diff --git a/grammar.txt b/grammar.txt
index dea4e6a..73af91c 100644
--- a/grammar.txt
+++ b/grammar.txt
@@ -21,8 +21,10 @@ operator ->
 	| "++" ;
 control ->
 	| if
-//	| when
+	| when
 if -> "if" expression "then" expression "else" expression ;
+variant_pattern -> "`" identifier ( "(" ( ( "_" | identifier ) ( "," ( "_" | identifier )* )? ) ")" )? ;
+when -> "when" expression "is" ( variant_pattern "=>" expression )+ ;
 compound -> variant | record | list ;
 variant -> "`" identifier ( "(" ( expression ( "," expression)* )? ")" )? ;
 base_record -> expression ( "with" identifier "=" expression ( "," "with" identifier "=" expression )* )?