Parse variants

ASTPrinter.cs

@@ -74,4 +74,13 @@ class ASTPrinter : IVisitor<string>
 	{
 		return parenthesize("list", expr.Elements);
 	}
+
+	public string visitVariantExpr(Variant expr)
+	{
+		if (expr.Argument == null)
+		{
+			return parenthesize("variant", new Identifier { Value = expr.Tag });
+		}
+		return parenthesize("variant", new Identifier { Value = expr.Tag }, expr.Argument);
+	}
 }

Expr.g.cs

@@ -19,6 +19,7 @@ public interface IVisitor<T> {
 	T visitIfExpr(If expr);
 	T visitIdentifierExpr(Identifier expr);
 	T visitListExpr(List expr);
+	T visitVariantExpr(Variant expr);
 	T visitSelectorExpr(Selector expr);
 }
 public class Sequence : Expr
@@ -91,6 +92,15 @@ public class List : Expr
 		return visitor.visitListExpr(this);
 	}
 }
+public class Variant : Expr
+{
+	public required Name Tag { get; init; }
+	public required Expr? Argument { get; init; }
+	public override T accept<T>(IVisitor<T> visitor)
+	{
+		return visitor.visitVariantExpr(this);
+	}
+}
 public class Selector : Expr
 {
 	public required Expr Left { get; init; }

Parser.cs

@@ -303,14 +303,31 @@ class Parser
 	}
 
 	private Expr? variant()
+	{
+		if (!match(TokenType.Backtick))
 		{
 			return null;
-		// throw new NotImplementedException("TODO variant");
+		}
+		Name? tag = name();
+		Expr? argument = null;
+		if (tag == null)
+		{
+			throw error(peek(), "Expect identifier after backtick.");
+		}
+		if (match(TokenType.LParen))
+		{
+			if (!match(TokenType.RParen))
+			{
+				argument = expression();
+				consume(TokenType.RParen, "Expect ')' after variant argument.");
+			}
+		}
+		return new Variant { Tag = tag, Argument = argument };
 	}
 
 	private Expr? record()
 	{
 		return null;
-		// throw new NotImplementedException("TODO record");
+		// TODO record
 	}
 }

ast_classes.fsx

@@ -26,6 +26,8 @@ let types =
         Fields = [ { Type = "Name"; Name = "Value" } ] }
       { Name = "List"
         Fields = [ { Type = "Expr[]"; Name = "Elements" } ] }
+      { Name = "Variant"
+        Fields = [ { Type = "Name"; Name = "Tag" }; { Type = "Expr?"; Name = "Argument" } ] }
       { Name = "Selector"
         Fields = [ { Type = "Expr"; Name = "Left" }; { Type = "Name"; Name = "FieldName" } ] } ]