feat: allow expressions in add-to-list statements

.claude/skills/mendix/write-microflows.md

@@ -420,6 +420,18 @@ commit $Product with events refresh;
 
 **Best Practice**: Use `with events` when you want before/after commit event handlers to execute. Use `refresh` when the committed object is displayed in the client and you want the UI to update immediately.
 
+## List Operations
+
+```mdl
+-- Existing variable form
+add $Item to $Items;
+
+-- Expression-valued add, useful when round-tripping Studio Pro list-add values
+add head($SourceItems) to $Items;
+```
+
+Use expression-valued `add` only when the expression returns an object compatible with the target list element type.
+
 ## Database Operations
 
 ### RETRIEVE Statement

cmd/mxcli/tui/watcher.go

@@ -4,6 +4,7 @@ import (
 	"os"
 	"path/filepath"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	tea "github.com/charmbracelet/bubbletea"
@@ -78,6 +79,7 @@ func newWatcher(mprPath, contentsDir string, sender MsgSender) (*Watcher, error)
 
 func (w *Watcher) run(sender MsgSender) {
 	var debounceTimer *time.Timer
+	var debounceSeq atomic.Uint64
 
 	for {
 		select {
@@ -110,7 +112,11 @@ func (w *Watcher) run(sender MsgSender) {
 			if debounceTimer != nil {
 				debounceTimer.Stop()
 			}
+			seq := debounceSeq.Add(1)
 			debounceTimer = time.AfterFunc(watchDebounce, func() {
+				if debounceSeq.Load() != seq {
+					return
+				}
 				sender.Send(MprChangedMsg{})
 			})
 

cmd/mxcli/tui/watcher_test.go

@@ -35,10 +35,11 @@ func TestWatcherDebounce(t *testing.T) {
 	}
 	defer w.Close()
 
-	// Rapidly write 5 times — should debounce into a single message
+	// Rapidly write 5 times — should debounce into a single message.
+	// Keep the burst tighter than the debounce window so slow CI machines do
+	// not accidentally let an intermediate timer fire.
 	for i := range 5 {
 		_ = os.WriteFile(unitFile, []byte{byte('a' + i)}, 0644)
-		time.Sleep(50 * time.Millisecond)
 	}
 
 	// Wait for debounce to fire (500ms + margin)

docs/01-project/MDL_QUICK_REFERENCE.md

@@ -224,6 +224,7 @@ authentication basic, session
 | Rollback | `rollback $entity [refresh];` | Reverts uncommitted changes |
 | Retrieve (DB) | `retrieve $Var from Module.Entity [where condition];` | Database XPath retrieve |
 | Retrieve (Assoc) | `retrieve $list from $Parent/Module.AssocName;` | Retrieve by association |
+| Add to list | `add expression to $list;` | Also accepts existing `add $item to $list;` form |
 | Call microflow | `$Result = call microflow Module.Name (Param = $value);` | |
 | Call nanoflow | `$Result = call nanoflow Module.Name (Param = $value);` | |
 | Call JS action | `$Result = call javascript action Module.Name (Param = $value);` | JavaScript action (nanoflow/microflow) |

docs/11-proposals/PROPOSAL_microflow_add_expression_to_list.md

@@ -0,0 +1,33 @@
+# Proposal: Microflow ADD Expression To List
+
+Status: Draft
+
+## Summary
+
+Allow `add` microflow statements to use any expression as the value being added to a list:
+
+```mdl
+add if $UseFirst then $FirstItem else $SecondItem to $TargetList;
+```
+
+The existing variable-only form remains valid:
+
+```mdl
+add $Item to $TargetList;
+```
+
+## Motivation
+
+Studio Pro stores the value of a list-add action as an expression string. Existing models can therefore contain a list-add value that is not a bare variable. MDL previously parsed only `add $Item to $List`, so describe/exec round-trips could not preserve expression-valued list additions.
+
+## Semantics
+
+The parser stores the add value as an expression. For compatibility, a bare variable expression also populates the legacy `Item` field in the AST. The builder writes the expression source to the Mendix `ChangeListAction.Value` field and falls back to the legacy item variable only when no expression is present.
+
+## Tests And Examples
+
+`mdl-examples/doctype-tests/add_expression_to_list.mdl` demonstrates adding an object-valued conditional expression to another list. Go regression tests cover parser behavior and builder output for both expression and simple-variable forms.
+
+## Open Questions
+
+- Should validation infer the list element type and reject expressions that cannot produce an object compatible with the target list?

docs/11-proposals/README.md

@@ -53,6 +53,7 @@ BSON schema Registry ◄──── multi-version Support
 | [XPath Gaps](xpath-gaps-proposal.md) | Partial | XPath constraint support gap analysis. ~85% complete, association paths and nested predicates remain | — |
 | [Microflow ENUM SPLIT Statement](PROPOSAL_microflow_enum_split_statement.md) | Implemented | Enumeration decision splits via `case $Var when Value then … end case;` | — |
 | [Microflow CHANGE Refresh Modifier](PROPOSAL_microflow_change_refresh_modifier.md) | Draft | Preserve `RefreshInClient` on change-object actions | — |
+| [Microflow ADD Expression To List](PROPOSAL_microflow_add_expression_to_list.md) | Draft | Preserve expression-valued list-add actions in microflow round-trips | — |
 | [LLM MDL Assistance](PROPOSAL_llm_mdl_assistance.md) | Proposed | Enhanced error messages with examples, reorganized skills by use case | — |
 
 ### Testing & Evaluation