finn-lang/Interpreter.cs

using System;
using System.Collections.Generic;
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 List
	{
		public required ImmutableList<object> Items { get; init; }

		public static List Empty = new List { Items = ImmutableList<object>.Empty };

		public object this[double i]
		{
			get { return Items[(int)i]; }
		}

		public bool IsEmpty
		{
			get { return Items.IsEmpty; }
		}

		public List Add(object item)
		{
			return new List { Items = Items.Add(item) };
		}

		public override string ToString()
		{
			System.IO.StringWriter sw = new System.IO.StringWriter();
			sw.Write("[");
			foreach (var item in this.Items)
			{
				sw.Write($" {item},");
			}
			sw.Write(" ]");
			return sw.ToString();
		}
	}

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

		public override string ToString()
		{
			System.IO.StringWriter sw = new System.IO.StringWriter();
			sw.Write("{");
			foreach ((var label, var values) in this.Fields)
			{
				foreach (var value in values)
				{
					sw.Write($" {label} = {value},");
				}
			}
			sw.Write(" }");
			return sw.ToString();
		}
	}

	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 List checkListOperand(Token op, Object operand)
	{
		if (operand is List l) return l;
		throw new RuntimeError(op, "Operand must be a record.");
	}

	private Record checkRecordOperand(Token op, Object operand)
	{
		if (operand is Record r) return r;
		throw new RuntimeError(op, "Operand must be a record.");
	}

	private double checkNumberOperand(Token op, Object operand)
	{
		if (operand is double d) return d;
		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.");
	}

	private Variant checkBoolOperand(Token op, object operand)
	{
		if (operand is Variant v)
		{
			if (v == Variant.True || v == Variant.False) return v;
		}
		throw new RuntimeError(op, "Operand must be <true,false>.");
	}

	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);
	}

	public object visitVariableExpr(AST.Variable expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitIfExpr(AST.If expr)
	{
		var cond = evaluate(expr.Condition);
		// TODO Maybe I should token info in the AST.
		var vb = checkBoolOperand(new Token(TokenType.If, "if", null, 1), cond);
		if (vb == Variant.True)
		{
			return evaluate(expr.Then);
		}
		else
		{
			return evaluate(expr.Else);
		}
	}

	public object visitIndexerExpr(AST.Indexer expr)
	{
		// TODO use real token
		var tok = new Token(TokenType.LBracket, "[", null, 1);
		var left = checkListOperand(tok, evaluate(expr.Left));
		var index = checkNumberOperand(tok, evaluate(expr.Index));
		try
		{
			var item = left[index];
			return new Variant("some", item);
		}
		catch { return new Variant("nothing", null); }
	}

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

	public object visitListExpr(AST.List expr)
	{
		// TODO use real token
		var tok = new Token(TokenType.LBracket, "[", null, 1);
		List l = List.Empty;
		foreach (var itemExpr in expr.Elements)
		{
			var item = evaluate(itemExpr);
			if (!l.IsEmpty)
			{
				try { checkTypesEqual(l[0], item); }
				catch { throw new RuntimeError(tok, "List items must all have same type."); }
			}
			l = l.Add(item);
		}
		return l;
	}

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

	public object visitRecordExpr(AST.Record expr)
	{
		// TODO use real token
		Token tok = new Token(TokenType.LBrace, "{", null, 1);
		Record rec = Record.Empty;
		if (expr.Base != null)
		{
			var baseRecValue = evaluate(expr.Base.Value);
			if (baseRecValue is not Record)
			{
				throw new RuntimeError(tok, "Base record must be a record.");
			}
			var baseRec = (Record)baseRecValue;

			// Updates
			HashSet<string> updateLabels = new HashSet<string>();
			foreach (AST.Field update in expr.Base.Updates)
			{
				var label = update.Name.Value;
				if (updateLabels.Contains(label))
				{
					throw new RuntimeError(tok, "Record updates must be to unique fields.");
				}
				updateLabels.Add(label);
				if (update.Value == null) throw new NotImplementedException();
				try
				{
					baseRec = baseRec.Update(label, evaluate(update.Value));
				}
				catch
				{
					throw new RuntimeError(tok, "Field update must have same type as previous value.");
				}
			}
			rec = baseRec;
		}

		// Extensions
		HashSet<string> extLabels = new HashSet<string>();
		foreach (AST.Field extension in expr.Extensions)
		{
			var label = extension.Name.Value;
			if (extLabels.Contains(label))
			{
				throw new RuntimeError(tok, "Record extensions must have unique field names.");

			}
			extLabels.Add(label);
			if (extension.Value == null) throw new NotImplementedException();
			rec = rec.Extend(label, evaluate(extension.Value));
		}

		return rec;
	}

	public object visitSelectorExpr(AST.Selector expr)
	{
		var left = evaluate(expr.Left);
		// TODO Use real token.
		var tok = new Token(TokenType.Period, ".", null, 1);
		var r = checkRecordOperand(tok, left);
		try
		{
			return r.Get(expr.FieldName.Value);
		}
		catch
		{
			throw new RuntimeError(tok, "Operand must have selected field.");
		}
	}

	public object visitSequenceExpr(AST.Sequence expr)
	{
		evaluate(expr.Left);
		return evaluate(expr.Right);
	}

	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 (checkBoolOperand(expr.Op, right) == Variant.True)
				{
					return Variant.False;
				}
				else
				{
					return Variant.True;
				}
			default:
				// Unreachable
				throw new Exception($"bad unary op: {expr.Op}");
		}
	}

	public object visitVariantExpr(AST.Variant expr)
	{
		return new Variant(expr.Tag.Value, expr.Argument);
	}

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