#+TITLE: Plugin / Slot System (v0.11.0)
#+DATE: 2026-05-11
#+AUTHOR: Amr Gharbeia / Hermes
#+STARTUP: content

* Overview

Extensible named slots. Applications and plugins register content into
named slots. The component tree renders whatever is registered.

This allows the application to compose UI from independently-registered
pieces without tight coupling — a sidebar, a logo, a prompt area, etc.

** Contract

- ~defslot name &key order render-fn mode~ — register a render function for a slot
- ~slot-render slot-name &rest args~ — call all registered render-fns, return combined output
- ~slot-p slot-name~ — check if a slot has registrations
- ~clear-slot slot-name~ — remove all registrations for a slot
- ~list-slots~ — return all slot names with registrations

** Slot modes

- ~:stack~ (default) — render all registered functions in ~:order~ sequence.
  Each ~defslot~ adds to the list. ~slot-render~ calls every function and
  returns a list of results. Use this for composable slots where multiple
  plugins contribute content (e.g., toolbar buttons, status bar segments).

- ~:replace~ — last registration wins, previous ones are discarded.
  Each ~defslot~ replaces the slot's entire entry list with the new
  registration. ~slot-render~ calls only the most recently registered
  function. Use this for exclusive slots where only one renderer should
  be active at a time (e.g., main content area, active panel).

- ~:single-winner~ — first registration wins, subsequent ones are ignored.
  Once a slot has an entry, further ~defslot~ calls for the same slot are
  no-ops. ~slot-render~ calls only the first (lowest-order) registered
  function. Use this for slots where the first plugin to register should
  own the spot (e.g., logo area, command palette).

The mode is set on the first ~defslot~ call for a slot. Subsequent calls
for the same slot ignore the ~:mode~ argument and use the established
mode — this prevents confusion when multiple plugins register into the
same slot with conflicting mode specifications.

* Implementation

** Package

The package provides the public API and exports all slot system symbols.
Clients :use this package or refer to symbols qualified.

#+BEGIN_SRC lisp :tangle ../src/components/slot-package.lisp
(defpackage :cl-tty.slot
  (:use :cl)
  (:export
   #:defslot
   #:slot-render
   #:slot-p
   #:clear-slot
   #:list-slots
   #:*slots*))
#+END_SRC

** Slot Storage: *slots*

The central registry is a hash table keyed by slot name (strings, for
case-insensitive lookup via ~equal~). Each value is a plist:

- ~:mode~ — the slot's mode keyword (~:stack~, ~:replace~, ~:single-winner~)
- ~:entries~ — list of ~(order . render-fn)~ cons cells, sorted by order

The ~:test #'equal~ ensures that ~:sidebar~ and ~"sidebar"~ map to the
same key.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(in-package :cl-tty.slot)

(defvar *slots* (make-hash-table :test 'equal)
  "Hash table mapping slot name (string) -> plist of slot data.
Each entry: (:mode <mode> :entries <(order . render-fn) list>).")
#+END_SRC

** defslot: Register a Render Function

~defslot~ inserts a new ~(order . render-fn)~ entry into the slot's
entry list. The behavior depends on the slot's mode, which is set on
the first call and frozen for subsequent calls:

- ~:stack~ — merge into existing entries, sorted by order
- ~:replace~ — clear all previous entries, keep only the new one
- ~:single-winner~ — no-op if the slot already has entries

The ~render-fn~ itself is returned so callers can use it inline.

The mode parameter is validated on first call via ~assert~ and then
frozen for subsequent calls. This prevents a later registration from
changing the slot's semantics out from under earlier registrations.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(defun defslot (name &key (order 0) render-fn (mode :stack))
  (let* ((key (string name))
         (slot (gethash key *slots*)))
    (if (null slot)
        ;; First registration — validate and set mode, create entry
        (progn
          (assert (member mode '(:stack :replace :single-winner)) ()
                  "Invalid slot mode: ~S (use :stack, :replace, or :single-winner)"
                  mode)
          (setf (gethash key *slots*)
                (list :mode mode
                      :entries (list (cons order render-fn)))))
        ;; Existing slot — respect frozen mode
        (let ((entries (getf slot :entries)))
          (ecase (getf slot :mode)
            (:stack
             (setf (getf slot :entries)
                   (sort (cons (cons order render-fn) entries)
                         #'< :key #'car)))
            (:replace
             (setf (getf slot :entries)
                   (list (cons order render-fn))))
            (:single-winner
             ;; First registration already present — no-op
             (values))))))
  render-fn)
#+END_SRC

** slot-render: Invoke Render Functions Per Mode

~slot-render~ dispatches on the slot's mode:

- ~:stack~ — call every non-nil render function in order, return a list
  of results. This is the most flexible mode, supporting multiple
  contributors per slot.

- ~:replace~ — call only the single registered function (the last one
  registered, since :replace clears earlier entries). Returns a single
  value, not a list.

- ~:single-winner~ — call only the first registered function (lowest
  order). Subsequent registrations were silently dropped during defslot.

