refactor: add Effect trait and apply_effects infrastructure

Define Effect trait in ui/effect.rs with concrete effect structs for
all model mutations, view changes, navigation, and app state updates.
Each effect implements apply(&self, &mut App). Add App::apply_effects
to apply a sequence of effects. No behavior change yet — existing
key handlers still work as before.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ui/app.rs

@@ -91,6 +91,12 @@ impl App {
         }
     }
 
+    pub fn apply_effects(&mut self, effects: Vec<Box<dyn super::effect::Effect>>) {
+        for effect in effects {
+            effect.apply(self);
+        }
+    }
+
     /// True when the model has no categories yet (show welcome screen)
     pub fn is_empty_model(&self) -> bool {
         self.model.categories.is_empty()

src/ui/effect.rs

@@ -0,0 +1,398 @@
+use std::fmt::Debug;
+use std::path::PathBuf;
+
+use crate::model::cell::{CellKey, CellValue};
+use crate::view::Axis;
+
+use super::app::{App, AppMode};
+
+/// A discrete state change produced by a command.
+/// Effects know how to apply themselves to the App.
+pub trait Effect: Debug {
+    fn apply(&self, app: &mut App);
+}
+
+// ── Model mutations ──────────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct AddCategory(pub String);
+impl Effect for AddCategory {
+    fn apply(&self, app: &mut App) {
+        let _ = app.model.add_category(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct AddItem {
+    pub category: String,
+    pub item: String,
+}
+impl Effect for AddItem {
+    fn apply(&self, app: &mut App) {
+        if let Some(cat) = app.model.category_mut(&self.category) {
+            cat.add_item(&self.item);
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct AddItemInGroup {
+    pub category: String,
+    pub item: String,
+    pub group: String,
+}
+impl Effect for AddItemInGroup {
+    fn apply(&self, app: &mut App) {
+        if let Some(cat) = app.model.category_mut(&self.category) {
+            cat.add_item_in_group(&self.item, &self.group);
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct SetCell(pub CellKey, pub CellValue);
+impl Effect for SetCell {
+    fn apply(&self, app: &mut App) {
+        app.model.set_cell(self.0.clone(), self.1.clone());
+    }
+}
+
+#[derive(Debug)]
+pub struct ClearCell(pub CellKey);
+impl Effect for ClearCell {
+    fn apply(&self, app: &mut App) {
+        app.model.clear_cell(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct AddFormula {
+    pub raw: String,
+    pub target_category: String,
+}
+impl Effect for AddFormula {
+    fn apply(&self, app: &mut App) {
+        if let Ok(formula) = crate::formula::parse_formula(&self.raw, &self.target_category) {
+            app.model.add_formula(formula);
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct RemoveFormula {
+    pub target: String,
+    pub target_category: String,
+}
+impl Effect for RemoveFormula {
+    fn apply(&self, app: &mut App) {
+        app.model.remove_formula(&self.target, &self.target_category);
+    }
+}
+
+// ── View mutations ───────────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct CreateView(pub String);
+impl Effect for CreateView {
+    fn apply(&self, app: &mut App) {
+        app.model.create_view(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct DeleteView(pub String);
+impl Effect for DeleteView {
+    fn apply(&self, app: &mut App) {
+        let _ = app.model.delete_view(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct SwitchView(pub String);
+impl Effect for SwitchView {
+    fn apply(&self, app: &mut App) {
+        let _ = app.model.switch_view(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct SetAxis {
+    pub category: String,
+    pub axis: Axis,
+}
+impl Effect for SetAxis {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().set_axis(&self.category, self.axis);
+    }
+}
+
+#[derive(Debug)]
+pub struct SetPageSelection {
+    pub category: String,
+    pub item: String,
+}
+impl Effect for SetPageSelection {
+    fn apply(&self, app: &mut App) {
+        app.model
+            .active_view_mut()
+            .set_page_selection(&self.category, &self.item);
+    }
+}
+
+#[derive(Debug)]
+pub struct ToggleGroup {
+    pub category: String,
+    pub group: String,
+}
+impl Effect for ToggleGroup {
+    fn apply(&self, app: &mut App) {
+        app.model
+            .active_view_mut()
+            .toggle_group_collapse(&self.category, &self.group);
+    }
+}
+
+#[derive(Debug)]
+pub struct HideItem {
+    pub category: String,
+    pub item: String,
+}
+impl Effect for HideItem {
+    fn apply(&self, app: &mut App) {
+        app.model
+            .active_view_mut()
+            .hide_item(&self.category, &self.item);
+    }
+}
+
+#[derive(Debug)]
+pub struct ShowItem {
+    pub category: String,
+    pub item: String,
+}
+impl Effect for ShowItem {
+    fn apply(&self, app: &mut App) {
+        app.model
+            .active_view_mut()
+            .show_item(&self.category, &self.item);
+    }
+}
+
+#[derive(Debug)]
+pub struct TransposeAxes;
+impl Effect for TransposeAxes {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().transpose_axes();
+    }
+}
+
+#[derive(Debug)]
+pub struct CycleAxis(pub String);
+impl Effect for CycleAxis {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().cycle_axis(&self.0);
