finn-lang/Interpreter.cs

using System;
using System.Collections.Generic;
using System.Collections.Immutable;

using AST = Finn.AST;

namespace Finn;

public class RuntimeError : Exception
{
	public readonly Token Token;

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

public class Env
{
	private readonly Env? enclosing;
	private readonly Dictionary<string, object> values = new Dictionary<string, object>();

	public Env()
	{
		this.enclosing = null;
	}

	public Env(Env enclosing)
	{
		this.enclosing = enclosing;
	}

	public object this[AST.Name name]
	{
		set
		{
			if (values.ContainsKey(name.Value))
			{
				// TODO use real location info
				var tok = new Token(TokenType.Identifier, name.Value, null, 1);
				throw new RuntimeError(tok, $"Cannot redefine variable {name} in same scope.");
			}
			values[name.Value] = value;
		}
		get
		{
			try
			{
				return values[name.Value];
			}
			catch
			{
				if (enclosing != null)
				{
					return enclosing[name];
				}
				// TODO use real location info
				var tok = new Token(TokenType.Identifier, name.Value, null, 1);
				throw new RuntimeError(tok, $"Undefined variable {name}.");
			}
		}
	}
}

public class Interpreter : AST.IExprVisitor<Env, object>
{
	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 PatternTagMismatchException : Exception
	{
		public PatternTagMismatchException(string patternTag, string valueTag) : base($"Pattern tag {patternTag} does not match value tag {valueTag}.") { }
	}

	private class PatternTypeMismatchException : Exception { }

	class PatternBinder : AST.IPatternVisitor<(object, Env), ValueTuple>
	{
		public ValueTuple visitFieldPatternPattern((object, Env) context, AST.FieldPattern pattern)
		{
			return (pattern.Pattern ?? new AST.SimplePattern(pattern.Name)).accept(context, this);
		}

		public ValueTuple visitRecordPatternPattern((object, Env) context, AST.RecordPattern pattern)
		{
			var (obj, env) = context;
			switch (obj)
			{
				case Record r:
					var removedLabels = new List<string>();
					foreach (var field in pattern.Fields)
					{
						removedLabels.Add(field.Name.Value);
						var fieldValue = r.Get(field.Name.Value);
						field.accept((fieldValue, env), this);
					}
					if (pattern.Rest != null)
					{
						var rest = r.Without(removedLabels);
						pattern.Rest.accept((rest, env), this);
					}
					return ValueTuple.Create();
			}
			throw new PatternTypeMismatchException();
		}

		public ValueTuple visitSimplePatternPattern((object, Env) context, AST.SimplePattern pattern)
		{
			var (obj, env) = context;
			if (pattern.Identifier == null)
			{
				return ValueTuple.Create();
			}
			env[pattern.Identifier] = obj;
			return ValueTuple.Create();
		}

		public ValueTuple visitVariantPatternPattern((object, Env) context, AST.VariantPattern pattern)
		{
			var (obj, env) = context;
			switch (obj)
			{
				case Variant v:
					if (v.Tag != pattern.Tag.Value)
					{
						throw new PatternTagMismatchException(pattern.Tag.Value, v.Tag);
					}
					if (v.Value == null && pattern.Argument == null)
					{
						return ValueTuple.Create();
					}
					if (v.Value != null && pattern.Argument != null)
					{
						return pattern.Argument.accept((v.Value, env), this);
					}
					throw new PatternTypeMismatchException();
			}
			// TODO throw a better exception
			throw new Exception($"Not a variant.");
		}
	}

	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 Without(IEnumerable<string> labels)
		{
			return new Record { Fields = Fields.RemoveRange(labels) };
		}

		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(new Env(), expression);
			Console.WriteLine(value);
		}
		catch (RuntimeError err)
		{
			Program.runtimeError(err);
		}
	}

	private object evaluate(Env env, AST.Expr expr)
	{
		return expr.accept(env, 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(Env env, AST.Binary expr)
	{
		var left = evaluate(env, expr.Left);
		var right = evaluate(env, 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(Env env, AST.Call expr)
	{
		throw new System.NotImplementedException();
	}

	public object visitGroupingExpr(Env env, AST.Grouping expr)
	{
		return evaluate(env, expr.Expression);
	}

	public object visitVariableExpr(Env env, AST.Variable expr)
	{
		return env[expr.Value];
	}

	public object visitIfExpr(Env env, AST.If expr)
	{
		var cond = evaluate(env, 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(env, expr.Then);
		}
		else
		{
			return evaluate(env, expr.Else);
		}
	}

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

	public object visitLetExpr(Env env, AST.Let expr)
	{
		var newEnv = new Env(env);
		foreach (var binding in expr.Bindings)
		{
			switch (binding)
			{
				case AST.VarBinding(var pattern, var valueExpr):
					var value = evaluate(env, valueExpr);
					try
					{
						pattern.accept((value, newEnv), new PatternBinder());
					}
					catch (Exception e)
					{
						// TODO use real info
						var tok = new Token(TokenType.Let, "let", null, 1);
						throw new RuntimeError(tok, e.Message);
					}
					break;
				default:
					throw new NotImplementedException("TODO function bindings");
			}
		}
		return evaluate(newEnv, expr.Body);
	}

	public object visitListExpr(Env env, 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(env, 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(Env env, AST.Literal expr)
	{
		return expr.Value;
	}

	public object visitRecordExpr(Env env, 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(env, 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);
				var updateValue = update.Value == null ? env[update.Name] : evaluate(env, update.Value);
				try
				{
					baseRec = baseRec.Update(label, updateValue);
				}
				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);
			var extensionValue = extension.Value == null ? env[extension.Name] : evaluate(env, extension.Value);
			rec = rec.Extend(label, extensionValue);
		}

		return rec;
	}

	public object visitSelectorExpr(Env env, AST.Selector expr)
	{
		var left = evaluate(env, 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(Env env, AST.Sequence expr)
	{
		evaluate(env, expr.Left);
		return evaluate(env, expr.Right);
	}

	public object visitUnaryExpr(Env env, AST.Unary expr)
	{
		object right = evaluate(env, 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(Env env, AST.Variant expr)
	{
		if (expr.Argument == null)
		{
			return new Variant(expr.Tag.Value, null);
		}
		return new Variant(expr.Tag.Value, evaluate(env, expr.Argument));
	}

	public object visitWhenExpr(Env env, AST.When expr)
	{
		var head = evaluate(env, expr.Head);
		// TODO use real info
		var tok = new Token(TokenType.When, "when", null, 1);
		foreach (var c in expr.Cases)
		{
			try
			{
				var newEnv = new Env(env);
				c.Pattern.accept((head, newEnv), new PatternBinder());
				return evaluate(newEnv, c.Value);
			}
			catch (PatternTagMismatchException)
			{
				continue;
			}
			catch (PatternTypeMismatchException e)
			{
				throw new RuntimeError(tok, e.Message);
			}
		}
		throw new RuntimeError(tok, "No matching patterns.");
	}
}