Moved the plugin directory

.idea/dequoter.iml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <module type="WEB_MODULE" version="4">
   <component name="Go" enabled="true" />
+  <component name="GoModuleSettings" enabled="true" />
   <component name="NewModuleRootManager">
     <content url="file://$MODULE_DIR$" />
     <orderEntry type="inheritedJdk" />

README.md

@@ -10,6 +10,33 @@ where all your chances will be lost when you close it.
 
 This project uses [Wails](https://wails.io) to build a desktop application with a Go backend and a Vite frontend.
 
+## Plugins
+
+Dequoter can be extended with new filters written in [UCL](https://ucl.lmika.dev).
+On startup, Dequoter creates `$HOME/Library/Application Support/dev.lmika.dequoter/plugins` if needed,
+then evaluates every file ending in `.ucl` found in it (including subdirectories).
+Filters are defined with the `d:filter` builtin:
+
+```
+d:filter "String: To Upper Case" { |in|
+  strs:to-upper $in
+}
+```
+
+`d:filter` takes a label, which is what appears in the command palette (Cmd+P), and a proc. The proc is
+called with the text to process as its only argument. The result of the proc becomes the filter output:
+strings are used as-is, other values are converted to their string form, and a proc that returns nothing
+is reported as an error. Errors raised with `error` are shown in the status bar.
+
+A plugin filter with the same label as a built-in filter replaces it in the palette. If two plugin files
+define the same label, the one loaded later wins (files are loaded in lexical path order). Files that
+fail to load are skipped and reported in the status bar; the remaining files are still loaded.
+
+After editing plugin files, choose File → Reload Plugins (Shift+Cmd+R) to rescan the directory without
+restarting. This discards the previous plugin filters and UCL interpreter state before loading the files again.
+Use File → Open Plugins in Finder or File → Open Plugins in Terminal to access the directory.
+If you have plugins in the old `$HOME/.config/dequoter/plugins` directory, move them to the new location.
+
 ## Download
 
 [Download Release](/dequoter-darwin-arm64.zip)

app.go

@@ -4,8 +4,10 @@ import (
 	"context"
 	"fmt"
 	"log"
+	"os/exec"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/wailsapp/wails/v2/pkg/runtime"
 	"lmika.dev/pkg/modash/moslice"
@@ -15,15 +17,23 @@ import (
 
 // App struct
 type App struct {
-	store   *Store
-	uclInst *ucl.Inst
+	store *Store
 
-	ctx context.Context
+	// mu guards uclInst, plugins and ctx, which are swapped together when plugins are reloaded.
+	mu      sync.RWMutex
+	uclInst *ucl.Inst
+	plugins *PluginFilters
+	ctx     context.Context
+
+	// baseCtx is the raw Wails context supplied to startup. ctx is derived from it.
+	baseCtx context.Context
 }
 
-// NewApp creates a new App application struct
-func NewApp(store *Store) *App {
-	uclInst := ucl.New(
+// newUCLInst creates a UCL interpreter with the standard modules plus the plugin module.
+func newUCLInst(plugins *PluginFilters) *ucl.Inst {
+	return ucl.New(
+		ucl.WithModule(plugins.Module()),
+		ucl.WithModule(builtins.Bytes()),
 		ucl.WithModule(builtins.CSV(nil)),
 		ucl.WithModule(builtins.Fns()),
 		ucl.WithModule(builtins.FS(nil)),
@@ -33,17 +43,140 @@ func NewApp(store *Store) *App {
 		ucl.WithModule(builtins.OS()),
 		ucl.WithModule(builtins.Strs()),
 		ucl.WithModule(builtins.Time()),
+		ucl.WithModule(builtins.URLs()),
 	)
-	return &App{
+}
+
+// NewApp creates a new App application struct
+func NewApp(store *Store) *App {
+	app := &App{
 		store:   store,
-		uclInst: uclInst,
+		baseCtx: context.Background(),
 	}
+	app.setUCLInst(newUCLInst(newPluginFilters()), nil)
+	return app
+}
+
+// setUCLInst installs a UCL interpreter and its plugin registry, rebuilding the app context so that
+// filters which fetch the interpreter from the context see the new instance. A nil plugins argument
+// is replaced with an empty registry.
+func (a *App) setUCLInst(inst *ucl.Inst, plugins *PluginFilters) {
+	if plugins == nil {
+		plugins = newPluginFilters()
+	}
+
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	a.uclInst = inst
+	a.plugins = plugins
+	a.ctx = context.WithValue(a.baseCtx, uclInstKey, inst)
+}
+
+// current returns the interpreter, plugin registry and context in use.
+func (a *App) current() (*ucl.Inst, *PluginFilters, context.Context) {
+	a.mu.RLock()
+	defer a.mu.RUnlock()
+	return a.uclInst, a.plugins, a.ctx
+}
+
+// loadPluginsFrom builds a fresh interpreter and plugin registry, loads the plugin files found under
+// dir into them, and installs them as the current instance. Returns the number of plugin filters
+// registered. Any load errors are available from the new registry's Errors method.
+func (a *App) loadPluginsFrom(dir string) int {
+	plugins := newPluginFilters()
+	inst := newUCLInst(plugins)
+
+	plugins.LoadDir(context.WithValue(a.baseCtx, uclInstKey, inst), inst, dir)
+	a.setUCLInst(inst, plugins)
+	return len(plugins.filters)
 }
 