+    }
+}
+
+#[derive(Debug)]
+pub struct SetNumberFormat(pub String);
+impl Effect for SetNumberFormat {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().number_format = self.0.clone();
+    }
+}
+
+// ── Navigation ───────────────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct SetSelected(pub usize, pub usize);
+impl Effect for SetSelected {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().selected = (self.0, self.1);
+    }
+}
+
+#[derive(Debug)]
+pub struct SetRowOffset(pub usize);
+impl Effect for SetRowOffset {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().row_offset = self.0;
+    }
+}
+
+#[derive(Debug)]
+pub struct SetColOffset(pub usize);
+impl Effect for SetColOffset {
+    fn apply(&self, app: &mut App) {
+        app.model.active_view_mut().col_offset = self.0;
+    }
+}
+
+// ── App state ────────────────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct ChangeMode(pub AppMode);
+impl Effect for ChangeMode {
+    fn apply(&self, app: &mut App) {
+        app.mode = self.0.clone();
+    }
+}
+
+#[derive(Debug)]
+pub struct SetStatus(pub String);
+impl Effect for SetStatus {
+    fn apply(&self, app: &mut App) {
+        app.status_msg = self.0.clone();
+    }
+}
+
+#[derive(Debug)]
+pub struct MarkDirty;
+impl Effect for MarkDirty {
+    fn apply(&self, app: &mut App) {
+        app.dirty = true;
+    }
+}
+
+#[derive(Debug)]
+pub struct SetYanked(pub Option<CellValue>);
+impl Effect for SetYanked {
+    fn apply(&self, app: &mut App) {
+        app.yanked = self.0.clone();
+    }
+}
+
+#[derive(Debug)]
+pub struct SetSearchQuery(pub String);
+impl Effect for SetSearchQuery {
+    fn apply(&self, app: &mut App) {
+        app.search_query = self.0.clone();
+    }
+}
+
+#[derive(Debug)]
+pub struct SetSearchMode(pub bool);
+impl Effect for SetSearchMode {
+    fn apply(&self, app: &mut App) {
+        app.search_mode = self.0;
+    }
+}
+
+#[derive(Debug)]
+pub struct SetPendingKey(pub Option<char>);
+impl Effect for SetPendingKey {
+    fn apply(&self, app: &mut App) {
+        app.pending_key = self.0;
+    }
+}
+
+// ── Side effects ─────────────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct Save;
+impl Effect for Save {
+    fn apply(&self, app: &mut App) {
+        if let Some(ref path) = app.file_path {
+            match crate::persistence::save(&app.model, path) {
+                Ok(()) => {
+                    app.dirty = false;
+                    app.status_msg = format!("Saved to {}", path.display());
+                }
+                Err(e) => {
+                    app.status_msg = format!("Save error: {e}");
+                }
+            }
+        } else {
+            app.status_msg = "No file path — use :w <path>".to_string();
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct SaveAs(pub PathBuf);
+impl Effect for SaveAs {
+    fn apply(&self, app: &mut App) {
+        match crate::persistence::save(&app.model, &self.0) {
+            Ok(()) => {
+                app.file_path = Some(self.0.clone());
+                app.dirty = false;
+                app.status_msg = format!("Saved to {}", self.0.display());
+            }
+            Err(e) => {
+                app.status_msg = format!("Save error: {e}");
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct ExportCsv(pub PathBuf);
+impl Effect for ExportCsv {
+    fn apply(&self, app: &mut App) {
+        let view_name = app.model.active_view.clone();
+        match crate::persistence::export_csv(&app.model, &view_name, &self.0) {
+            Ok(()) => {
+                app.status_msg = format!("Exported to {}", self.0.display());
+            }
+            Err(e) => {
+                app.status_msg = format!("Export error: {e}");
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct SetPanelOpen {
+    pub panel: Panel,
+    pub open: bool,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub enum Panel {
+    Formula,
+    Category,
+    View,
+}
+
+impl Effect for SetPanelOpen {
+    fn apply(&self, app: &mut App) {
+        match self.panel {
+            Panel::Formula => app.formula_panel_open = self.open,
+            Panel::Category => app.category_panel_open = self.open,
+            Panel::View => app.view_panel_open = self.open,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct SetPanelCursor {
+    pub panel: Panel,
+    pub cursor: usize,
+}
+impl Effect for SetPanelCursor {
+    fn apply(&self, app: &mut App) {
+        match self.panel {
+            Panel::Formula => app.formula_cursor = self.cursor,
+            Panel::Category => app.cat_panel_cursor = self.cursor,
+            Panel::View => app.view_panel_cursor = self.cursor,
+        }
+    }
+}
+
+// ── Convenience constructors ─────────────────────────────────────────────────
+
+pub fn mark_dirty() -> Box<dyn Effect> {
+    Box::new(MarkDirty)
+}
+
+pub fn set_status(msg: impl Into<String>) -> Box<dyn Effect> {
+    Box::new(SetStatus(msg.into()))
+}
+
+pub fn change_mode(mode: AppMode) -> Box<dyn Effect> {
+    Box::new(ChangeMode(mode))
+}
+
+pub fn set_selected(row: usize, col: usize) -> Box<dyn Effect> {
+    Box::new(SetSelected(row, col))
+}

src/ui/mod.rs

@@ -1,5 +1,6 @@
 pub mod app;
 pub mod category_panel;
+pub mod effect;
 pub mod formula_panel;
 pub mod grid;
 pub mod help;