Implemented pattern matching in let-exprs

Interpreter.cs

@@ -75,6 +75,79 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 		}
 	}
 
+	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; }
@@ -138,6 +211,11 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			return values.Peek();
 		}
 
+		public Record Without(IEnumerable<string> labels)
+		{
+			return new Record { Fields = Fields.RemoveRange(labels) };
+		}
+
 		public Record Remove(string name)
 		{
 			ImmutableStack<object>? values;
@@ -364,17 +442,16 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 			switch (binding)
 			{
 				case AST.VarBinding(var pattern, var valueExpr):
-					switch (pattern)
+					var value = evaluate(env, valueExpr);
+					try
 					{
-						case AST.SimplePattern(var identifier):
-							var value = evaluate(env, valueExpr);
-							if (identifier != null)
-							{
-								newEnv[identifier] = value;
-							}
-							break;
-						default:
-							throw new NotImplementedException("TODO moar patternz");
+						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:
@@ -509,7 +586,11 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 	public object visitVariantExpr(Env env, AST.Variant expr)
 	{
-		return new Variant(expr.Tag.Value, expr.Argument);
+		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)