 // startup is called when the app starts. The context is saved
 // so we can call the runtime methods
 func (a *App) startup(ctx context.Context) {
-	a.ctx = context.WithValue(ctx, uclInstKey, a.uclInst)
+	a.baseCtx = ctx
+
+	dir, err := ensurePluginDir()
+	if err != nil {
+		plugins := newPluginFilters()
+		plugins.recordError(err)
+		a.setUCLInst(newUCLInst(plugins), plugins)
+		return
+	}
+	a.loadPluginsFrom(dir)
+}
+
+// domReady is called once the frontend has loaded. Reports any plugin load errors in the status bar.
+func (a *App) domReady(ctx context.Context) {
+	_, plugins, _ := a.current()
+	a.reportPluginStatus(len(plugins.filters), false)
+}
+
+// ReloadPlugins discards the current plugin filters and UCL interpreter, rescans the plugin
+// directory, and refreshes the command palette. Invoked from the File menu.
+func (a *App) ReloadPlugins() {
+	dir, err := ensurePluginDir()
+	if err != nil {
+		_, _, ctx := a.current()
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
+			Message: fmt.Sprintf("Cannot prepare plugin directory: %v", err),
+			Error:   true,
+		})
+		return
+	}
+
+	count := a.loadPluginsFrom(dir)
+
+	_, _, ctx := a.current()
+	runtime.EventsEmit(ctx, "processors-changed")
+	a.reportPluginStatus(count, true)
+}
+
+// openPluginDir opens the plugin directory using the specified macOS application.
+func (a *App) openPluginDir(application string) {
+	dir, err := ensurePluginDir()
+	if err == nil {
+		err = exec.Command("/usr/bin/open", "-a", application, dir).Run()
+	}
+	if err != nil {
+		_, _, ctx := a.current()
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
+			Message: fmt.Sprintf("Cannot open plugins in %s: %v", application, err),
+			Error:   true,
+		})
+	}
+}
+
+// reportPluginStatus shows plugin load errors in the status bar. If there are no errors and
+// reloaded is true, a confirmation message is shown instead.
+func (a *App) reportPluginStatus(filterCount int, reloaded bool) {
+	_, plugins, ctx := a.current()
+
+	if errs := plugins.Errors(); len(errs) > 0 {
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
+			Message: fmt.Sprintf("%d plugin file(s) failed to load: %v", len(errs), errs[0]),
+			Error:   true,
+		})
+		return
+	}
+
+	if reloaded {
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
+			Message: fmt.Sprintf("Plugins reloaded: %d filter(s)", filterCount),
+		})
+	}
+}
+
+// lookupProcessor finds a text processor by action key, preferring plugin-defined filters.
+func (a *App) lookupProcessor(name string) (TextProcessor, bool) {
+	_, plugins, _ := a.current()
+
+	if proc, ok := plugins.filters[name]; ok {
+		return proc, true
+	}
+	proc, ok := TextFilters[name]
+	return proc, ok
 }
 
 func (a *App) LoadCurrentBuffer() (string, error) {
@@ -55,7 +188,17 @@ func (a *App) SaveCurrentBuffer(buffer string) error {
 }
 
 func (a *App) ListProcessors() (resp []ListProcessorsResponse) {
+	_, plugins, _ := a.current()
+
+	pluginLabels := make(map[string]struct{}, len(plugins.filters))
+	for k, v := range plugins.filters {
+		pluginLabels[v.Label] = struct{}{}
+		resp = append(resp, ListProcessorsResponse{Name: k, Label: v.Label})
+	}
 	for k, v := range TextFilters {
+		if _, shadowed := pluginLabels[v.Label]; shadowed {
+			continue
+		}
 		resp = append(resp, ListProcessorsResponse{Name: k, Label: v.Label})
 	}
 	sort.Slice(resp, func(i, j int) bool { return resp[i].Label < resp[j].Label })
@@ -63,13 +206,15 @@ func (a *App) ListProcessors() (resp []ListProcessorsResponse) {
 }
 
 func (a *App) TriggerTextProcess(action string) {
-	runtime.EventsEmit(a.ctx, "request-text-process", RequestTextProcess{
+	_, _, ctx := a.current()
+	runtime.EventsEmit(ctx, "request-text-process", RequestTextProcess{
 		Action: action,
 	})
 }
 
