Parse indexing expressions

ASTPrinter.cs

@@ -125,4 +125,9 @@ class ASTPrinter : IVisitor<string>
 
 		}
 	}
+
+	public string visitIndexerExpr(Indexer expr)
+	{
+		return parenthesize("index", expr.Left, expr.Index);
+	}
 }

Expr.g.cs

@@ -22,6 +22,7 @@ public interface IVisitor<T> {
 	T visitVariantExpr(Variant expr);
 	T visitRecordExpr(Record expr);
 	T visitSelectorExpr(Selector expr);
+	T visitIndexerExpr(Indexer expr);
 }
 public class Sequence : Expr
 {
@@ -120,3 +121,12 @@ public class Selector : Expr
 		return visitor.visitSelectorExpr(this);
 	}
 }
+public class Indexer : Expr
+{
+	public required Expr Left { get; init; }
+	public required Expr Index { get; init; }
+	public override T accept<T>(IVisitor<T> visitor)
+	{
+		return visitor.visitIndexerExpr(this);
+	}
+}

Parser.cs

@@ -220,7 +220,9 @@ class Parser
 
 		if (match(TokenType.LBracket))
 		{
-			throw new NotImplementedException("TODO index");
+			var index = expression();
+			consume(TokenType.RBracket, "Expect '[' after expression.");
+			return new Indexer { Left = expr, Index = index };
 		}
 
 		if (match(TokenType.LParen))

ast_classes.fsx

@@ -30,10 +30,13 @@ let types =
         Fields = [ { Type = "Name"; Name = "Tag" }; { Type = "Expr?"; Name = "Argument" } ] }
       { Name = "Record"
         Fields =
-          [ { Type = "Field[]"; Name = "Extensions" }
+          [ { Type = "Field[]"
+              Name = "Extensions" }
             { Type = "BaseRecord?"; Name = "Base" } ] }
       { Name = "Selector"
-        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Name"; Name = "FieldName" } ] } ]
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Name"; Name = "FieldName" } ] }
+      { Name = "Indexer"
+        Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Expr"; Name = "Index" } ] } ]
 
 let visitorMethod baseName t = $"visit{t.Name}{baseName}"