Refactor MRU EXWM workspace using cycle/focus-previous!

Instead of consuming `cycle/previous-focus`, define a function
`cycle/focus-previous!` that "focuses" the element at `previous-index` and
returns that element.

This function greatly simplified the code in window-manager.el and eliminated
the unnecessary `exwm/previous-workspace` variable that was managing the state.

configs/shared/.emacs.d/wpc/cycle.el

@@ -75,6 +75,19 @@
         (nth i (cycle-xs cycle))
       nil)))
 
+;; TODO: Consider adding "!" to the function name herein since many of them
+;; mutate the collection, and the APIs are beginning to confuse me.
+(defun cycle/focus-previous! (xs)
+  "Jump to the item in XS that was most recently focused; return the cycle.
+This will error when previous-index is nil.  This function mutates the
+underlying struct."
+  (let ((i (cycle-previous-index xs)))
+    (if (maybe/some? i)
+        (progn
+          (cycle/jump i xs)
+          (cycle/current xs))
+      (error "Cannot focus the previous element since cycle-previous-index is nil"))))
+
 (defun cycle/next (xs)
   "Return the next value in `XS' and update `current-index'."
   (let* ((current-index (cycle-current-index xs))
@@ -135,7 +148,8 @@
     (prelude/assert (= 1 (->> xs (cycle/jump 0) cycle/current)))
     (prelude/assert (= 2 (->> xs (cycle/jump 1) cycle/current)))
     (prelude/assert (= 3 (->> xs (cycle/jump 2) cycle/current)))
-    (prelude/assert (= 2 (cycle/previous-focus xs)))))
+    (prelude/assert (= 2 (cycle/previous-focus xs)))
+    (prelude/assert (= 2 (cycle/focus-previous! xs)))))
 
 (provide 'cycle)
 ;;; cycle.el ends here

configs/shared/.emacs.d/wpc/window-manager.el

@@ -249,22 +249,12 @@
 (defun exwm/next-workspace ()
   "Cycle forwards to the next workspace."
   (interactive)
-  (exwm-workspace-switch
+  (exwm/change-workspace (cycle/next exwm/workspaces)))
-   (exwm/named-workspace-index (cycle/next exwm/workspaces)))
-  (window-manager/alert
-   (string/concat
-    "Current workspace: "
-    (exwm/named-workspace-label (cycle/current exwm/workspaces)))))
 
 (defun exwm/prev-workspace ()
   "Cycle backwards to the previous workspace."
   (interactive)
-  (exwm-workspace-switch
+  (exwm/change-workspace (cycle/prev exwm/workspaces)))
-   (exwm/named-workspace-index (cycle/prev exwm/workspaces)))
-  (window-manager/alert
-   (string/concat
-    "Current workspace: "
-    (exwm/named-workspace-label (cycle/current exwm/workspaces)))))
 
 ;; TODO: Create friendlier API for working with EXWM.
 
@@ -534,17 +524,26 @@ Currently using super- as the prefix for switching workspaces."
      (kbd/for 'workspace (s-capitalize key))
      handler)))
 
+(defun exwm/change-workspace (workspace)
+  "Switch EXWM workspaces to the WORKSPACE struct."
+  (exwm-workspace-switch (exwm/named-workspace-index workspace))
+  (window-manager/alert
+   (string/format "Switched to: %s" (exwm/named-workspace-label workspace))))
+
 (defun exwm/switch (label)
   "Switch to a named workspaces using LABEL."
-  (cycle/focus
+  (cycle/focus (lambda (x)
-   (lambda (x)
                  (equal label
                         (exwm/named-workspace-label x)))
                exwm/workspaces)
-  (exwm-workspace-switch
+  (exwm/change-workspace (cycle/current exwm/workspaces)))
-   (exwm/named-workspace-index (cycle/current exwm/workspaces)))
+
-  (window-manager/alert
+;; TODO: Assign an easy-to-remember keybinding to this.
-   (string/concat "Switched to: " label)))
+(exwm-input-set-key (kbd "C-S-f") #'exwm/toggle-previous)
+(defun exwm/toggle-previous ()
+  "Focus the previously active EXWM workspace."
+  (interactive)
+  (exwm/change-workspace (cycle/focus-previous! exwm/workspaces)))
 
 (defun exwm/ivy-switch ()
   "Use ivy to switched between named workspaces."