 func (a *App) PromptUser(label string, resp func(string)) {
-	runtime.EventsOnce(a.ctx, "prompt-response", func(data ...interface{}) {
+	_, _, ctx := a.current()
+	runtime.EventsOnce(ctx, "prompt-response", func(data ...interface{}) {
 		if len(data) == 0 {
 			return
 		}
@@ -79,21 +224,29 @@ func (a *App) PromptUser(label string, resp func(string)) {
 		}
 		resp(ans)
 	})
-	runtime.EventsEmit(a.ctx, "prompt-request", PromptRequest{
+	runtime.EventsEmit(ctx, "prompt-request", PromptRequest{
 		Label: label,
 	})
 }
 
 func (a *App) ProcessText(req ProcessTextRequest) {
-	filter, ok := TextFilters[req.Action]
+	// Capture the context once so the whole operation runs against a single interpreter, even if
+	// plugins are reloaded part way through.
+	_, _, ctx := a.current()
+
+	filter, ok := a.lookupProcessor(req.Action)
 	if !ok {
 		log.Printf("Unknown filter: [%s]", req.Action)
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
+			Message: fmt.Sprintf("Unknown filter: %s", req.Action),
+			Error:   true,
+		})
 		return
 	}
 
 	applyFilter := func(filter TextFilter) {
 		resp, err := moslice.MapWithError(req.Input, func(span TextSpan) (TextSpan, error) {
-			outRes, err := filter(a.ctx, span.Text)
+			outRes, err := filter(ctx, span.Text)
 			if err != nil {
 				return TextSpan{}, err
 			}
@@ -106,14 +259,14 @@ func (a *App) ProcessText(req ProcessTextRequest) {
 			}, nil
 		})
 		if err != nil {
-			runtime.EventsEmit(a.ctx, "set-statusbar-message", SetStatusbarMessage{
+			runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
 				Message: fmt.Sprintf("Error running filter: %v", err.Error()),
 				Error:   true,
 			})
 			return
 		}
 
-		runtime.EventsEmit(a.ctx, "process-text-response", ProcessTextResponse{
+		runtime.EventsEmit(ctx, "process-text-response", ProcessTextResponse{
 			Output: resp,
 		})
 	}
@@ -128,25 +281,25 @@ func (a *App) ProcessText(req ProcessTextRequest) {
 			inBfr.WriteString(span.Text)
 		}
 
-		msg, err := filter.Analyze(a.ctx, inBfr.String())
+		msg, err := filter.Analyze(ctx, inBfr.String())
 		if err != nil {
-			runtime.EventsEmit(a.ctx, "set-statusbar-message", SetStatusbarMessage{
+			runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
 				Message: fmt.Sprintf("Error running analysis: %v", err.Error()),
 				Error:   true,
 			})
 			return
 		}
 
-		runtime.EventsEmit(a.ctx, "set-statusbar-message", SetStatusbarMessage{
+		runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
 			Message: msg,
 		})
 	case filter.Filter != nil:
 		applyFilter(filter.Filter)
 	case filter.FilterWithArg != nil:
 		a.PromptUser(filter.FilterWithArg.Label, func(ans string) {
-			filter, err := filter.FilterWithArg.OnConfirm(a.ctx, ans)
+			filter, err := filter.FilterWithArg.OnConfirm(ctx, ans)
 			if err != nil {
-				runtime.EventsEmit(a.ctx, "set-statusbar-message", SetStatusbarMessage{
+				runtime.EventsEmit(ctx, "set-statusbar-message", SetStatusbarMessage{
 					Message: fmt.Sprintf("Error running filter: %v", err.Error()),
 					Error:   true,
 				})

frontend/src/controllers/commands_controller.js

@@ -12,15 +12,33 @@ export class CommandsController extends Controller {
 
     async connect() {
         this._lastCommand = null;
+        this._options = [];
 
+        await this._loadProcessors();
+        window.runtime.EventsOn("processors-changed", () => {
+            this._loadProcessors();
+        });
+    }
+
+    async _loadProcessors() {
         let processors = await ListProcessors();
-        processors.forEach((processor) => {
+
+        let options = processors.map((processor) => {
             let option = document.createElement("option");
             option.value = processor.name;
             option.text = processor.label;
-            this.commandSelectTarget.appendChild(option);
+            return option;
         });
-        this._options = Array.from(this.commandSelectTarget.options);
+        this._options = options;
+        this.commandSelectTarget.replaceChildren(...options);
+
+        if (this._lastCommand !== null && !options.some(opt => opt.value === this._lastCommand)) {
+            this._lastCommand = null;
+        }
+
+        if (this.element.open) {
+            this._filterOptions(this.commandInputTarget.value);
+        }
     }
 
     showCommands(ev) {

frontend/wailsjs/go/main/App.d.ts

@@ -10,6 +10,8 @@ export function ProcessText(arg1:main.ProcessTextRequest):Promise<void>;
 
 export function PromptUser(arg1:string,arg2:any):Promise<void>;
 
