Update RIDDL language reference for when/else/end syntax

EBNF grammar:
- Updated when_condition to support literal_string | ["!"] identifier
- Updated when_statement to include optional else block

Language reference:
- Added documentation for else block in when statements
- Added section on condition types (literal strings, identifiers, negated)
- Documented when/else/end syntax with examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: reidspencer

docs/riddl/references/ebnf-grammar.md

@@ -191,7 +191,8 @@ statement = when_statement | match_statement | send_statement | tell_statement |
 entity_statement = statement | morph_statement | become_statement ;
 
 (* Control flow *)
-when_statement = "when" literal_string "then" pseudo_code_block "end" ;
+when_condition = literal_string | ["!"] identifier ;
+when_statement = "when" when_condition "then" pseudo_code_block ["else" pseudo_code_block] "end" ;
 match_statement = "match" literal_string "{" {match_case}+ ["default" "{" {statement} "}"] "}" ;
 match_case = "case" literal_string "{" {statement} "}" ;
 

docs/riddl/references/language-reference.md

@@ -378,8 +378,30 @@ when "condition" then {
 } end
 ```
 
-The `end` keyword is required to terminate when statements. Note: there is no
-`else` clause - use multiple `when` statements for different conditions.
+The `end` keyword is required to terminate when statements.
+
+**Condition Types:**
+The `when` statement accepts three forms of conditions:
+- Literal string: `when "user is authenticated" then`
+- Identifier reference: `when authorized then` (using a `let` binding)
+- Negated identifier: `when !authorized then`
+
+**Optional Else Block:**
+An `else` clause can handle the false case:
+```
+let authorized = "user has permission"
+when authorized then {
+  send event ActionCompleted to outlet Events
+} else {
+  error "User not authorized"
+} end
+```
+
+Alternatively, use multiple `when` statements with negation:
+```
+when authorized then { ... } end
+when !authorized then { ... } end
+```
 
 ### Match Statement
 Pattern matching for multiple conditions: