Overhaul inf-elixir-send-* buffer lookup

This is a relatively complex flow that I've wanted to revisit for some
time. I will be uploading a diagram to the wiki that encapsulates most
of the flow.

Also introduces a new `inf-elixir-set-repl` command which allows the
user to manually set what REPL they want the current buffer to send
to. This choice is remembered until the buffer is killed or the
command is run again.

inf-elixir.el

@@ -32,6 +32,7 @@
 
 (require 'comint)
 (require 'subr-x)
+(require 'map)
 
 
 ;;; Customization
@@ -69,11 +70,25 @@ NOTE: Changing this variable will not affect running REPLs."
   :type 'string
   :group 'inf-elixir)
 
+(defcustom inf-elixir-repl-buffer nil
+  "Override for what REPL buffer code snippets should be sent to.
+
+If this variable is set and the corresponding REPL buffer exists
+and has a living process, all `inf-elixir-send-*' commands will
+send to it.  If this variable is unset (the default) or the
+indicated buffer is dead or has a dead process, a warning will be
+printed instead."
+  :type 'buffer
+  :group 'inf-elixir)
+
 
 ;;; Mode definitions and configuration
 (defvar inf-elixir-project-buffers (make-hash-table :test 'equal)
   "A mapping of projects to buffers with running Elixir REPL subprocesses.")
 
+(defvar inf-elixir-unaffiliated-buffers '()
+  "A list of Elixir REPL buffers unaffiliated with any project.")
+
 ;;;###autoload
 (define-minor-mode inf-elixir-minor-mode
   "Minor mode for Elixir buffers that allows interacting with the REPL.")
@@ -154,18 +169,66 @@ Always returns a REPL buffer for DIR."
       (with-current-buffer
           (apply #'make-comint-in-buffer buf-name nil (car cmd) nil (cdr cmd))
         (inf-elixir-mode)
-        (when dir (inf-elixir--set-project-buffer dir (current-buffer)))
+        (if dir
+            (inf-elixir--set-project-buffer dir (current-buffer))
+          (add-to-list 'inf-elixir-unaffiliated-buffers (current-buffer)))
         (current-buffer)))))
 
-(defun inf-elixir--send (command)
-  "Send COMMAND to the REPL process in BUF."
-  (let* ((proj-dir (inf-elixir--find-project-root))
-         (proj-buf (inf-elixir--get-project-buffer proj-dir)))
-    (if (process-live-p (get-buffer-process proj-buf))
-        (with-current-buffer proj-buf
-          (comint-add-to-input-history command)
-          (comint-send-string proj-buf (concat command "\n")))
-      (message (concat "No REPL running in " (inf-elixir--project-name proj-dir))))))
+(defun inf-elixir--send (cmd)
+  "Determine where to send CMD and send it."
+  (when-let ((buf (inf-elixir--determine-repl-buf)))
+    (with-current-buffer buf
+      (comint-add-to-input-history cmd)
+      (comint-send-string buf (concat cmd "\n")))
+    (pop-to-buffer buf)))
+
+(defun inf-elixir--determine-repl-buf ()
+  "Determines where to send a cmd when `inf-elixir-send-*' are used."
+  (if inf-elixir-repl-buffer
+      (if (process-live-p (get-buffer-process inf-elixir-repl-buffer))
+          inf-elixir-repl-buffer
+        (inf-elixir--prompt-repl-buffers "`inf-elixir-repl-buffer' is dead, please choose another REPL buffer: "))
+    (if-let ((proj-dir (inf-elixir--find-project-root)))
+        (inf-elixir--determine-project-repl-buf proj-dir)
+      (inf-elixir--prompt-repl-buffers))))
+
+(defun inf-elixir--determine-project-repl-buf (proj-dir)
+  "Determines where to send a cmd when `inf-elixir-send-*' are used inside the PROJ-DIR Mix project."
+  (if-let ((proj-buf (inf-elixir--get-project-buffer proj-dir)))
+      (if (process-live-p (get-buffer-process proj-buf))
+          proj-buf
+        (if (y-or-n-p "A project REPL buffer exists, but the process is dead.  Start a new one? ")
+            (inf-elixir-project)
+          (inf-elixir--prompt-repl-buffers)))
+    (if (y-or-n-p "No REPL exists for this project yet.  Start one? ")
+        (inf-elixir-project)
+      (inf-elixir--prompt-repl-buffers))))
+
+(defun inf-elixir--prompt-repl-buffers (&optional prompt)
+  "Prompt the user to select an inf-elixir REPL buffers or create an new one.
+
+Returns the select buffer (as a buffer object).
+
+If PROMPT is supplied, it is used as the prompt for a REPL buffer.
+
+When the user selects a REPL, it is set as `inf-elixir-repl-buffer' locally in
+the buffer so that the choice is remembered for that buffer."
+  ;; Cleanup
+  (setq inf-elixir-unaffiliated-buffers (seq-filter 'buffer-live-p inf-elixir-unaffiliated-buffers))
+  (setq inf-elixir-project-buffers
+        (map-into
+         (map-filter (lambda (_dir buf) (buffer-live-p buf)) inf-elixir-project-buffers)
+         '(hash-table :test equal)))
+  ;; Actual functionality
+  (let* ((repl-buffers (append
+                       '("Create new")
+                       (mapcar (lambda (buf) `(,(buffer-name buf) . buf)) inf-elixir-unaffiliated-buffers)
+                       (mapcar (lambda (buf) `(,(buffer-name buf) . buf)) (hash-table-values inf-elixir-project-buffers))))
+         (prompt (or prompt "Which REPL?"))
+         (selected-buf (completing-read prompt repl-buffers (lambda (_) t) t)))
+    (setq-local inf-elixir-repl-buffer (if (equal selected-buf "Create new")
+                                           (inf-elixir)
+                                         selected-buf))))
 
 
 ;;; Public functions
@@ -222,6 +285,11 @@ be prompted for the REPL command.  The default is provided by
         (inf-elixir-run-cmd default-directory cmd))
     (message "Could not find project root! Try `inf-elixir' instead.")))
 
+(defun inf-elixir-set-repl ()
+  "Select which REPL to use for this buffer."
+  (interactive)
+  (inf-elixir--prompt-repl-buffers))
+
 (defun inf-elixir-send-line ()
   "Send the region to the REPL buffer and run it."
   (interactive)