+export function ReloadPlugins():Promise<void>;
+
 export function SaveCurrentBuffer(arg1:string):Promise<void>;
 
 export function TriggerTextProcess(arg1:string):Promise<void>;

frontend/wailsjs/go/main/App.js

@@ -18,6 +18,10 @@ export function PromptUser(arg1, arg2) {
   return window['go']['main']['App']['PromptUser'](arg1, arg2);
 }
 
+export function ReloadPlugins() {
+  return window['go']['main']['App']['ReloadPlugins']();
+}
+
 export function SaveCurrentBuffer(arg1) {
   return window['go']['main']['App']['SaveCurrentBuffer'](arg1);
 }

go.mod

@@ -9,7 +9,7 @@ require (
 	gopkg.in/yaml.v3 v3.0.1
 	lmika.dev/pkg/modash v0.1.0
 	lmika.dev/pkg/progdoc v0.0.0-20260207235039-984257fb414b
-	ucl.lmika.dev v0.1.3
+	ucl.lmika.dev v0.1.7
 )
 
 require (

go.sum

@@ -796,5 +796,5 @@ lmika.dev/pkg/modash v0.1.0/go.mod h1:8NDl/yR1eCCEhip9FJlVuMNXIeaztQ0Ks/tizExFcT
 lmika.dev/pkg/progdoc v0.0.0-20260207235039-984257fb414b h1:uarBEkpjAnpzO98btsA6ZDw9oGnJX8zSb8gZnCFl+6Y=
 lmika.dev/pkg/progdoc v0.0.0-20260207235039-984257fb414b/go.mod h1:GefQ+wxW6L0p2TfB4fzoP7ihOgEt5Lb/1qgXhruVHos=
 rsc.io/binaryregexp v0.2.0/go.mod h1:qTv7/COck+e2FymRvadv62gMdZztPaShugOCi3I+8D8=
-ucl.lmika.dev v0.1.3 h1:Y2m+ORnLMA8boQmkXpW41VbkR3bwmnZjDLGVTYbZQic=
-ucl.lmika.dev v0.1.3/go.mod h1:Hjl6Udph2yuOPcBSytz4SFJVt5MZbM5JyiSAyTMTbNU=
+ucl.lmika.dev v0.1.7 h1:3yfvWOhBnWpQIm4NhKrUBh5PajB1wzUWzoPPa8ltaVo=
+ucl.lmika.dev v0.1.7/go.mod h1:Hjl6Udph2yuOPcBSytz4SFJVt5MZbM5JyiSAyTMTbNU=

main.go

@@ -37,9 +37,20 @@ func main() {
 		appMenu.Append(menu.AppMenu()) // On macOS platform, this must be done right after `NewMenu()`
 	}
 	fileMenu := appMenu.AddSubmenu("File")
+	fileMenu.AddText("Reload Plugins", keys.Combo("r", keys.CmdOrCtrlKey, keys.ShiftKey), func(_ *menu.CallbackData) {
+		app.ReloadPlugins()
+	})
+	fileMenu.AddText("Open Plugins in Finder", nil, func(_ *menu.CallbackData) {
+		app.openPluginDir("Finder")
+	})
+	fileMenu.AddText("Open Plugins in Terminal", nil, func(_ *menu.CallbackData) {
+		app.openPluginDir("Terminal")
+	})
+	fileMenu.AddSeparator()
 	fileMenu.AddText("Quit", keys.CmdOrCtrl("q"), func(_ *menu.CallbackData) {
 		// `rt` is an alias of "github.com/wailsapp/wails/v2/pkg/runtime" to prevent collision with standard package
-		rt.Quit(app.ctx)
+		_, _, ctx := app.current()
+		rt.Quit(ctx)
 	})
 
 	if runtime.GOOS == "darwin" {
@@ -57,6 +68,7 @@ func main() {
 		},
 		BackgroundColour: &options.RGBA{R: 27, G: 38, B: 54, A: 1},
 		OnStartup:        app.startup,
+		OnDomReady:       app.domReady,
 		Bind: []interface{}{
 			app,
 		},

