Empty = new Record
		{
			Fields = ImmutableDictionary<string, ImmutableStack<object>>.Empty,
		};
		public required ImmutableDictionary<string, ImmutableStack<object>> Fields { get; init; }

		public Record Update(string name, object value)
		{
			ImmutableStack<object>? values;
			if (!Fields.TryGetValue(name, out values))
			{
				throw new ArgumentException($"no such field: {name}");
			}
			checkTypesEqual(value, values.Peek());
			return new Record { Fields = Fields.SetItem(name, values!.Pop().Push(value)) };
		}

		public object Get(string name)
		{
			ImmutableStack<object>? values;
			if (!Fields.TryGetValue(name, out values))
			{
				throw new ArgumentException($"no such field: {name}");
			}
			return values.Peek();
		}

		public Record Remove(string name)
		{
			ImmutableStack<object>? values;
			if (!Fields.TryGetValue(name, out values))
			{
				throw new ArgumentException($"no such field: {name}");
			}
			var value = values.Peek();
			var popped = values.Pop();
			if (popped.IsEmpty)
			{
				return new Record { Fields = Fields.Remove(name) };
			}
			return new Record { Fields = Fields.SetItem(name, popped) };
		}

		public Record Extend(string name, object value)
		{
			var values = Fields.GetValueOrDefault(name, ImmutableStack<object>.Empty);
			return new Record { Fields = Fields.SetItem(name, values.Push(value)) };
		}
	}

	private record Variant(string Tag, object? Value)
	{
		public static readonly Variant True = new Variant("true", null);
		public static readonly Variant False = new Variant("false", null);
		public static Variant FromBool(bool b)
		{
			return b ? True : False;
		}
		public bool IsEmpty { get { return Value == null; } }

		public override string ToString()
		{
			if (Value == null)
			{
				return $"`{Tag}";
			}
			else
			{
				return $"`{Tag}({Value})";
			}
		}
	}

	public void Interpret(AST.Expr expression)
	{
		try
		{
			var value = evaluate(expression);
			Console.WriteLine(value);
		}
		catch (RuntimeError err)
		{
			Program.runtimeError(err);
		}
	}

	private object evaluate(AST.Expr expr)
	{
		return expr.accept(this);
	}

	private void checkNumberOperand(Token op, Object operand)
	{
		if (operand is double) return;
		throw new RuntimeError(op, "Operand must be a number.");
	}

	private void checkNumberOperands(Token op, object left, object right)
	{
		if (left is double && right is double) return;
		throw new RuntimeError(op, "Operands must be numbers.");
	}

	private void checkStringOperands(Token op, object left, object right)
	{
		if (left is string && right is string) return;
		throw new RuntimeError(op, "Operands must be strings.");
	}

	public object visitBinaryExpr(AST.Binary expr)
	{
		var left = evaluate(expr.Left);
		var right = evaluate(expr.Right);

		switch (expr.Op.type)
		{
			case TokenType.Minus:
				checkNumberOperands(expr.Op, left, right);
				return (double)left - (double)right;
			case TokenType.Plus:
				checkNumberOperands(expr.Op, left, right);
				return (double)left + (double)right;
			case TokenType.Slash:
				checkNumberOperands(expr.Op, left, right);
				return (double)left / (double)right;
			case TokenType.Asterisk:
				checkNumberOperands(expr.Op, left, right);
				return (double)left * (double)right;

			case TokenType.PlusPlus:
				checkStringOperands(expr.Op, left, right);
				return (string)left + (string)right;

			case TokenType.Greater:
				checkNumberOperands(expr.Op, left, right);
				return Variant.FromBool((double)left > (double)right);
			case TokenType.GreaterEqual:
				checkNumberOperands(expr.Op, left, right);
				return Variant.FromBool((double)left >= (double)right);
			case TokenType.Less:
				checkNumberOperands(expr.Op, left, right);
				return Variant.FromBool((double)left < (double)right);
			case TokenType.LessEqual:
				checkNumberOperands(expr.Op, left, right);
				return Variant.FromBool((double)left <= (double)right);

			case TokenType.BangEqual:
				return Variant.FromBool((left, right) switch
				{
					(String l, String r) => l != r,
					(double l, double r) => l != r,
					_ => throw new ArgumentException(),
				});
			case TokenType.DoubleEqual:
				return Variant.FromBool((left, right) switch
				{
					(String l, String r) => l == r,
					(double l, double r) => l == r,
					_ => throw new ArgumentException(),
				});
		}

		throw new ArgumentException($"bad binary op: {expr.Op}");
	}

	public object visitCallExpr(AST.Call expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitGroupingExpr(AST.Grouping expr)
	{
		return evaluate(expr.Expression);
		throw new System.NotImplementedException();
	}

	public object visitIdentifierExpr(AST.Identifier expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitIfExpr(AST.If expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitIndexerExpr(AST.Indexer expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitLetExpr(AST.Let expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitListExpr(AST.List expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitLiteralExpr(AST.Literal expr)
	{
		return expr.Value;
	}

	public object visitRecordExpr(AST.Record expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitSelectorExpr(AST.Selector expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitSequenceExpr(AST.Sequence expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitUnaryExpr(AST.Unary expr)
	{
		object right = evaluate(expr.Right);
		switch (expr.Op.type)
		{
			case TokenType.Minus:
				checkNumberOperand(expr.Op, right);
				return -(double)right;
			case TokenType.Bang:
				if (right is Variant v)
				{
					if (v == Variant.True)
					{
						return Variant.False;
					}
					else if (v == Variant.False)
					{
						return Variant.True;
					}
				}
				throw new RuntimeError(expr.Op, "Boolean must be <true, false>");
			default:
				// Unreachable
				throw new Exception($"bad unary op: {expr.Op}");
		}
	}

	public object visitVariantExpr(AST.Variant expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitWhenExpr(AST.When expr)
	{
		throw new System.NotImplementedException();
	}
}