Returns ~nil~ if the slot has no registrations or if the handler is nil.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(defun slot-render (slot-name &rest args)
  (let ((slot (gethash (string slot-name) *slots*)))
    (when slot
      (let ((mode (getf slot :mode))
            (entries (getf slot :entries)))
        (ecase mode
          (:stack
           (mapcar (lambda (entry)
                     (let ((fn (cdr entry)))
                       (when fn (apply fn args))))
                   entries))
          (:replace
           (let ((fn (cdar (last entries))))
             (when fn (apply fn args))))
          (:single-winner
           (let ((fn (cdar entries)))
             (when fn (apply fn args)))))))))
#+END_SRC

** slot-p: Check Slot Existence

Uses ~nth-value 1~ of ~gethash~ which returns ~t~ if the key is
present (even if the value is ~nil~) or ~nil~ if absent. This is the
canonical Common Lisp idiom for testing hash-table membership.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(defun slot-p (slot-name)
  (nth-value 1 (gethash (string slot-name) *slots*)))
#+END_SRC

** clear-slot: Remove All Registrations

Calls ~remhash~ to delete the slot's entry from the hash table
entirely. After this call ~slot-p~ returns false and ~slot-render~
returns nil for the given slot name.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(defun clear-slot (slot-name)
  (remhash (string slot-name) *slots*))
#+END_SRC

** list-slots: Enumerate Registered Slots

Iterates over all hash keys in ~*slots*~ and returns them as a list.
Only slots that have been registered (i.e. have at least one entry)
appear in the result.

#+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
(defun list-slots ()
  (loop for key being the hash-keys of *slots* collect key))
#+END_SRC

** Tests

The test suite uses FiveAM and exercises each public function,
including mode-specific behavior.

*** Test Package and Suite

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(defpackage :cl-tty-slot-test (:use :cl :cl-tty.slot :fiveam))
(in-package :cl-tty-slot-test)

(def-suite slot-suite :description "Slot system tests")
(in-suite slot-suite)
#+END_SRC

*** defslot-register: Registering a slot makes it visible

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test defslot-register ()
  (clear-slot :test-slot)
  (defslot :test-slot :order 1 :render-fn (lambda () "hello"))
  (is-true (slot-p :test-slot)))
#+END_SRC

*** slot-render-calls: Stack mode calls all functions in order

Verifies that ~:stack~ mode preserves multiple registrations and calls
them in ascending order sequence.

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test slot-render-calls ()
  (clear-slot :test-slot)
  (defslot :test-slot :order 1 :render-fn (lambda () "a"))
  (defslot :test-slot :order 2 :render-fn (lambda () "b"))
  (is (equal '("a" "b") (slot-render :test-slot))))
#+END_SRC

*** slot-render-empty: Unregistered slot returns nil

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test slot-render-empty ()
  (clear-slot :ghost)
  (is-false (slot-render :ghost)))
#+END_SRC

*** clear-slot-removes: Clearing a slot makes it absent

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test clear-slot-removes ()
  (clear-slot :test-slot)
  (defslot :test-slot :order 1 :render-fn (lambda () "x"))
  (clear-slot :test-slot)
  (is-false (slot-p :test-slot)))
#+END_SRC

*** stack-mode-multiple-entries: Stack keeps all registrations

Verifies that ~:stack~ mode (default) accumulates entries across
multiple ~defslot~ calls.

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test stack-mode-multiple-entries ()
  (clear-slot :stack-test)
  (defslot :stack-test :order 1 :render-fn (lambda () "first"))
  (defslot :stack-test :order 2 :render-fn (lambda () "second"))
  (defslot :stack-test :order 3 :render-fn (lambda () "third"))
  (is (equal '("first" "second" "third") (slot-render :stack-test))))
#+END_SRC

*** replace-mode-last-wins: Replace keeps only the last registration

Verifies that ~:replace~ mode discards previous entries on each new
~defslot~ call.

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test replace-mode-last-wins ()
  (clear-slot :replace-test)
  (defslot :replace-test :mode :replace :order 1 :render-fn (lambda () "old"))
  (defslot :replace-test :mode :replace :order 2 :render-fn (lambda () "new"))
  (is (equal "new" (slot-render :replace-test))))
#+END_SRC

*** single-winner-mode-first-wins: Single-winner keeps only the first

Verifies that ~:single-winner~ mode ignores subsequent registrations.

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test single-winner-mode-first-wins ()
  (clear-slot :winner-test)
  (defslot :winner-test :mode :single-winner :order 1
           :render-fn (lambda () "alpha"))
  (defslot :winner-test :mode :single-winner :order 2
           :render-fn (lambda () "beta"))
  (is (equal "alpha" (slot-render :winner-test))))
#+END_SRC

*** clear-slot-removes-mode: Clearing resets mode, allowing new mode

Verifies that clearing a slot removes the mode lock, so a subsequent
~defslot~ can set a new mode.

#+BEGIN_SRC lisp :tangle ../tests/slot-tests.lisp
(def-test clear-slot-removes-mode ()
  (clear-slot :mode-test)
  (defslot :mode-test :mode :replace :render-fn (lambda () "only"))
  (clear-slot :mode-test)
  (defslot :mode-test :mode :stack :render-fn (lambda () "fresh"))
  (is-true (slot-p :mode-test))
  (is (equal '("fresh") (slot-render :mode-test))))
#+END_SRC