plugins.go

@@ -0,0 +1,202 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"log"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+
+	"ucl.lmika.dev/ucl"
+)
+
+const pluginKeyPrefix = "plugin:"
+
+// PluginFilters is the registry of text filters defined by UCL plugin files.
+type PluginFilters struct {
+	filters map[string]TextProcessor
+	errors  []error
+}
+
+func newPluginFilters() *PluginFilters {
+	return &PluginFilters{
+		filters: make(map[string]TextProcessor),
+	}
+}
+
+// Module returns the "d" UCL module, exposing d:filter.
+func (p *PluginFilters) Module() ucl.Module {
+	return ucl.Module{
+		Name: "d",
+		Builtins: map[string]ucl.BuiltinHandler{
+			"filter":    p.filterBuiltin,
+			"map-lines": p.mapLinesBuiltin,
+		},
+	}
+}
+
+// filterBuiltin implements `d:filter LABEL PROC`. PROC receives the filter input as its only
+// argument and its result becomes the filter output.
+func (p *PluginFilters) filterBuiltin(ctx context.Context, args ucl.CallArgs) (any, error) {
+	var (
+		label string
+		proc  ucl.Invokable
+	)
+
+	if err := args.Bind(&label, &proc); err != nil {
+		return nil, fmt.Errorf("d:filter: expected LABEL and PROC: %w", err)
+	}
+	if strings.TrimSpace(label) == "" {
+		return nil, errors.New("d:filter: label must not be empty")
+	}
+	if proc.IsNil() {
+		return nil, errors.New("d:filter: PROC must not be nil")
+	}
+
+	p.filters[pluginKeyPrefix+slugify(label)] = TextProcessor{
+		Label: label,
+		Filter: func(ctx context.Context, input string) (TextFilterResponse, error) {
+			res, err := proc.Invoke(ctx, input)
+			if err != nil {
+				return TextFilterResponse{}, err
+			}
+
+			switch v := res.(type) {
+			case nil:
+				return TextFilterResponse{}, errors.New("filter returned no value")
+			case string:
+				return TextFilterResponse{Output: v}, nil
+			default:
+				return TextFilterResponse{Output: fmt.Sprint(v)}, nil
+			}
+		},
+	}
+	return nil, nil
+}
+
+// mapLinesBuiltin implements `d:map-lines INPUT PROC` which applies a transform for each line of the input.
+func (p *PluginFilters) mapLinesBuiltin(ctx context.Context, args ucl.CallArgs) (any, error) {
+	var (
+		input string
+		proc  ucl.Invokable
+	)
+
+	if err := args.Bind(&input, &proc); err != nil {
+		return nil, fmt.Errorf("d:map-lines: expected INPUT and PROC: %w", err)
+	}
+
+	var (
+		dst     bytes.Buffer
+		dstLine bytes.Buffer
+	)
+
+	scnr := bufio.NewScanner(strings.NewReader(input))
+	isFirst := true
+	for scnr.Scan() {
+		dstLine.Reset()
+		line := scnr.Text()
+
+		out, err := proc.Invoke(ctx, line)
+		if err != nil {
+			return TextFilterResponse{}, err
+		} else if out == nil {
+			continue
+		} else if strOut, ok := out.(fmt.Stringer); ok {
+			dstLine.WriteString(strOut.String())
+		} else {
+			dstLine.WriteString(fmt.Sprint(out))
+		}
+
+		if isFirst {
+			isFirst = false
+		} else {
+			dst.WriteString("\n")
+		}
+
+		dst.WriteString(dstLine.String())
+	}
+
+	return dst.String(), nil
+}
+
+// LoadDir evaluates every *.ucl file found under dir (recursively, in lexical order). Files
+// that fail to load are recorded in Errors() and skipped; loading continues with the rest.
+// A missing directory is not an error.
+func (p *PluginFilters) LoadDir(ctx context.Context, inst *ucl.Inst, dir string) {
+	if _, err := os.Stat(dir); err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			p.recordError(fmt.Errorf("%s: %w", dir, err))
+		}
+		return
+	}
+
+	_ = filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			p.recordError(fmt.Errorf("%s: %w", path, err))
+			return nil
+		}
+		if d.IsDir() || !strings.EqualFold(filepath.Ext(path), ".ucl") {
+			return nil
+		}
+
+		if err := p.loadFile(ctx, inst, path); err != nil {
+			p.recordError(fmt.Errorf("%s: %w", path, err))
+		}
+		return nil
+	})
+}
+
+func (p *PluginFilters) loadFile(ctx context.Context, inst *ucl.Inst, path string) error {
+	f, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, err = inst.Eval(ctx, f)
+	return err
+}
+
+func (p *PluginFilters) recordError(err error) {
+	log.Printf("plugin load error: %v", err)
+	p.errors = append(p.errors, err)
+}
+
+// Errors returns the errors encountered while loading plugin files.
+func (p *PluginFilters) Errors() []error {
+	return p.errors
+}
+
+// pluginDir returns the plugin directory under macOS Application Support.
+func pluginDir() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, "Library", "Application Support", "dev.lmika.dequoter", "Plugins"), nil
+}
+
+func ensurePluginDir() (string, error) {
+	dir, err := pluginDir()
+	if err != nil {
+		return "", err
+	}
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return "", fmt.Errorf("create plugin directory: %w", err)
+	}
+	return dir, nil
+}
+
+var nonSlugChars = regexp.MustCompile(`[^a-z0-9]+`)
+
+// slugify converts a label into a key-safe slug: lower-cased, with runs of characters other than
+// [a-z0-9] replaced by a single hyphen and leading/trailing hyphens removed.
+func slugify(label string) string {
+	return strings.Trim(nonSlugChars.ReplaceAllString(strings.ToLower(label), "-"), "-")
+}

