Add a state monad as the foundation of Cubes

* Each cube will be a state monad and the description of how to
  change the state.

core/fg42/cube.el

@@ -0,0 +1,45 @@
+;;; Cube --- Cube library of FG42 -*- lexical-binding: t; -*-
+;;
+;; Copyright (c) 2010-2020 Sameer Rahmani & Contributors
+;;
+;; Author: Sameer Rahmani <lxsameer@gnu.org>
+;; URL: https://gitlab.com/FG42/FG42
+;; Version: 3.0.0
+;;
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+;;
+;;; Commentary:
+;; Cubes are the building blocks of any `FG42' editor.  Each `cube' is a
+;; unit which defines different abilities in a deterministic and idempotent
+;; way.  Cubes are composable and a composition of cubes creates an editor.
+;;
+;;; Code:
+(require 'fg42/utils)
+(require 'fg42/state)
+
+
+(defun fg42/cube-apply (state cube)
+  "Apply the given CUBE to the given STATE.
+It returns a cons of (new-state . cube)."
+  (cons state cube))
+
+
+(defun fg42/cube-bind (m f)
+  "Bind the state monad M with the given function F."
+  (let ((binder (fg42/state-bind-maker #'fg42/cube-apply)))
+    (funcall binder m f)))
+
+
+(provide 'fg42/cube)
+;;; cube.el ends here

core/fg42/state.el

@@ -0,0 +1,84 @@
+;; State --- State library of FG42 -*- lexical-binding: t; -*-
+;;
+;; Copyright (c) 2010-2020 Sameer Rahmani & Contributors
+;;
+;; Author: Sameer Rahmani <lxsameer@gnu.org>
+;; URL: https://gitlab.com/FG42/FG42
+;; Version: 3.0.0
+;;
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+;;
+;;; Commentary:
+;; Cubes are the building blocks of any `FG42' editor.  Each `cube' is a
+;; unit which defines different abilities in a deterministic and idempotent
+;; way.  Cubes are composable and a composition of cubes creates an editor.
+;;
+;;; Code:
+(require 'fg42/utils)
+
+
+(defun fg42/state-return (x)
+  "Return a state monad containing X."
+  (lambda (state)
+    (cons state x)))
+
+
+(defun fg42/state-bind-maker (binder-fn)
+  "Return a bind function for state monad.
+It creates a bind function that binds monadic functions by applying BINDER-FN
+to the return state and the value inside the monad before binding them.
+
+The BINDER-FN has to return a cons in form of (state . prev-v)."
+  (lambda (m f)
+    "Applys F on the M monad."
+    (lambda (state)
+      (let* ((v (funcall m state))
+             (new-v (funcall (f (cdr v)) (car v))))
+        (funcall binder-fn
+                 ;; State
+                 (car new-v)
+                 ;; Value of in the monad from M
+                 (cdr new-v))))))
+
+
+(defun fg42/state-unit ()
+  "Create a new state monad.
+This monad describes the entire editor configuration"
+  ;; Why not a cl-struct ?
+  ;; Because first, we want it to be human readable
+  (lambda (state)
+    (cons
+     (list '(:mode->keybindings . ())
+           '(:mode->prefix . ())
+           '(:mode->fn . ())
+           '(:cubes . ()))
+     ())))
+
+
+(defun fg42/state-get (k &optional default)
+  "Return the value of the given K from the STATE.
+It will return DEFAULT in case of a missing key."
+  (lambda (state)
+    (or (cdr (assoc k state)) default)))
+
+
+
+(defun fg42/state-cons (k v)
+  "Add the given value V to the list addressed by key K on STATE."
+  (lambda (state)
+    (cons (cons k (cons v (or (fg42/state-get state k) '()))) state)))
+
+
+(provide 'fg42/state)
+;;; state.el ends here