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[Token identifier]
	{
		set
		{
			var name = (string)identifier.Literal!;
			if (values.ContainsKey(name))
			{
				throw new RuntimeError(identifier, $"Cannot redefine variable {name} in same scope.");
			}
			values[name] = value;
		}
		get
		{
			var name = (string)identifier.Literal!;
			try
			{
				return values[name];
			}
			catch
			{
				if (enclosing != null)
				{
					return enclosing[identifier];
				}
				throw new RuntimeError(identifier, $"Undefined variable {name}.");
			}
		}
	}
}

public class Interpreter : AST.IExprVisitor<Env, object>
{
	private class PatternMismatchException : Exception
	{
		public readonly Token Start;
		public PatternMismatchException(Token start, string message) : base(message)
		{
			Start = start;
		}
	}

	private class VariantTagMismatchException : PatternMismatchException
	{
		public VariantTagMismatchException(Token start, string patternTag, string valueTag) : base(start, $"Pattern tag {patternTag} does not match value tag {valueTag}.") { }
	}

	class PatternBinder : AST.IPatternVisitor<(object, Env), ValueTuple>
	{
		public ValueTuple visitFieldPatternPattern((object, Env) context, AST.FieldPattern pattern)
		{
			return (pattern.Pattern ?? new AST.SimplePattern(pattern.Name, 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)
					{
						string name = (string)field.Name.Literal!;
						removedLabels.Add(name);
						var fieldValue = r.Get(name);
						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 PatternMismatchException(pattern.Start, "Matched value {obj} is not a record.");
		}

		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:
					var tag = (string)pattern.Tag.Literal!;
					if (v.Tag != tag)
					{
						throw new VariantTagMismatchException(pattern.Start, tag, 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 PatternMismatchException(pattern.Start, "Variant pattern arity does not match variant value.");
			}
			throw new PatternMismatchException(pattern.Start, $"Matched value {obj} is 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}");
			}
			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);
		var vb = checkBoolOperand(expr.Condition.Start, cond);
		if (vb == Variant.True)
		{
			return evaluate(env, expr.Then);
		}
		else
		{
			return evaluate(env, expr.Else);
		}
	}

	public object visitIndexerExpr(Env env, AST.Indexer expr)
	{
		var left = checkListOperand(expr.Left.Start, evaluate(env, expr.Left));
		var index = checkNumberOperand(expr.Index.Start, 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)
					{
						var start = e is PatternMismatchException ?
							((PatternMismatchException)e).Start :
							binding.Start;
						throw new RuntimeError(start, e.Message);
					}
					break;
				default:
					throw new NotImplementedException("TODO function bindings");
			}
		}
		return evaluate(newEnv, expr.Body);
	}

	public object visitListExpr(Env env, AST.List expr)
	{
		List l = List.Empty;
		foreach (var itemExpr in expr.Elements)
		{
			l = l.Add(evaluate(env, itemExpr));
		}
		return l;
	}

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

	public object visitRecordExpr(Env env, AST.Record expr)
	{
		Record rec = Record.Empty;
		if (expr.Base != null)
		{
			var baseRecValue = evaluate(env, expr.Base.Value);
			if (baseRecValue is not Record)
			{
				throw new RuntimeError(expr.Base.Value.Start, "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 = (string)update.Name.Literal!;
				if (updateLabels.Contains(label))
				{
					throw new RuntimeError(update.Name, "Record updates must be to unique fields.");
				}
				updateLabels.Add(label);
				(var updateValue, var updateValueToken) =
					update.Value == null ?
						(env[update.Name], update.Name) :
						(evaluate(env, update.Value), update.Value.Start);
				try
				{
					baseRec = baseRec.Update(label, updateValue);
				}
				catch
				{
					throw new RuntimeError(updateValueToken, "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 = (string)extension.Name.Literal!;
			if (extLabels.Contains(label))
			{
				throw new RuntimeError(extension.Name, "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);
		var r = checkRecordOperand(expr.Left.Start, left);
		try
		{
			return r.Get((string)expr.FieldName.Literal!);
		}
		catch
		{
			throw new RuntimeError(expr.FieldName, "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)
	{
		var tag = (string)expr.Tag.Literal!;
		if (expr.Argument == null)
		{
			return new Variant(tag, null);
		}
		return new Variant(tag, evaluate(env, expr.Argument));
	}

	public object visitWhenExpr(Env env, AST.When expr)
	{
		var head = evaluate(env, expr.Head);
		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 (VariantTagMismatchException)
			{
				continue;
			}
			catch (PatternMismatchException e)
			{
				throw new RuntimeError(e.Start, e.Message);
			}
		}
		throw new RuntimeError(expr.Start, "No matching patterns.");
	}
}