plugins_test.go

@@ -0,0 +1,412 @@
+package main
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"ucl.lmika.dev/ucl"
+	"ucl.lmika.dev/ucl/builtins"
+)
+
+func newTestPlugins() (*PluginFilters, *ucl.Inst) {
+	plugins := newPluginFilters()
+	inst := ucl.New(
+		ucl.WithModule(plugins.Module()),
+		ucl.WithModule(builtins.Strs()),
+	)
+	return plugins, inst
+}
+
+func writePluginFile(t *testing.T, dir, name, content string) {
+	t.Helper()
+	path := filepath.Join(dir, name)
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestPluginFilters_LoadDir(t *testing.T) {
+	ctx := context.Background()
+
+	t.Run("registers filter from upper-case example", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+		dir := t.TempDir()
+		writePluginFile(t, dir, "upper.ucl", `
+d:filter "String: To Upper Case" { |in|
+  strs:to-upper $in
+}
+`)
+
+		plugins.LoadDir(ctx, inst, dir)
+
+		if len(plugins.Errors()) != 0 {
+			t.Fatalf("unexpected errors: %v", plugins.Errors())
+		}
+		proc, ok := plugins.filters["plugin:string-to-upper-case"]
+		if !ok {
+			t.Fatalf("filter not registered, have %v", plugins.filters)
+		}
+		if proc.Label != "String: To Upper Case" {
+			t.Errorf("label = %q", proc.Label)
+		}
+		res, err := proc.Filter(ctx, "abc")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if res.Output != "ABC" || res.Append {
+			t.Errorf("got %+v", res)
+		}
+	})
+
+	t.Run("finds nested files and ignores non-ucl files", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+		dir := t.TempDir()
+		writePluginFile(t, dir, "sub/dir/nested.ucl", `d:filter "Nested" { |in| $in }`)
+		writePluginFile(t, dir, "notes.txt", `d:filter "Ignored" { |in| $in }`)
+		writePluginFile(t, dir, "README.md", `this is not ucl {`)
+
+		plugins.LoadDir(ctx, inst, dir)
+
+		if len(plugins.Errors()) != 0 {
+			t.Fatalf("unexpected errors: %v", plugins.Errors())
+		}
+		if _, ok := plugins.filters["plugin:nested"]; !ok {
+			t.Error("nested filter not registered")
+		}
+		if _, ok := plugins.filters["plugin:ignored"]; ok {
+			t.Error("non-ucl file should have been ignored")
+		}
+	})
+
+	t.Run("missing directory is not an error", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+
+		plugins.LoadDir(ctx, inst, filepath.Join(t.TempDir(), "does-not-exist"))
+
+		if len(plugins.Errors()) != 0 {
+			t.Fatalf("unexpected errors: %v", plugins.Errors())
+		}
+		if len(plugins.filters) != 0 {
+			t.Errorf("unexpected filters: %v", plugins.filters)
+		}
+	})
+
+	t.Run("bad file is reported and sibling still loads", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+		dir := t.TempDir()
+		writePluginFile(t, dir, "a-bad.ucl", `d:filter "Broken" { |in| `)
+		writePluginFile(t, dir, "b-good.ucl", `d:filter "Good" { |in| $in }`)
+		writePluginFile(t, dir, "c-bad-usage.ucl", `d:filter "Only Label"`)
+
+		plugins.LoadDir(ctx, inst, dir)
+
+		errs := plugins.Errors()
+		if len(errs) != 2 {
+			t.Fatalf("expected 2 errors, got %v", errs)
+		}
+		if !strings.Contains(errs[0].Error(), "a-bad.ucl") {
+			t.Errorf("first error should name the file: %v", errs[0])
+		}
+		if !strings.Contains(errs[1].Error(), "c-bad-usage.ucl") {
+			t.Errorf("second error should name the file: %v", errs[1])
+		}
+		if _, ok := plugins.filters["plugin:good"]; !ok {
+			t.Error("good filter not registered")
+		}
+		if _, ok := plugins.filters["plugin:broken"]; ok {
+			t.Error("broken filter should not be registered")
+		}
+	})
+
+	t.Run("later definition with same label wins", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+		dir := t.TempDir()
+		writePluginFile(t, dir, "01-first.ucl", `d:filter "Dup" { |in| "first" }`)
+		writePluginFile(t, dir, "02-second.ucl", `d:filter "Dup" { |in| "second" }`)
+
+		plugins.LoadDir(ctx, inst, dir)
+
+		if len(plugins.filters) != 1 {
+			t.Fatalf("expected 1 filter, got %v", plugins.filters)
+		}
+		res, err := plugins.filters["plugin:dup"].Filter(ctx, "x")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if res.Output != "second" {
+			t.Errorf("output = %q", res.Output)
+		}
+	})
+
+	t.Run("procs defined in plugin files are reusable", func(t *testing.T) {
+		plugins, inst := newTestPlugins()
+		dir := t.TempDir()
+		writePluginFile(t, dir, "helpers.ucl", `
+proc shout { |s| strs:to-upper $s }
+d:filter "Shout" { |in| shout $in }
+`)
+
+		plugins.LoadDir(ctx, inst, dir)
+
+		if len(plugins.Errors()) != 0 {
+			t.Fatalf("unexpected errors: %v", plugins.Errors())
+		}
+		res, err := plugins.filters["plugin:shout"].Filter(ctx, "hi")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if res.Output != "HI" {
+			t.Errorf("output = %q", res.Output)
+		}
+	})
+}
+
+func TestPluginFilters_ProcResults(t *testing.T) {
+	ctx := context.Background()
+
+	tests := []struct {
+		name    string
+		script  string
+		input   string
+		want    string
+		wantErr string
+	}{
+		{
+			name:   "string result used as-is",
+			script: `d:filter "F" { |in| strs:to-upper $in }`,
+			input:  "abc",
+			want:   "ABC",
+		},
+		{
+			name:   "non-string result is stringified",
+			script: `d:filter "F" { |in| add 40 2 }`,
+			input:  "ignored",
+			want:   "42",
+		},
+		{
+			name:    "nil result is an error",
+			script:  `d:filter "F" { |in| }`,
+			input:   "abc",
+			wantErr: "filter returned no value",
+		},
+		{
+			name:    "error raised in proc propagates",
+			script:  `d:filter "F" { |in| error "boom" }`,
+			input:   "abc",
+			wantErr: "boom",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			plugins, inst := newTestPlugins()
+			if _, err := inst.EvalString(ctx, tt.script); err != nil {
+				t.Fatal(err)
+			}
+			proc, ok := plugins.filters["plugin:f"]
+			if !ok {
+				t.Fatalf("filter not registered")
+			}
+
+			res, err := proc.Filter(ctx, tt.input)
+			if tt.wantErr != "" {
+				if err == nil || !strings.Contains(err.Error(), tt.wantErr) {
+					t.Fatalf("expected error containing %q, got %v (res %+v)", tt.wantErr, err, res)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if res.Output != tt.want {
+				t.Errorf("output = %q, want %q", res.Output, tt.want)
+			}
+		})
+	}
+}
+
+func TestPluginFilters_FilterBuiltinValidation(t *testing.T) {
+	ctx := context.Background()
+
+	tests := []struct {
+		name   string
+		script string
+	}{
+		{name: "missing proc", script: `d:filter "Label"`},
+		{name: "no args", script: `d:filter`},
+		{name: "empty label", script: `d:filter "  " { |in| $in }`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			plugins, inst := newTestPlugins()
+			if _, err := inst.EvalString(ctx, tt.script); err == nil {
+				t.Fatal("expected an error")
+			}
+			if len(plugins.filters) != 0 {
+				t.Errorf("no filter should be registered, got %v", plugins.filters)
+			}
+		})
+	}
+}
+
+func TestSlugify(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string
+	}{
+		{"String: To Upper Case", "string-to-upper-case"},
+		{"  Lines: Go Template…  ", "lines-go-template"},
+		{"already-a-slug", "already-a-slug"},
+		{"MiXeD CaSe 123", "mixed-case-123"},
+		{"---", ""},
+	}
+	for _, tt := range tests {
+		if got := slugify(tt.in); got != tt.want {
+			t.Errorf("slugify(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}
+
+func TestApp_PluginIntegration(t *testing.T) {
+	ctx := context.Background()
+	app := NewApp(nil)
+	dir := t.TempDir()
+	writePluginFile(t, dir, "upper.ucl", `d:filter "String: To Upper Case" { |in| strs:to-upper $in }`)
+	writePluginFile(t, dir, "rev.ucl", `d:filter "Custom: Echo" { |in| $in }`)
+
+	if n := app.loadPluginsFrom(dir); n != 2 {
+		t.Fatalf("expected 2 filters loaded, got %d", n)
+	}
+	_ = ctx
+
+	t.Run("ListProcessors shadows built-in by label", func(t *testing.T) {
+		resp := app.ListProcessors()
+
+		byLabel := map[string][]string{}
+		for _, r := range resp {
+			byLabel[r.Label] = append(byLabel[r.Label], r.Name)
+		}
+		if names := byLabel["String: To Upper Case"]; len(names) != 1 || names[0] != "plugin:string-to-upper-case" {
+			t.Errorf("expected only the plugin entry for the shadowed label, got %v", names)
+		}
+		if names := byLabel["Custom: Echo"]; len(names) != 1 || names[0] != "plugin:custom-echo" {
+			t.Errorf("expected plugin entry, got %v", names)
+		}
+		if names := byLabel["String: To Lower Case"]; len(names) != 1 || names[0] != "lower-case" {
+			t.Errorf("built-in should be unaffected, got %v", names)
+		}
+		for i := 1; i < len(resp); i++ {
+			if resp[i-1].Label > resp[i].Label {
+				t.Errorf("response not sorted by label at %d: %q > %q", i, resp[i-1].Label, resp[i].Label)
+			}
+		}
+	})
+
+	t.Run("lookupProcessor prefers plugin then falls back to built-in", func(t *testing.T) {
+		if proc, ok := app.lookupProcessor("plugin:custom-echo"); !ok || proc.Label != "Custom: Echo" {
+			t.Errorf("plugin lookup failed: %v %v", proc, ok)
+		}
+		if proc, ok := app.lookupProcessor("lower-case"); !ok || proc.Label != "String: To Lower Case" {
+			t.Errorf("built-in lookup failed: %v %v", proc, ok)
+		}
+		if _, ok := app.lookupProcessor("nope"); ok {
+			t.Error("unknown key should not be found")
+		}
+	})
+}
+
+func TestApp_ReloadPlugins(t *testing.T) {
+	ctx := context.Background()
+	app := NewApp(nil)
+	dir := t.TempDir()
+
+	writePluginFile(t, dir, "a.ucl", `
+proc helper { |s| strs:to-upper $s }
+d:filter "Filter A" { |in| helper $in }
+`)
+
+	if n := app.loadPluginsFrom(dir); n != 1 {
+		t.Fatalf("expected 1 filter, got %d", n)
+	}
+
+	firstInst, firstPlugins, firstCtx := app.current()
+	if uclInstFromContext(firstCtx) != firstInst {
+		t.Fatal("context does not carry the current interpreter")
+	}
+	if res, err := firstInst.EvalString(ctx, `helper "x"`); err != nil || res != "X" {
+		t.Fatalf("helper proc should be defined: %v %v", res, err)
+	}
+	filterA, ok := firstPlugins.filters["plugin:filter-a"]
+	if !ok {
+		t.Fatal("Filter A not registered")
+	}
+
+	// Replace a.ucl with b.ucl and reload.
+	if err := os.Remove(filepath.Join(dir, "a.ucl")); err != nil {
+		t.Fatal(err)
+	}
+	writePluginFile(t, dir, "b.ucl", `d:filter "Filter B" { |in| strs:to-lower $in }`)
+
+	if n := app.loadPluginsFrom(dir); n != 1 {
+		t.Fatalf("expected 1 filter after reload, got %d", n)
+	}
+
+	secondInst, secondPlugins, secondCtx := app.current()
+	if secondInst == firstInst {
+		t.Error("interpreter should have been rebuilt")
+	}
+	if uclInstFromContext(secondCtx) != secondInst {
+		t.Error("context should carry the new interpreter")
+	}
+	if _, ok := secondPlugins.filters["plugin:filter-a"]; ok {
+		t.Error("Filter A should have been deregistered")
+	}
+	if _, ok := app.lookupProcessor("plugin:filter-a"); ok {
+		t.Error("lookupProcessor should no longer find Filter A")
+	}
+	if proc, ok := app.lookupProcessor("plugin:filter-b"); !ok || proc.Label != "Filter B" {
+		t.Errorf("Filter B should be registered, got %v %v", proc, ok)
+	}
+	if _, err := secondInst.EvalString(ctx, `helper "x"`); err == nil {
+		t.Error("helper proc from the removed file should no longer be defined")
+	}
+
+	t.Run("filter captured before reload still works", func(t *testing.T) {
+		res, err := filterA.Filter(ctx, "abc")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if res.Output != "ABC" {
+			t.Errorf("output = %q", res.Output)
+		}
+	})
+
+	t.Run("reload with bad file reports fresh errors", func(t *testing.T) {
+		writePluginFile(t, dir, "c.ucl", `d:filter "Broken" { |in|`)
+
+		n := app.loadPluginsFrom(dir)
+
+		_, plugins, _ := app.current()
+		if n != 1 || len(plugins.filters) != 1 {
+			t.Errorf("expected only Filter B to survive, got n=%d filters=%v", n, plugins.filters)
+		}
+		if errs := plugins.Errors(); len(errs) != 1 || !strings.Contains(errs[0].Error(), "c.ucl") {
+			t.Errorf("expected one error naming c.ucl, got %v", errs)
+		}
+
+		if err := os.Remove(filepath.Join(dir, "c.ucl")); err != nil {
+			t.Fatal(err)
+		}
+		app.loadPluginsFrom(dir)
+		if _, plugins, _ := app.current(); len(plugins.Errors()) != 0 {
+			t.Errorf("errors should be cleared after a clean reload, got %v", plugins.Errors())
+		}
+	})
+}